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Cannonmn
11-28-2016, 05:33 PM
Friend will sell me a gear head drill, runs on 460v, I only have 220, so for slightly more he'll throw in a step-up transformer after he removes it from the machine onto which it is now bolted. Never had 460v. before, so tell me if there are any special challenges, or everyone does it and no sweat.

flylo
11-28-2016, 05:54 PM
No problem with a transformer. I don't know why but most radial drills I've seen are 460-480V & not convertable like most 3phase machines. The Swiss one I got with the machine shop is but it's the only one I've seen.

Fasttrack
11-28-2016, 05:59 PM
My BIL has a K&T 2D on 460 volts through a step up transformer. No problems or special issues with it...

J Tiers
11-28-2016, 06:27 PM
The 480 is still in the UL voltage range, which is up to 600V. Insulated wire for 600V is commonly available.

It DOES involve you in connectors which can be significantly more expensive, especially if at higher currents, so hard wiring with disconnect may be better.

Above that is "medium voltage", which is not so nice..... Motors running on 4160V? Not preferred in my shop, thanks.....

PStechPaul
11-28-2016, 06:55 PM
Also make sure any fuses or circuit breakers on the 460V line are properly rated. Usually 600V, but some are 500V. Don't use the more common 300V rated protection devices or switches. You may be able to hard wire from the 460V secondary of the autotransformer and use fuses and switches on the 230V primary side. This should be OK if the autotransformer is located close to the machine and proper wiring is used to the motor. Armored cable might be a good idea, but make sure the cut ends are properly terminated to keep ground continuity and protect the wires inside.

Jon Heron
11-28-2016, 09:42 PM
Fuse the primary at 125% of the primary current rating ((KVA/831)*1.25) or fuse the primary up to 300% of the primary current and fuse the secondary at 125% secondary current.
The latter is more reliable but the first is likely all you will need for a machine tool.
If its an auto transformer (likely) no provinsions for grounding the secondary are required as you will have a reference through the tranny.
Other than that there s nothing to worry about, 480/277 is the standard industrial voltage int he US, in Canada its 600/347.
Cheers,
Jon

CalM
11-28-2016, 10:34 PM
Friend will sell me a gear head drill, runs on 460v, I only have 220, so for slightly more he'll throw in a step-up transformer after he removes it from the machine onto which it is now bolted. Never had 460v. before, so tell me if there are any special challenges, or everyone does it and no sweat.

Do you have 3ph on the wall?

Going 220 1ph, to 440V 3ph takes "extra equipment".

Cannonmn
11-29-2016, 12:08 AM
Do you have 3ph on the wall?

Going 220 1ph, to 440V 3ph takes "extra equipment".

Thanks, yes have 220 3ph on the wall, is an industrial-type building. Have a friend who hooks up the machines, he doesn't like plugs etc. so everything gets hard-wired to the wall off a breaker in the main panel with appropriate rating.

CalM
11-29-2016, 12:20 AM
Then it is only a matter of a suitable transformer

hitnmiss
11-29-2016, 10:13 AM
my Feeler HLV copy lathe is 440 3 Phase, I have a rotary making 3 phase 220, then I use a step down transformer backwards to power the lathe. Works great and the wires (factory on the lathe) are tiny!

Go for it, 440 stuff is cheap cause home guys are scared of it.

lakeside53
11-29-2016, 11:44 AM
Friend will sell me a gear head drill, runs on 460v, I only have 220, so for slightly more he'll throw in a step-up transformer after he removes it from the machine onto which it is now bolted. Never had 460v. before, so tell me if there are any special challenges, or everyone does it and no sweat.

Do you have 208 or 240 3 phase? 220 would be odd ball unless you have buck/boost transformers in line. Most common is 208 (wye). If that the case you just need 208-> 480 transformer. or.. Change the motor. Don't worry about the "460" verses "480". 460 is the motor design criteria so it can develop full power with an assumed voltage drop on input.

Take great care with 480. You can die. If you have no real experience, get an electrician to wire it for you.

Cannonmn
11-29-2016, 12:04 PM
Thanks, yes, cheap, Swedish-made large bench model, works fine, $350 incl transformer. Think it takes M3 drills. Speed adjusts from very slow to 1000 rpm. Yes I think our voltage is actually 208. I think we'll attach the transformer box to the drill to eliminate need for 440v wires around the shop.

Michael Edwards
11-29-2016, 12:20 PM
I have a rotary making 3 phase 220, then I use a step down transformer backwards to power the lathe. Works great

I did the same thing for a Kasto power hacksaw, it was easier than finding a suitable 2-speed replacement motor.

ME

Seastar
11-29-2016, 12:22 PM
Lots of years ago when I was at Purdue University taking my Bachelors degree I had a professor in an electrical machines class that had helped several States design their electric chairs. Back then it was the preferred method of execution.
He stated that 110/115 Volts AC was just about the lowest voltage that wold kill an adult human under ideal conditions.
440 is well above that threshold.
You should insure all of the wiring meets code. If you don't know how then hire a professional electrician.
I have been shocked by 440 and I was out for about 5 minutes.
Fortunately it was not under ideal conditions to induce current flow through my heart so I survived without ill effects. I do have a scar on my arm to this day.
Bill

Arcane
11-29-2016, 12:43 PM
Do you have 208 or 240 3 phase? 220 would be odd ball unless you have buck/boost transformers in line. Most common is 208 (wye). If that the case you just need 208-> 480 transformer. or.. Change the motor. Don't worry about the "460" verses "480". 460 is the motor design criteria so it can develop full power with an assumed voltage drop on input.

Take great care with 480. You can die. If you have no real experience, get an electrician to wire it for you.

It would be nice if people would use the proper values for describing voltages. Electric utilities use 120 volts as a base voltage reference when it comes to setting and adjusting the voltage output of substation transformers, not 110V, not 117V.

Seastar
11-29-2016, 12:50 PM
I'm old fashioned, in my early day as an engineer it was called 110, 220, 440 and so on.
Generally those were the voltages measured at outlets in homes, businesses and factories under half fuse load. There were not many circuit breakers. The voltages have been adjusted several times since we first installed AC in this country. Hence the 110, 117, 120 designations.
Most people know what you mean when you say any of these.
Bill

Arcane
11-29-2016, 05:31 PM
I'm old fashioned, in my early day as an engineer it was called 110, 220, 440 and so on.
Generally those were the voltages measured at outlets in homes, businesses and factories under half fuse load. There were not many circuit breakers. The voltages have been adjusted several times since we first installed AC in this country. Hence the 110, 117, 120 designations.
Most people know what you mean when you say any of these.
Bill

From reading several posts over the past few years it seems a lot of people don't.

Paul Alciatore
11-29-2016, 06:19 PM
Scared of it? When you have worked with 40,000 and 50,000 Volts, that is child's play.




my Feeler HLV copy lathe is 440 3 Phase, I have a rotary making 3 phase 220, then I use a step down transformer backwards to power the lathe. Works great and the wires (factory on the lathe) are tiny!

Go for it, 440 stuff is cheap cause home guys are scared of it.

Seastar
11-29-2016, 08:49 PM
Scared of it? When you have worked with 40,000 and 50,000 Volts, that is child's play.
Yup!
That stuff is a whole nuther world!
Long poles and insulated gloves in a bucket.
Bill

J Tiers
11-29-2016, 09:37 PM
Yup!
That stuff is a whole nuther world!
Long poles and insulated gloves in a bucket.
Bill

In Paul's case, it would be referring to plate voltage on the transmitter finals.

flylo
11-29-2016, 11:42 PM
my Feeler HLV copy lathe is 440 3 Phase, I have a rotary making 3 phase 220, then I use a step down transformer backwards to power the lathe. Works great and the wires (factory on the lathe) are tiny!

