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View Full Version : Electrical branch switching... mutually exclusive.



Arthur.Marks
01-07-2011, 05:22 PM
I have two 30A 220V circuits that runs into my garage. One runs my welder. The other runs an electric heater in the winter. I am looking at getting a 60gal. 3HP air compressor soon. There are no slots left on my main box for another 220V line---and I wouldn't want the expense anyway. 99% of the time only one of those two circuits is running at a time. I would like to make a branch circuit hard-wired in line with the outlet used for the heater.

So the question: How to wire it so you have a (manual) switch that allows only one of those two connections to be live? I guess the ideal would be something of a disconnect box that has three settings: Off/disconnected, Line 1, Line 2. So if you had the box set on "Line 1", one connection is live and the other completely disconnected. This would eliminate both machines from ever running at once.

I'm just not familiar with what is available outside of simple, single line disconnect boxes. Is there such a thing that will do what I'm talking about above? What is it called? I've looked through McMaster's selection and couldn't find a match.

Any info is appreciated.

bruto
01-07-2011, 05:39 PM
What you need is a double pole, double throw switch. I'm guessing you won't find one cheap.

Here's one: http://www.galesburgelectriclighting.com/store/product.php?productid=3878


As an alternative which might end up cheaper, if you can dispense with the center-off function (or put up with a separate master off switch), you could find two double pole single throw switches of the correct amperage, and mount them side by side in a single box. Mount one right side up, and the other upside down, and gang the toggles together.

edit...note: you'll often find the switch types abbreviated. What you'd be scanning for in listings is DPDT.

darryl
01-07-2011, 05:44 PM
There was a similar request here not long ago. Guy wanted to have the water pump take precedence over the air compressor, with both having to run from the same feed. In your case, the air compressor would take precedence over the heater.

It can be done using a relay and not much else, except a housing, connections, etc. I'll see if I can find that thread.

Black_Moons
01-07-2011, 05:44 PM
Need some of these: http://i205.photobucket.com/albums/bb146/mlypnkw/Halloween/knife-switch_002.jpg
Just joking.

Unfortualy, past 20A you can't buy standard 'light switch' style switchs, and the price goes way up.

It might even be cheaper to buy a 30A relay with 120v coils, and just wire a light switch to the coils.

You could also wire it up so that the thermastat or pressure switch actualy controls the relay, So that either the heater or compressor can override the power as needed. (Make SURE you have a good popoff safty valve on the compressor if you do this, Relays are known to 'stick' sometimes, Of course, So are pressure switchs since they are not much more then a pressure activated relay)

darryl
01-07-2011, 05:49 PM
Here's a link to the thread I was thinking about.

http://bbs.homeshopmachinist.net/showthread.php?t=44596

macona
01-07-2011, 06:12 PM
They make breakers that serve two 120v circuits on one breaker. Get a couple of these and consolidate a couple of your 120 circuits and use the newly opened slots for a new 240 breaker.

Edit: Like these:

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&productId=100356518&langId=-1&catalogId=10053&ci_src=14110944&ci_sku=100356518&cm_mmc=shopping-_-googlebase-_-D27X-_-100356518&locStoreNum=4018&marketID=54

Optics Curmudgeon
01-07-2011, 06:14 PM
Look into generator transfer switches, they are well interlocked to prevent both circuits from being connected at once.

Arthur.Marks
01-07-2011, 06:47 PM
Hmmm... the switch that bruno linked to would be a great, simple solution. The spec's are a little confusing, though. Generally, it is listed as 30A. Then further down it specifies Max. Amp. as 24 and 2HP AC motor limit. See HERE (http://www.leviton.com/OA_HTML/ibeCCtpItmDspRte.jsp?item=453286&section=10046&minisite=10051&language=US). I did find THIS (http://www.surplussales.com/Electrical/Boxes.html)(DPDT 2 Pole Fusible Switch Box), but it is a bit like Black_Moons' joke... with possible asbestos content :eek: :D

The generator transfer switches sound good, but they're friggin' expensive!! $500+ ain't gonna happen. So far it sounds like the option of taking two switches wired opposite and then linked together is where I'm currently leaning:

As an alternative which might end up cheaper, if you can dispense with the center-off function (or put up with a separate master off switch), you could find two double pole single throw switches of the correct amperage, and mount them side by side in a single box. Mount one right side up, and the other upside down, and gang the toggles together.