Go for it, 440 stuff is cheap cause home guys are scared of it.

I was going to mention that a step up or step down will work but thought someone might object. It's a lot easier to find a step down around here.I have a 9K as it was all copper inside & the larger 12K wasn't.

J Tiers
11-29-2016, 11:57 PM
Step down or step up are the same unit.... the magnetics cares nothing for direction of power. Most transformers are reversible, aside from low power ones which have compensation built-in*.

The new NEC says you cannot reverse the transformer unless it is marked to allow that, which nothing very old is.

Maybe there is a point to that, and maybe not. Most of us have used transformers either way for decades without issue. I am not certain what the reasoning behind the rule is. Could be insulation, could be a current rating issue, could be an issue of fault currents and the bracing of the wiring and coils. Or a combo of all that.

* A transformer with compensation built-in will have the wrong ratio if reversed, since the forward turns ratio is adjusted to account for losses and the "series impedance", so as to produce true voltage under load. This is normally not an issue for units over a few hundred VA.

Jon Heron
11-30-2016, 07:54 AM
The new NEC says you cannot reverse the transformer unless it is marked to allow that
Where does the NEC state that?
I have never seen a power or distribution transformer marked for use either way as it is redundant. Both here and in the US transformers are used as step up or step down with no special markings or other requirements.
Cheers,
Jon

Alistair Hosie
11-30-2016, 09:29 AM
Question Should I worry about 460 volt machines ?
Answer yes you should send them all post paid to yours truly
YOUR'S TRULY ALISTAIR
ADDRESS somewhere in Scotland near to the middle. The postie knows me ! that should do.
or if you'd rather send cash that would be very acceptable too .

J Tiers
11-30-2016, 09:49 AM
Where does the NEC state that?
I have never seen a power or distribution transformer marked for use either way as it is redundant. Both here and in the US transformers are used as step up or step down with no special markings or other requirements.
Cheers,
Jon

Section 450-11 B.

States the transformer may be "supplied at the marked secondary voltage, provided the installation is in accordance with the manufacturer's instructions". Connection information is generally on the nameplate, which is where that "instruction" would be placed. I suppose it could also be on a data sheet.

Point is, the manufacturer must rate it as bidirectional.

pinstripe
11-30-2016, 10:16 AM
That stuff is a whole nuther world!
Long poles and insulated gloves in a bucket.

Excuse my ignorance. What's the bucket for? To make sure the gloves aren't contaminated with something conductive?

J Tiers
11-30-2016, 10:49 AM
Excuse my ignorance. What's the bucket for? To make sure the gloves aren't contaminated with something conductive?

He's thinking High voltage power line.

"Bucket truck"..... Bucket is the thing you stand in as the arm raises you up to work. Then you use "hot sticks" to work with the wires etc from a safe distance beyond arc length. Or the bucket is insulated from the truck, in which case it usually has a conductive liner that gets clipped to the power line. then you are at power line potential, just like a bird sitting on the line, and can work without being shocked, although you may feel the voltage leak off you into the air.

pinstripe
11-30-2016, 11:20 AM
Got it, thanks.

Black Forest
11-30-2016, 11:32 AM
Every tool in my shop except for power hand tools uses 380v 3phase. It can bite real hard if something goes wrong. It happened to me and it took over three years for me to get over the damage. The doctor couldn't understand why I lived! I still don't have 100% usage of my right arm.

Be careful but then you should be careful with 120v also.

lakeside53
11-30-2016, 01:32 PM
From reading several posts over the past few years it seems a lot of people don't.


Yes, and it becomes particularly important when you don't have standard utility voltages. I'm working with a system that is actually 220v 3 phase (has buck transformers to reduce the 240 output) so to convert that to 208/120 require a specific transformer and resized input breakers. So easy to just "assume" a voltage that is incorrect. I recently ran into this with a large Architect plan set specifying equipment on multiple floors at 240 3 phase. Hmmm... not many building have that. You see 240/120 single phase, but most "low volt" three phase is 208/120.

J Tiers
11-30-2016, 01:33 PM
It still seems strange to me that other places let 415V 3 phase into residences.

We have 240V, but the important fact there is that it is only 120V to ground. Much less chance of getting across 2 hots than across a hot and ground (earth). There are lots of grounded things, and only a few things that are at mains voltage.

In apartment buildings, we have 208V 3 phase, but again, 120V to earth.

pinstripe
11-30-2016, 02:09 PM
It still seems strange to me that other places let 415V 3 phase into residences.

It's common here, but you won't find it running through all the walls of the house. I would say that in nearly all cases, it's only used for air conditioning. From the street to the meter box and to the a/c. No sockets, it's hardwired and only touched when the air conditioning needs replacing.

Doing your own electrical work is also not allowed here, so only licensed electricians should be working on the circuits, even the 240V ones.

My lathe and mill are hardwired with isolators. If I wanted three-phase wall sockets, the electrician would have had to install an RCD, and there was no room in the meter box. You can get wall sockets with inbuilt RCDs, but $$$.

415V is becoming less common with inverter air conditioners. You have to pay extra to get three-phase, so people are opting for inverters instead. When my brother bought his a few years ago, the inverter was cheaper than the three-phase. More parts, but I guess they make up for it in volume.

vincemulhollon
11-30-2016, 02:24 PM
The new NEC says you cannot reverse the transformer

In the old days you could get fuse-less wall wart transformers that were protected by the mfgr embedding a fuse or thermal cutout in the primary windings.

Perhaps the Chinese are starting to ship fuse-less transformers and people are hooking them up backwards and frying because the protection would then be in the secondary not primary so something like a primary to frame short would be very exciting. Or even some shorted secondary turns could be exciting.

J Tiers
11-30-2016, 03:31 PM
In the old days you could get fuse-less wall wart transformers that were protected by the mfgr embedding a fuse or thermal cutout in the primary windings.

Perhaps the Chinese are starting to ship fuse-less transformers and people are hooking them up backwards and frying because the protection would then be in the secondary not primary so something like a primary to frame short would be very exciting. Or even some shorted secondary turns could be exciting.

The NEC generally does not apply to wall warts and so forth, those are plug-in devices and covered by UL instead. The NEC is generally involved with the permanent wiring of a facility. As such, it would cover transformers for significant power, that are wired into the facility to convert voltages, such as obtaining 208/120V from a 480V 3 phase service, in a factory. Such transformers do not generally have that sort of internal protection.

lakeside53
11-30-2016, 03:41 PM
Another aspect of reverse connection of a transformer is UL. If it's not UL listed for reverse connection, it's not legal in many (most?) jurisdictions.

J Tiers
11-30-2016, 03:53 PM
And that means the NEC clause is to some degree un-needed.

Underlying the NEC is the idea that all "equipment" employed in a wiring system is "listed for the purpose". Effectively that means it complies with UL requirements and carries a UL recognition with "conditions of use" that include how it is actually being used.

Jon Heron
11-30-2016, 07:51 PM
States the transformer may be "supplied at the marked secondary voltage, provided the installation is in accordance with the manufacturer's instructions". Connection information is generally on the nameplate, which is where that "instruction" would be placed. I suppose it could also be on a data sheet.

Point is, the manufacturer must rate it as bidirectional.
That is a misinterpretation, on the contrary it is stating that you can feed it reverse, providing the instructions do not say that you cant.
This is is done on a daily basis in the industry and I can assure you it is code compliant, unless the mfg instructions state you cant.
I would challenge you to find a transformer that states you can feed it reverse, as it is a redundant statement.
I am a 3rd generation electrician who has been in the trade for just over 25 years now. I now specialize in applying and interpreting both the Canadian and US (NEC, NFPA) codes and standards.
You can take that to the bank.
Cheers,
Jon

Jon Heron
11-30-2016, 07:55 PM
120V kills more people world wide than any other voltage. It is not enough to throw you off but it is enough to contract your muscles enough that you cant let go whereas 240 - 480 volts will typically throw you off.
The stats are out there if you check it out you may be surprised.
Cheers,
Jon

J Tiers
11-30-2016, 08:29 PM
That is a misinterpretation, on the contrary it is stating that you can feed it reverse, providing the instructions do not say that you cant.
This is is done on a daily basis in the industry and I can assure you it is code compliant, unless the mfg instructions state you cant.
I would challenge you to find a transformer that states you can feed it reverse, as it is a redundant statement.
I am a 3rd generation electrician who has been in the trade for just over 25 years now. I now specialize in applying and interpreting both the Canadian and US (NEC, NFPA) codes and standards.
You can take that to the bank.
Cheers,
Jon

You can take it TO the bank, but you may not be able to DEPOSIT it....

1) not all transformers exhibit the same ratio in reverse, as mentioned above.

2) Primary windings are insulated for lightning surges. Secondary windings may NOT be (or they may be) insulated suitably, and could therefore break down if the secondary is connected to the incoming power.

This is for transformers conductively connected to the incoming power line, as for example a 480 -208 transformer for supplying office power in a factory for which the incoming line is 480V.

OK.

Looking at, for example, the Hammond Power Solutions (Guelph Ont) catalog for the HPS "Sentinel" line........

The listings in the catalog PLAINLY IDENTIFY the "Primary" voltage intended, and the "secondary" voltage produced. The installation manuals do not have anything but general precautions in them.

http://www.hammondpowersolutions.com/files/DOE-LV-Catalog11.pdf

Now, unless my education is sadly lacking, and primary and secondary do not mean what I think, the intent is that the "primary" shall receive input, and the "secondary" shall provide output.

That all said, in most cases there will be NO problems in a reverse connection. I have done it, and probably will again. Physics-wise, the transformer is bidirectional. But the specific manufacture of any particular unit may not allow it to be reversed in all applications.

The NEC just covers the subject with a blanket statement, and takes no account of specific cases where it might be of no consequence. But the NEC is the rule to follow IF you need to have the inspector sign off on the installation. It IS something that the inspector CAN "call out" as unsuitable, at least in the US, where the NEC is generally given the force of law by the local government entity (city, county).

Here are some specific comments about reversing (backfeeding) from Hammond.

http://www.hammondpowersolutions.com/innovation/troubleshooting-transformer-has-been-backfed/

I had forgotten the issue of inrush currents, which they specifically mention.

Again, I have done it, even with an HPS transformer, with no ill effects. But it is NOT allowed unless the manufacturer allows it.

Obviously the general rule from HPS is that one should NOT do it.

lakeside53
11-30-2016, 09:25 PM
120V kills more people world wide than any other voltage. It is not enough to throw you off but it is enough to contract your muscles enough that you cant let go whereas 240 - 480 volts will typically throw you off.
The stats are out there if you check it out you may be surprised.
Cheers,
Jon

We've been though this before... The "stats" are hugely skewed by the accessible 240/120 volt nodes verse 480

Jon Heron
11-30-2016, 09:33 PM
Jerry, I gave you the correct interpretation, its up to you to accept it or not but your way out of your bailiwick here.
A better man may have just said "thanks for the correction" rather than throwing out all this nonsense trying to keep from appearing to have made a misstatement.
Either way it makes no difference to me I was merely trying to set the record strait.

You can take it TO the bank, but you may not be able to DEPOSIT it....

1) not all transformers exhibit the same ratio in reverse, as mentioned above.
We are talking power and distribution transformers here not multi tap secondary control transformers so that statement is wrong.
2) Primary windings are insulated for lightning surges. Secondary windings may NOT be (or they may be) insulated suitably, and could therefore break down if the secondary is connected to the incoming power.
This is for transformers conductively connected to the incoming power line, as for example a 480 -208 transformer for supplying office power in a factory for which the incoming line is 480V.

Completely false, primary and secondary and all mains connected equipment less than 750V are subject to the same dielectric strength testing of 2 X the applied voltage + 1000v for one minute.
OK.

Looking at, for example, the Hammond Power Solutions (Guelph Ont) catalog for the HPS "Sentinel" line........

The listings in the catalog PLAINLY IDENTIFY the "Primary" voltage intended, and the "secondary" voltage produced. The installation manuals do not have anything but general precautions in them.

http://www.hammondpowersolutions.com/files/DOE-LV-Catalog11.pdf

Now, unless my education is sadly lacking, and primary and secondary do not mean what I think, the intent is that the "primary" shall receive input, and the "secondary" shall provide output.

That all said, in most cases there will be NO problems in a reverse connection. I have done it, and probably will again. Physics-wise, the transformer is bidirectional. But the specific manufacture of any particular unit may not allow it to be reversed in all applications.
I am not sure of your education but it is surly lacking in the codes and standards that apply to the installation of power and distribution transformers.
A transformer is a turns ratio machine, period.
If a mfg does not want their transformer used in reverse they MUST state so in the installation instructions and on the winding diagram on the cover. There is nothing in the NEC or CEC which says you cant, period.
Interesting you mention HPS which is 15 minutes from my house and a regular customer of mine. They use me as an accredited third party to witness heat runs and dielectric strength testing of HV, MV and standard power and distribution transformers that do not fall under the scope of their CSA certification listing. They also have a cousin in Waterloo (original brothers had a falling out) Hammond Manufacturing that makes control transformers and grid simulators which is also a good customer of mine.
The NEC just covers the subject with a blanket statement, and takes no account of specific cases where it might be of no consequence. But the NEC is the rule to follow IF you need to have the inspector sign off on the installation. It IS something that the inspector CAN "call out" as unsuitable, at least in the US, where the NEC is generally given the force of law by the local government entity (city, county).If the code is silent it does not mean you cant do something, it means you can.
Yep the installation will need to meet the minimum requirements of the NEC, and it will unless specifically not allowed by the mfg's instructions as I have already explained.
I am well aware of the US requirements beyond just the requirements of the NEC too.
Coincidentally I just got home last night from Orlando Florida where I underwent 2 days of auditing by TUVR (an NRTL) on my NEC, UL508, NFPA79 and few other electrical standards...
Here are some specific comments about reversing (backfeeding) from Hammond.

http://www.hammondpowersolutions.com/innovation/troubleshooting-transformer-has-been-backfed/

I had forgotten the issue of inrush currents, which they specifically mention.

Again, I have done it, even with an HPS transformer, with no ill effects. But it is NOT allowed unless the manufacturer allows it.