CCWKen
01-07-2011, 06:54 PM
Well, I did a search but couldn't find the model number and I just got back from the barn. (Not going back out again.) I have a 50A/240v switch that comes in a metal box and powers my compressor. That's how it's turned on and off. I'm pretty sure I bought it at Home Depot because that's were I did most of my shopping for boxes and wire. It's a simple slide switch in a 5x7 metal box. As I recall, they were less than $25.

You could use two of those but then you'd have to provide the "mutually exclusive" logic. :cool:

Weston Bye
01-07-2011, 08:29 PM
Put a small 2-circuit breaker box at the end of the wire. Connect each device to it's own breaker, then design a mechanical blocker flag that, when in one position blocks one breaker and allows the other, and when in the other position does the opposite. Just make it so that both breakers can't be turned on at once.

I see you are in Chicago - don't know if such a contraption would pass the City of Chicago electrical code, though.

mickeyf
01-07-2011, 08:43 PM
make it so that both breakers can't be turned on at once.
While this would work, my understanding is that breakers are not actually designed to be used as switches. In a typical lifetime they would have many orders of magnitude fewer cycles than a switch. You might want to compare specs and see if you feel comfortable with that approach first.

Arthur.Marks
01-07-2011, 09:29 PM
Okay, I've done a little more investigating.... :)

I think I am misleading myself WRT the original link to the DPDT easy solution by Bruto: http://www.galesburgelectriclighting.com/store/product.php?productid=3878
The switch is rated 30A. My breaker is 30A. The heater plug / outlet is rated 30A... and the motor on the "3.2HP" compressor is rated 15A. So who cares? Everything should work fine with that switch. I think I'll follow the simplest route.

Thanks again, guys! This has been very helpful.

Arthur.Marks
01-07-2011, 09:57 PM
Then again, this is the same thing at 1/3rd the price, right?
http://www.alliedelec.com/search/productdetail.aspx?SKU=8700591

lakeside53
01-07-2011, 10:08 PM
You need to make sure that any switch you use is rated for the HP (not the plate current) of the motor, and has a rating at least as high as the current of your heater. I don't see any HP rating on that switch. The Levitron link showed a max hp of 2... Of course, depends on the actual motor used on your "3.2hp" compressor. If it really is 3.2, just use a contactor driven by a low rated switch.

bruto
01-07-2011, 10:31 PM
Circuit breakers that are switch rated, or carry the "SWD" designation, can be used as switches without problems.

As far as I know all the standard Square D breakers are suited to switching flourescent lights whether or not they bear the SWD, but I haven't been able to wade through the literature on line yet to see if that means that they are "switch rated" in general.

Square D does list among its accessories an interlock "QO2DTI" to do exactly what is called for here.

I've been using the Square D breakers in my shop as light switches for 22 years with no problems, but again, I don't know whether this would apply to other types of loads.

lakeside53
01-07-2011, 10:38 PM
They make breakers that serve two 120v circuits on one breaker. Get a couple of these and consolidate a couple of your 120 circuits and use the newly opened slots for a new 240 breaker.

Edit: Like these:

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&productId=100356518&langId=-1&catalogId=10053&ci_src=14110944&ci_sku=100356518&cm_mmc=shopping-_-googlebase-_-D27X-_-100356518&locStoreNum=4018&marketID=54


And... you can also get quad or "double 240" volt breakers for most panels - 2 x 240 volt breakers in place of your exisiting breaker. If it was mine I'd just add breakers or put in a sub panel. Neither are expensive.