Obviously the general rule from HPS is that one should NOT do it.
What general rule? That's a trouble shooting guide not a rule or code or standard.
I am sure they would love to sell you a transformer with a custom nameplate but nobody in their right mind would purchase one.
The standard practice regardless of which way its fed, is to use HRC fuses at 300% of the primary calculated current and fuses at 125% or the secondary current (or input and output current if thats easier to visualize). This makes a robust circuit with no nuisance tripping from inrush unlike when you just fuse the primary at 125% with no secondary fuses, which is also allowed but more subject to nuisance tripping.
Your interpretations are erroneous.
Cheers,
Jon

Jon Heron
11-30-2016, 09:42 PM
We've been though this before... The "stats" are hugely skewed by the accessible 240/120 volt nodes verse 480
Then have a look at the stats on fatality's of electrical workers whom are exposed to the gamut on a daily basis. I believe the IAEI keeps pretty good records of this though I cant be bothered to look into it.
Just repeating something taught to me many years ago and its something that rings true for me as I have been hung up on 120V before while the 600, 347 and 14000v contacts I have made hurt way worse and left marks, the 120 was the closest to killing me. My father has a good story as well where if his muscles didnt contract enough to pull his legs up off hte ladder he was on thus pulling the drill out of the hole he would be dead cause he couldn't let go of the drill. Though he said he wished he was dead after landing on his carry all and breaking 4 ribs...
Either way, it can all kill you, even 12V so just be careful with the ol electrons and play safe!
Cheers,
Jon

lakeside53
11-30-2016, 10:21 PM
And there is the added joy of "arc flash". To work on our big live 480 panels you dress up in the rubber suit. Urgh...

Our local inspection requires arc flash calcs on even a small (30kva) 480 - 208/120 transformer. The disconnect calculated at 6g TNT with initial blast force of 35lb/sq ft.

I love how it's expressed in gm/TNT equiv. I'd hate to see what our 1600amp panel calcs out at. ;)

J Tiers
11-30-2016, 10:57 PM
Jerry, I gave you the correct interpretation, its up to you to accept it or not but your way out of your bailiwick here.
A better man may have just said "thanks for the correction" rather than throwing out all this nonsense trying to keep from appearing to have made a misstatement.
....
Jon

I am not "trying " to do any such. That is YOUR problem, not mine.

And, in fact, far from "misinterpreting" anything, it is literally the first time I have ever looked at that clause, so I am not reading anything into it. It is NOT "my" interpretation. If it is anyone's interpretation, it is the interpretation of the other engineers who have passed that "interpretation" to me, when I mentioned doing a backfed connection.

The relevant clause is a recent addition to the NEC. It is widely interpreted as meaning exactly what I said, which as mentioned is NOT "my" interpretation.

Looking at your statements, and noting that you are in Canada, while the NEC is a US document....I said, well here's a statement contrary to others, let's see. So I went to look at Hammond, which is a manufacturer I have dealt with, to see if indeed they were actually ambiguous about their connections, and failed to comment on using a step down as a step-up unit.

Well, they are NOT. They state the Primary voltages, and list the definition of "primary" as the winding to which power is supplied. Pretty clear.

They also state that they "do not recommend" backfed connections. That is also pretty clear.

You can argue over the issue of their statements being a "prohibition", or not a "prohibition", but when they state they do not recommend it, that seems clear.

Incidentally, the comment about "removing ground straps" is particularly telling. As you are probably aware, the transformers are UL listed. The UL listing is apparently contingent on the star winding being re-eestablished as a neutral, i.e. "grounded".

So, if used without that connection, if that does, as Hammond states, "violate the UL listing" by being contrary to the "conditions of use", the transformer would NOT be UL recognized if so connected. And as such, it would therefore NOT be "listed for the purpose", and so would not comply with the NEC requirement that equipment be "listed for the purpose".

There is another point to be made here.... IF the removal of the ground strap IS contrary to the conditions of listing, that is pretty good evidence that the secondary insulation is intended ONLY for the normal secondary voltage, and is NOT intended to stand up to the impulse voltages that are present on the incoming power line from the utility.

In another section with regard to backfeeding, Hammond says the following:

"NEC requirement statement c/w exclusions

Include the 3 level of notes:

– No note = step down

– Suitable for step-up (as per NEC)

– Bi-directional"

Notice that this

1) admits the need for the note "per NEC"

2) States that no note means "step down only"

3) Shows a specific wording that indicates it is suitable for bi-directional usage.

Did you really NEED any more information? This other section I did not find thru the site, but via google. It may not be complete, and does not show some of the info from the other note. I strongly suspect that the notes quoted directly above are actually a list of things to be added to the section, but that does not decrease their validity.....

http://www.hammondpowersolutions.com/faq/can-a-transformer-be-back-fed-or-used-in-reverse/

Full disclosure here; I have designed transformers and do understand both their operation, and also many of the insulation requirements applied to transformers. However, I have been doing other things, and I was not aware of that new issue in the NEC until relatively recently.

Jon Heron
12-01-2016, 04:58 PM
And there is the added joy of "arc flash". To work on our big live 480 panels you dress up in the rubber suit. Urgh...

Our local inspection requires arc flash calcs on even a small (30kva) 480 - 208/120 transformer. The disconnect calculated at 6g TNT with initial blast force of 35lb/sq ft.

I love how it's expressed in gm/TNT equiv. I'd hate to see what our 1600amp panel calcs out at. ;)
Yup, its a different world out there now! In my day we were trained how to safely work on live equipment and now it has gone full circle that it is illegal to work on live equipment without wearing a space suit, which in my professional opinion has its own hazards as far as precision and clumsiness with the tools...
It all started in the US with the NFPA 70E and now its here in Canada with the CSA Z462 which was based on the NFPA document. For the most part the intent is good with these documents but like most things they go overboard....
One day robots will have to do live work, mark my words as these standards get reviewed every few years or so and the the folks doing the reviewing feel they aren't doing their jobs unless they have something to add to the standard... This is regardless of whether or not anyone has any practical experience in the field or not, but I digress...
Cheers,
Jon

Jon Heron
12-01-2016, 05:20 PM
Jerry, I am done with this discussion, your not listening. I would suggest you go talk to any seasoned electrician about this acceptable practice rather than an EE.
By the way NEC450.11 (b) is not new, its also in the 2014 edition (I didn't bother to look in earlier additions as its redundant anyways) and it also specifically states the same as the 2017 code that you can feed the secondary. Thats pretty damn clear if you ask me.
Further definitions for you: "do not recommend" is not equal to "do not", by any stretch.
Removing the ground strap is a specific requirement from the code if your back feeding a delta/star or if your not grounding the XO, for obvious reasons you must boot the XO and tag it. This does not invalidate the listing and is a common practice also as any sparky worth their salt can confirm for you since you dont believe me.
Believe whatever you want though.
Have fun!
Jon

J Tiers
12-01-2016, 05:43 PM
Jon, I am done also.

YOU have not listened nor read.

I DO BACKFEED.

I recognize that the local inspector CAN call it out. (And I would only do it for lab setups. We had one running several years no issues. That cuts no ice with the NEC and a persnickety inspector.)

So does Hammond Power Solutions, since they show the notes and refer to needing a specific note per NEC. Looks like you are on the wrong side of this one, despite your protests.

HPS ALSO states that the UL is contingent on the grounding, which indicates a probable insulation issue. They insulate for the need, not the maximum. Line side can be 6kV or better, secondary to ground may be less

I never said it was entirely sensible, but it IS in the code. Yes, IIRC 2014 was the first mention of it, Have not seen the 2017 yet, it will be a while before the local yokels get around to accepting it by law.

Arcane
12-01-2016, 10:37 PM
And there is the added joy of "arc flash". To work on our big live 480 panels you dress up in the rubber suit. Urgh...

Our local inspection requires arc flash calcs on even a small (30kva) 480 - 208/120 transformer. The disconnect calculated at 6g TNT with initial blast force of 35lb/sq ft.

I love how it's expressed in gm/TNT equiv. I'd hate to see what our 1600amp panel calcs out at. ;)

Back when it became a known concern for us (provincial utility Linemen) we were told our most dangerous arc flash would occur if the 208/120V secondary on a 500Kva padmount 3ph transformer was shorted out at the transformer bushings. IIRC anyone within 20 feet (or was it 30 feet?) of it could experience various degrees of harm if they weren't wearing the proper PPE. Closer than 3 feet without PPE was supposed to result in death.

We had those suits too, we called them "Homer suits".

garyhlucas
12-01-2016, 10:42 PM
This same kind of question came up on a CNC newsgroup. The issue here is simple. If you are talking a three phase transformer to step up from 208 to 480 volts it will work but it won't be legal in the US. That's because the 208 volt side is Y connected and the 480 volt side is delta connected. The reason they are delta y is because it reduces the transmission of noise. Because the 480 side is delta connected you will only be able to get ungrounded 480 power which is not legal and is quite dangerous in the event of a ground fault.
To get legal 480 from 208 one way is to use 3 single phase transformers with a 120/240 primary and a 240 secondary along with 3 buckboost transformers to get the voltage up to 480 Y with a ground tap. A bit messy but I've done it for a customer.

J Tiers
12-01-2016, 10:58 PM
Well, if THAT is the only problem, you can go with corner grounded. Perfectly safe, although it can be confusing to look at.

As far as I know that is not completely outlawed, even if the power companies will not install that service anymore. There are issues if you want a corner grounded service, but we are talking a machine, or possibly a couple machines. At worst a branch circuit, not a service.

You DO have to un-bond the wye secondary to use it as a primary, and there you may run into insulation issues. There are also fault current mechanical forces, which the winding would not see as a secondary, but as a primary it could see large transient currents that could damage the winding and cause more trouble.

Then, compensated windings are a possible issue. Assume a 240-480 transformer, with 5% impedance, and full compensation. If reversed, it could produce an output voltage about 10% lower than the expected 480V, with a true 240V input. With an input at the -10% minimum voltage, the output would be 20% low, maybe more if the load current goes up due to low voltage (motor load).

If these type things are not an issue for a certain unit, then it would be "suitable for use as a step-up".

Now, Jon is correct about one thing. The basic operation of a transformer is bidirectional. It is the way it is implemented that affect the ability to really use it as bidirectional.

lakeside53
12-01-2016, 11:10 PM
Yes, corner grounding 480 gives 480 to ground on 2 legs, zero on the other.

It's even more confusing when you ground the center point of one Delta leg. I recently put in a 45kva 480-240 Delta-Delta. The secondary ground point is not a corner (could have been) but the center of one of the legs (centered to derive 120-120 from one side if needed). You get 120 to ground on two legs, and 208 on the other. ;)

Why do this ? Big production welders have lower input current at 240 than 208... and the transformer being and odd-ball was close to free.

J Tiers
12-02-2016, 12:42 AM
That's actually very common, as you know. It is just "wild leg" / "High leg" / "lighting tap" / "farm" 240V 3 phase.

Many names for it, but all mean the same thing. I see it fairly often in places where a smallish 3 phase load exists and some 120 is wanted as well. It would not justify a 208 3 phase, or, as in your case, it is undesirable to have 208.

The one difficulty is that the 3 phase is not balanced, because the 120/240 adds load to that phase that is not present on the others. So it is usually done open delta with a larger transformer on the 120-0-120 side.

Common or not, it can be confusing.

By the way, it is also exactly what you get from an RPC.

lakeside53
12-02-2016, 01:04 AM
yes, 120v loads are typically limited to 5% of output. I didn't use them at all.

Not exactly what you get from an rpc. Ground reference from the wild leg isn't 208 and the measurement of that is pretty much invalid. The transformer split winding ground establishes a new ground/neutral reference point.

J Tiers
12-02-2016, 01:14 AM
yes, 120v loads are typically limited to 5% of output. I didn't use them at all.

Not exactly what you get from an rpc. Ground reference from the wild leg isn't 208 and the measurement of that is pretty much invalid. The transformer split winding ground establishes a new ground/neutral reference point.

Pretty much exact, I'd say.....

The supply to the RPC is equivalent to the 120-0-120 winding, and is grounded the same way, the neutral is grounded at the service in each case. The idler then produces an output equivalent to the 3rd wire in the wild leg.

And, it IS just about 208V from the neutral to the generated leg. Often lower just because the motor does not produce a perfect output.

The "non-valid" deal is more because that voltage gives no particularly useful information relative to how the RPC is working. If it is working right, it ideally produces a system exactly like the wild leg service. Differences would have to be small ones related to voltage drops in the neutral wire.

Jon Heron
12-02-2016, 05:11 PM
This same kind of question came up on a CNC newsgroup. The issue here is simple. If you are talking a three phase transformer to step up from 208 to 480 volts it will work but it won't be legal in the US. That's because the 208 volt side is Y connected and the 480 volt side is delta connected. The reason they are delta y is because it reduces the transmission of noise. Because the 480 side is delta connected you will only be able to get ungrounded 480 power which is not legal and is quite dangerous in the event of a ground fault.
To get legal 480 from 208 one way is to use 3 single phase transformers with a 120/240 primary and a 240 secondary along with 3 buckboost transformers to get the voltage up to 480 Y with a ground tap. A bit messy but I've done it for a customer.

That is not correct, it is perfectly legal here and in the US and the Y or XO has zero to do with noise, its only a center tap used to create a reference to ground and to split the voltage.
The purpose of the XO (Y) is to create a reference to ground/neutral. So for us canucks you get 600/347 or 120/240 on a wye system where the 347 or 120 is between one phase and the XO/neutral and the 600 or 240 is phase to phase.
However a delta system is by no means illegal and there are many entire plants up here and in the US that are still run on delta systems, you just need to have ground fault indicator lights for the ungrounded delta system, or if it is for a dedicated machine and the ungrounded circuit supplies only the machine, the lights are not even required though they are recommended. The only danger with a delta system is to the equipment as when you do have a fault it is a bolted fault phase to phase which is more destructive. A delta system is actually safer for us humans as the only way to get a lifter is to get between 2 phases.
When you back feed wye to delta you must isolate, boot and tag the XO as I explained earlier and this method is specifically addressed in the code and is permitted.
Cheers,
Jon

J Tiers
12-02-2016, 06:25 PM
You are only required to ground as far as the NEC is concerned ("shall be grounded") if by grounding you can have a line to ground voltage of no more than 150V OR there is a 3 phase wye system with a neutral used as a conductor, OR it is a wild leg delta with the center tap of one phase used as a circuit conductor.

That DOES cover virtually all the usual services, so effectively pretty much all of them shall be grounded. It is forgivable for folks to not know that, or to be careless in describing what needs to be grounded, if they are not dealing with industrial electric systems.

A pure delta is "permitted to be" grounded, by corner grounding, as are some other systems. So "Corner grounding" is allowed but not required for 3 phase delta

NOT required to be grounded are circuits supplying ONLY electric furnaces (not applicable here) circuits supplying ONLY VFDs, certain control circuits, and a few other things, but they require ground detectors.

So, yes, Jon is technically correct. It does not matter a lot, because the situation does not come up much. In general, "separately derived systems" need to be grounded becaus most fall under the "shall be" considerations above.

Note that 480 pure delta would NOT be in the "shall be", it is in the "permitted to be" category, because there is no way of getting all conductors to be 150 or less volts from ground. So in the case of a transformer used to make 480 delta there would not have to be a ground.

BTW, Jon: I checked around with folks I know who are PEs and deal with it all the time. They ALL agree that there has to be a manufacturer statement somewhere that the specific step-down transformer is suitable for step up use. 100% agreement on the interpretation. Could be a catalog spec, or an after-the-fact letter covering older models, so it need not be actually marked on the unit.

However, nobody reported having an inspector ask for a suitability statement..... It's new, and relatively obscure, so I guess that is not surprising. And, nobody piped up and reminded the inspector, either.......;) Might also be that the 2014 code is not in force there yet.... it can take a couple or 3 years to get sround to updating the law. I think the city here is still on 2011, or was until recently.

Jon Heron
12-02-2016, 07:36 PM
Christ this is frustrating...
I either hate to see the endless dissemination of false information on the ol interweb or I have a personality defect which does not allow me to pass by this stuff... Perhaps both! :rolleyes:

You are only required to ground as far as the NEC is concerned ("shall be grounded") if by grounding you can have a line to ground voltage of no more than 150V OR there is a 3 phase wye system with a neutral used as a conductor, OR it is a wild leg delta with the center tap of one phase used as a circuit conductor.

That DOES cover virtually all the usual services, so effectively pretty much all of them shall be grounded. It is forgivable for folks to not know that, or to be careless in describing what needs to be grounded, if they are not dealing with industrial electric systems.

A pure delta is "permitted to be" grounded, by corner grounding, as are some other systems. So "Corner grounding" is allowed but not required for 3 phase delta
Right, no requirement to corner ground and delta systems are perfectly legal (and common though not as much as they used to be) providing ground fault indicator lights are used (unless its for a dedicated machine as I already pointed out).
NOT required to be grounded are circuits supplying ONLY electric furnaces (not applicable here) circuits supplying ONLY VFDs, certain control circuits, and a few other things, but they require ground detectors.

So, yes, Jon is technically correct. It does not matter a lot, because the situation does not come up much. In general, "separately derived systems" need to be grounded becaus most fall under the "shall be" considerations above.
I am 100% correct, these are not my opinions but facts based on the code(s), of which I am an expert and which are now an integral part of my profession. A "separately derived system" is created any time a transformer with an air gap is utilized (not an auto) and they are permitted to not be grounded.
Note that 480 pure delta would NOT be in the "shall be", it is in the "permitted to be" category, because there is no way of getting all conductors to be 150 or less volts from ground. So in the case of a transformer used to make 480 delta there would not have to be a ground.
Correct, no need to ground it, ever. Only ground fault indicator lights are required, this has been the same since the beginning of time.
A delta system has no referance to ground so the voltage to ground will always be 0 under normal operating conditions (no fault).

BTW, Jon: I checked around with folks I know who are PEs and deal with it all the time. They ALL agree that there has to be a manufacturer statement somewhere that the specific step-down transformer is suitable for step up use. 100% agreement on the interpretation. Could be a catalog spec, or an after-the-fact letter covering older models, so it need not be actually marked on the unit.
Completely false, neither the NEC nor the CEC requires a transformer to state it can be back fed. It only states that if the instructions state you cant back feed it that you cant, period. Here is the code verbatim out of the 2014 NEC

450.11 Marking.
(A) General. Each transformer shall be provided with a
nameplate giving the following information:
(0 Name of manufacturer
(2) Rated kilovolt-amperes
(3) Frequency
(4) Primary and secondary voltage
(5) Impedance of transformers 25 kVA and larger
(6) Required clearances for transformers with ventilating
openings
(7) Amount and kind of insulating liquid where used
(8) For dry-type transformers, temperature class for the in-
sulation system
(B) Source Marking. A transformer shall be permitted to
be supplied at the marked. secondary voltage, provided that
the installation is in accordance with the manufacturer's
instructions.
Which means if the mfg instructions dont say you cant back feed it..... YOU CAN! If the code stated "SHALL be marked as suitable for back feed" or similar then it would be required to be marked as such, but IT DOES NOT STATE THAT ANYWHERE.
However, nobody reported having an inspector ask for a suitability statement..... Of course not, it is not required and I bet you cant find a set of install instructions which states it can be back fed, as it is redundant.It's new, and relatively obscure, so I guess that is not surprising. It is NOT new. And, nobody piped up and reminded the inspector, either.......;) Might also be that the 2014 code is not in force there yet.... it can take a couple or 3 years to get sround to updating the law. I think the city here is still on 2011, or was until recently.
Again, I would suggest you talk to an electrician not an engineer, its the electricians who install and service this type of equipment and whom have to deal with inspection to get it passed, not design engineers...
Cheers,
Jon

J Tiers
12-02-2016, 09:10 PM
Christ this is frustrating...
I either hate to see the endless dissemination of false information on the ol interweb or I have a personality defect which does not allow me to pass by this stuff... Perhaps both! :rolleyes:

Again, I would suggest you talk to an electrician not an engineer, its the electricians who install and service this type of equipment and whom have to deal with inspection to get it passed, not design engineers...
Cheers,
Jon

Not design engineers. The PEs who sign off on the installation and are present during the inspection of large facilities.

With all due respect, the electrician is the LAST person to ask. Electricians generally are very good about following the code. It's their business. Reasons behind the code? Not so good in general. Grasp of the theory that is behind those reasons? Not good at all in most cases.

It's the PEs who write the code, who know the problems. Us design guys also know the issues, and I can tell you straight up and NO BS... There are transformers that CAN be reversed off the incoming line safely, and there are transformers that SHOULD NOT be used that way.

YOU , as the installer, have no way of knowing which is which, unless it is made clear. That's the point behind the clause, however poorly worded. You can't tell the players apart without the playbook, so the idea is to identify the ones that are OK.

That's basically why the provision was inserted, as I understand it. It's kinda weak and mealy-mouthed as to the wording, but the intent is, and is interpreted as being, that the unit shall be identified in some way as suitable. Otherwise it is presumed NOT to be suitable.

Will they blow up when power is applied? No.... Not if connected up properly to rated voltage.

Will they stand up to lightning surges etc that occur on the incoming line? You do not know unless you are told. Neither do I unless I know details of the design, which is unlikely.

Will they stand up to the mechanical forces experienced during a fault condition-induced overcurrent? You do not know, and I do not know either unless the thing was designed to a certain fault current level ON THE SECONDARY which is now the primary.... which may not be done, because the secondary receives less abuse.... anything which saturates the core will not get through without being limited.

You see the issue, if you can calm down long enough to comprehend it.

Some units won't work well just because they won't have the right voltages if reversed. You could compensate for that, maybe, but if not intended for reversal, they are almost certainly not suitably designed and made.

Some unit will work, but will not withstand common line side conditions of overvoltage. Again, not designed for the purpose.

Some units will work, but will fail catastrophically in a fault condition. Yet another not designed for the purpose.

It's much like the meter you probably have. It is marked as a CAT II, III, or IV, most likely. Each category is protected against more sever transients and higher fault current capability that increase as you go from a 120V outlet back towards the incoming power line. Lower rated meters will "work", but in a fault or other abnormal conditions, some meters will fireball right in your hand. I've seen the tests.....

The "suitable" transformers will be,among other things, suited to the conditions they would experience when the intended secondary is connected as a primary basically to the incoming line on the facility side of some overcurrent protection.

The secondary is often essentially rated like a CAT I or CAT II meter. The primary has to be rated for the conditions of CAT III or even CAT IV, so it is not an issue. When you reverse the thing, you may expose a lower rated winding to abuses it is not designed to withstand safely.

lakeside53
12-02-2016, 09:25 PM
I know from bitter experience that the inspectors here take the position that unless it's labeled (and/or backed up by exacting manf. data) for a certain use it can't be used. Period. To say that because it explicitly doesn't say it can't be used so therefore it can.. lol.. good luck with that argument. OK, you can appeal to the AHJ, but who has time for that?

lakeside53
12-02-2016, 09:45 PM
Pretty much exact, I'd say.....

The supply to the RPC is equivalent to the 120-0-120 winding, and is grounded the same way, the neutral is grounded at the service in each case. The idler then produces an output equivalent to the 3rd wire in the wild leg.

And, it IS just about 208V from the neutral to the generated leg. Often lower just because the motor does not produce a perfect output.

The "non-valid" deal is more because that voltage gives no particularly useful information relative to how the RPC is working. If it is working right, it ideally produces a system exactly like the wild leg service. Differences would have to be small ones related to voltage drops in the neutral wire.


I beg to differ. No way has the generated leg of any rpc I've built or measured been anywhere near 208 to ground...


A person way smarter them me produced the following as an explanation.

http://i238.photobucket.com/albums/ff150/lakeside53/misc%20linked%20uploads/Phasor_zps0594225e.jpg (http://s238.photobucket.com/user/lakeside53/media/misc%20linked%20uploads/Phasor_zps0594225e.jpg.html)

Jon Heron
12-02-2016, 10:06 PM
Jerry, That is some epic BS there, through and through lol!
You have no idea what your talking about.
Gave me a grin though lol.
Have fun.
Jon

Jon Heron
12-02-2016, 10:21 PM
I know from bitter experience that the inspectors here take the position that unless it's labeled (and/or backed up by exacting manf. data) for a certain use it can't be used. Period. To say that because it explicitly doesn't say it can't be used so therefore it can.. lol.. good luck with that argument. OK, you can appeal to the AHJ, but who has time for that?
Believe whatever you want but the facts stand.
Show me a transformer manual that says it can be backfed, if this was required as you guys are asserting, they should be readily available as they are not uncommon and are being installed on a daily basis in a step up configuration.
The term "shall" is a code-speak term or legalese if you will, if the transformer was required to be marked as suitable to be backfed it would have a "shall be marked" clause, period.
I sit on numerous code committees, (despite Jerry's complete fallacy that engineers write the codes, they are only a part of the working groups) and this is how these things are written.
I could dig into the product standard's that transformers are built to and show no special clause for backfed marking too but whats the point, you boys already have your answer figured eh.
Cheers,
Jon

J Tiers
12-03-2016, 12:16 AM
Lakeside:

Which are you talking about, the true neutral or the 3 phase? Your guy (Peter Haas) proves my point nicely with respect to the voltage from "N" to "B".

Simplest way to see it is to think of a "Scott-T" connection for 2 phase to 3 phase conversion, such as 240V 2 phase to 240V 3 phase. In that setup, a center tapped 1:1 ratio transformer (#1) has a winding from another transformer (#2) connected from the center tap (neutral, here) to the "B" phase wire in your diagram.

The #2 transformer has a turns ratio of 0.866:1, and is fed with phase voltage B from the 2 phase 240V source on the primary side. The secondary is the 0.866 side, and is connected from your "N" to "B".

So there is no way to escape having 0.866 x the 240V phase voltage from "N" to "B", which is 208V, there is one unambiguous source for the voltage, the #2 transformer and it is supplying 0.866 of the 240V. It makes true 3 phase at 240V.

If you add up the voltages Mr Haas shows from N to N' and from N' to B, it comes out the same.

If your RPC does not produce that voltage (mine does not , quite, either), or something reasonably close, then to that extent it is not producing perfect good 3 phase, there is probably some phase imbalance that a meter will not show you. Mine produces about 10% higher, unloaded. I have not checked it with a load on it, and I have not checked it with an oscilloscope to see if there is a phase error. It happily runs the motors, and they don't seem to overheat, so I have not gotten too far into it, I have other things to do.


Jerry, That is some epic BS there, through and through lol!
You have no idea what your talking about.
Gave me a grin though lol.
Have fun.
Jon

Suit yourself.

It is far from BS, but if you do not wish to believe any of it, that is YOUR problem and is not mine.

There is strong technical background to what I wrote. The more you protest that that is BS, the more you frankly show your ignorance. Ignorance is nothing to be ashamed of. Willful ignorance is another thing entirely. You are presently exhibiting willful ignorance........... unwillingness to learn.

I am perfectly willing to learn. It would please me to find that the NEC does NOT say what it appears to say. That would make things easier for me.

So, I have on one hand, a number of experienced PEs telling me it DOES say that, and on the other, one experienced electrician saying it does not. The electrician also calls sound technical info "BS", which does not enhance his credibility.

It's still coming down on the side of it being a requirement. A blamed nuisance, actually, but that's not uncommon for standards... they get more nit-picky with time.

Yes, the code committees are made up of a variety of people. I have been on code committees also, and each party brings their specialty to the table. Engineers bring the technical basis of whatever is being discussed, after all that is what we do. The other representatives, from the trades etc, bring a different set of experiences and thoughts to the table, making sure that the technical folks stay "grounded"... pun intended... and do not end up requiring impossible things, or creating contradictions in practical work. Do not mistake one for the other.

Time for this side issue to just go away.... it's gotten way off topic.

Final comment:

I KNOW they "work". The business of code committees seems to be to take the safest path, and then double down on it. As in this case.

The point is NOT that you can't backfeed some (not all) transformers and get what is expected. The point seems to be that if you are going to use a device "off data sheet", and the device is safety related, that it should be rated to do what you are trying to do, and you should be able to KNOW that.

Which basically means the usage should be allowed by the data sheet. With data sheets, "no news" is not "good news". If it ain't on the data sheet, or on some sort of technical addendum sheet, or marked on the product, it is not guaranteed. Period.

Seems reasonable.

The fact that we have all been getting away with it for years isn't the issue. The issue is that it seems to be intended that we should KNOW when we can get away with it successfully. And anyhow, the legal document states (in what I admit is poor wording) that we HAVE to know it is OK, in the opinion of folks who should know, since they have to sign their name to the plans.

This is more so now that there are so many more sources for transformers and other devices, not all of whom over-build things the way they have been made in the past by US makers.

PStechPaul
12-03-2016, 12:26 AM
The diagram in post #60 shows that the wild leg of a 120-N-120 (240 center tapped) is 240 * cos(30) = 207.8 volts. BTW, cos(30) = sqrt(3)/2.

I can understand how a reverse fed step-up transformer might exhibit different characteristics under fault conditions. Normally the primary is wound first, closest to the core, and the secondary on top of that. When a high current is applied or drawn, the coil will tend to expand. If the overcurrent is in the inner winding, it will put outward pressure on the outer winding, which may help contain this mechanical force. If reversed, with overcurrent applied to the outer winding, it may tend to separate from the inner winding. But in most cases the ampere-turns will be similar (up to saturation, at least).

It may be best to connect a transformer as an autotransformer, and for three phase two or three can be used. I have used three 120/240 autotransformers on a 3-phase 120/208V supply for 240/416, and this was connected to a three-deck "Variac" in boost mode to obtain close to 277/480. But that was just a temporary bench test setup.

J Tiers
12-03-2016, 06:05 PM
I was not going to post any more about this, but I was sent a copy of the minutes regarding this issue. I think they adequately dispose if the matter by showing the intent is precisely what I said it was. Manufacturer must state it is allowed, but need not mark it on the transformer itself.

Note that the wording used was used SPECIFICALLY TO AVOID requiring a mark on the transformer itself.



"Submitter: Gaylord Poe, Inspection Bureau, Inc.
Recommendation: Revise text to read as follows:
2014 NEC Proposed - 450.11 - Marking. Each transformer shall be
provided with a nameplate giving with the following information:
1. The name of the manufacturer.,
2. Rated kilovolt-amperes.,
3. Frequency.,
4. Primary and secondary voltage.,
5. For dry type transformers, if the transformer is permitted to be connected
in either direction, it shall be marked as “Bi-Directional”. Installation
instructions shall be provided by the manufacturer detailing how the
transformer should be connected when the primary and secondary are reversed.
6. The impedance of transformers 25kVA and larger.,
7. Required clearances for transformers with ventilating openings., and
8. The amount and kind of insulating liquid where used.

In addition, the nameplate of each dry-type transformer shall include the
temperature class for the insulation system.

Substantiation: Although it is common industry practice to reverse wire drytype
transformers, the marking requirements in the current NEC do not make
the installer aware of the fact that UL 1561 does not support this practice. UL
1561 provides that the testing of these transformers includes “...step up, step
down, and autotransformer type...” with the supply being connected to the
primary and the load being connected to the secondary, thus, making
connections in any other manner a violation of 110.3(B). Changing the marking
requirements of 450.11 will provide clarification and enhance electrical safety.

Panel Meeting Action: Accept in Principle
Revise the text to read as follows:
450.11 - Marking.
(A) General. Each transformer shall be provided with a nameplate giving the
following information:
1. The name of the manufacturer
2. Rated kilovolt-amperes
3. Frequency
4. Primary and secondary voltage
5. The impedance of transformers 25kVA and larger
6. Required clearances for transformers with ventilating openings
7. The amount and kind of insulating liquid where used
8. For dry-type transformers, the temperature class for the insulation system

(B) Source Marking. A transformer shall be permitted to be supplied at the
marked secondary voltage provided the installation is in accordance with the
manufacturers’ instructions.

Panel Statement: The Panel action corrects a violation of the NEC Style
Manual relative to the formatting of lists and meets the submitter’s intent. CMP
9 concludes that a specific marking as opposed to general instructions from the
manufacturer is excessive. Submitter’s proposal was limited only to dry-type
transformers without adequate substantiation.
Number Eligible to Vote: 12
Ballot Results: Affirmative: 12 "

Jon Heron
12-05-2016, 05:24 PM
While it appears that the intent may have been for the MFG to specifically state that the transformer can be backfed, they failed.
Stating is must comply with the MFG's instructions is just that, unless it says you cant, you can, simple and this how it works with all things in the code, from simple toggle switches to complex control panels, etc.
If they wanted specific wording to state that it can be backfed then they must have the "shall" in there and the product standard would have to be revised to include any required testing and marking.
Further to that, the UL1561, CSA C9 or No.47 or the C88 product standards have no provisions or type testing for a transformer to be backfed, therefore you will never see a transformer with that marking, if its not in the product standard its not a requirement so who would do it.
Its pretty obvious to me why these test and markings are not in the standards, because its redundant which way they are fed, there are no compensation coils or other widgets used in these types of transformers and there is no difference to the insulation requirements of either side.
The most common transformer used for this application is auto transformers due to the individual machine use they are typically used for and there is no XO to isolate and tag, not that the code doesn't allow that too...
If an a-hole know nothing inspector asked for specific marking and the primary and secondary was marked as primary and secondary rather then H and L, I suppose the only option would be to appeal.
In the real world these are being installed and accepted regularly without any special wording, that I bet does not even exist.

Jon

lakeside53
12-05-2016, 06:27 PM
I don't have my boots on and don't want to wade knee deep into the argument, but a small data point.

Here's the exact transformer I use - back fed from a 240v Phase Perfect to give 480v.

http://attachments.temcoindustrialpower.com/product_info/Acme_Sec1_G.pdf

Extract : Reverse connectable: Yes

Cannonmn
12-10-2016, 05:08 PM
I don't have my boots on and don't want to wade knee deep into the argument, but a small data point.

Here's the exact transformer I use - back fed from a 240v Phase Perfect to give 480v.

http://attachments.temcoindustrialpower.com/product_info/Acme_Sec1_G.pdf

Extract : Reverse connectable: Yes

Here's the beast head down for transport. https://springfieldarsenal.files.wordpress.com/2016/12/img_0845.jpg

https://springfieldarsenal.files.wordpress.com/2016/12/img_0847.jpg

Cannonmn
12-10-2016, 05:41 PM
The electrical path will be to take one phase of the 208v. From the wall to a single-phase step-up xformer, then the output of that to a s.s. Inverter that outputs 3 phase to the drill. The xformer is square, 0ne foot on each side, weighs about 150 lbs, rated at 7.5 kva. Inverter is very light alum. And plastic box about size of a. Pint milk carton, supposed to handle 5 hp before de-rating which I'm told is about 50% in this situation. Since drill speed is set via gear train I'm guessing the 120 rpm slowest speed will be able to produce lots of torque.

Cannonmn
12-12-2016, 08:22 AM
I read what was posted above about the Temco xformer being reverse-connectable but I don't know what they cost and don't know of a vendor; all I'd need would be a 3kva model. However I found this one cheap ($325?) but can't find anything saying it can be used in reverse. Where Temco is Delta/Delta, this is Delta/Wye, does that matter?

https://www.amazon.com/Dongan-Transformer-63-0303-3-Phase-General/dp/B013EL3J68/ref=sr_1_25?ie=UTF8&qid=1481521937&sr=8-25&keywords=3-phase+transformer#feature-bullets-btf

Black Forest
12-12-2016, 08:48 AM
That looks like either it was made by Strand or Arboga in Sweden. A nice medium duty drill press.

J Tiers
12-12-2016, 10:31 AM
I read what was posted above about the Temco xformer being reverse-connectable but I don't know what they cost and don't know of a vendor; all I'd need would be a 3kva model. However I found this one cheap ($325?) but can't find anything saying it can be used in reverse. Where Temco is Delta/Delta, this is Delta/Wye, does that matter?
....

Being from china, it may not meet any of the requirements. Even some UL marked units from china have been, to my knowledge, not actually properly tested, so the UL was not valid.

However, leaving that aside.... No idea if that one SHOULD be reverse connectable or not. There is a general statement of UL recognition, so it SHOULD be reasonable. But, as noted in the minutes above, the UL test is performed using the marked primary and secondary as input and output, and does not involve the reverse connection, meaning that no testing whatever was done in reverse connected operation (presumably nless the manufacturer stated that it should be tested in both directions).

My suggestion is to go with it. You might check that the output in reverse connection is generally correct. If the transformer is "compensated", the output in reverse will be very noticeably low, which might cause you problems. The kVA is low enough that it might be compensated.

Jon Heron
12-12-2016, 08:07 PM
It is not required to state suitable for reverse direction as the code does not demand it. Instructions are not required to lay out all of the suitable installation methods but rather the limitations of the unit. If the intent was to require that transformers had that marking it would have to spell it out and it clearly does not.
That being said you should be looking for an auto transformer, they have a smaller footprint and are cheaper than a standard air gap transformer. With an auto you also do not have to worry about delta-wye, grounding and so forth.
If you are concerned about your inspection passing the best thing you can do is contact your inspector and ask him outright before you spend any money.
Cheers,
Jon

Jon Heron
12-12-2016, 08:10 PM
BTW, nice looking machine!
Cheers,
Jon