Arthur.Marks
01-07-2011, 11:05 PM
I've been looking around for the last two hours and learning a lot in the process. Wouldn't this be a simple plug-n-play option? Take the incoming connection now going to the heater outlet and re-route it into this panel first: http://www.homedepot.com/Electrical-Breakers-Distribution-Load-Centers-Load-Centers/QO/h_d1/N-5yc1vZ1xi3Zbm2wZ17u/R-100096290/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053T Then connect the heater outlet to one breaker and the compressor to the other breaker. It already has the mechanical interlock. Yeah, the breakers are each 60A instead of 30A, but the main panel would trip as it would right now if something went awry.

bruto
01-08-2011, 11:45 AM
I've been looking around for the last two hours and learning a lot in the process. Wouldn't this be a simple plug-n-play option? Take the incoming connection now going to the heater outlet and re-route it into this panel first: http://www.homedepot.com/Electrical-Breakers-Distribution-Load-Centers-Load-Centers/QO/h_d1/N-5yc1vZ1xi3Zbm2wZ17u/R-100096290/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053T Then connect the heater outlet to one breaker and the compressor to the other breaker. It already has the mechanical interlock. Yeah, the breakers are each 60A instead of 30A, but the main panel would trip as it would right now if something went awry.I don't see any reason why that would not work. My only reservation here would be how the breakers are rated as switches. The mechanical cycling itself should be no problem, but you might need to take care not to switch them with the compressor turned on. So far I haven't found definitive answers on this in the Square D literature.

lakeside53
01-08-2011, 12:32 PM
Look at your wire size to the heater - if it's suitable, just change out the source breaker and put a sub-panel on that wire. If there is no neutral (2 wires plus ground), it will be 240 only, but...

As I was mentioned before, the easy way is just to rearrange your breakers in the existing panel, and add a 240 circuit for your new need.

Tinkerer
01-08-2011, 12:53 PM
How bout a rotary switch?

30 amp 125/250 volt (motor rated)

http://www.mcmaster.com/#ul-recognized-rotary-switches/=ai43td

Arthur.Marks
01-08-2011, 01:38 PM
Look at your wire size to the heater - if it's suitable, just change out the source breaker and put a sub-panel on that wire. If there is no neutral (2 wires plus ground), it will be 240 only, but...

As I was mentioned before, the easy way is just to rearrange your breakers in the existing panel, and add a 240 circuit for your new need.

Thank you for reasserting this option. I was thinking last night that this would entail adding the sub-panel near the main box and running wire all the way to the compressor location. I am now reading this that if the wire is of sufficient size (I'll have to go look.. give me a few) it can branch off in the garage. The sub-panel could go there, which would in the end allow all to be running safely at once. A mutually exclusive switch would no longer be needed.

If it seems like I'm juggling all the options, it is because I am trying to do this with the least cost and installation while still making it very convenient to operate. I sort of liked the Square D generator box because it would effectively be "free". You see, the whole reasoning behind this compressor purchase is that I have a slew of Home Depot gift cards redeemed from my cc reward points at the end of the year. The compressor will cost me nothing out-of-pocket. That said, I have no problem spending cash on the switch if it keeps everything up to code with dumb-proof safety. I tend to go overboard in that regard because if an electrical inspector ever walks through here, I want to show clear intent. My main shop (not in the garage) was wired by a licensed electrician. I have since come to learn that even he could not decipher every code (mainly inconsequential ones, but nonetheless). I hope that shows my perspective and explains my responses thus far.

Then again, maybe I should just fall back on the ultra basic, original, original plan :p Stick a plug on a lead to the compressor and call it a day. The outlet is behind a table and near the ground, but hey! It would clearly be 1) inexensive 2)Code compliant... Too bad it would be very inconvenient :(

alchymist
01-08-2011, 05:54 PM
Find another sub-panel, smallest you can find, and put it at the end of the existing run. Put two breakers in it, and wire both devices into it. Then just swap breakers on/off for whichever device you want powered at any given time. I have seen these boxes as small as 4 breakers, perfect for 2 - 220 volt doubles. You can interlock the breakers if so desired.

gda
01-08-2011, 06:03 PM
I opted for the plug-unplug-plug route.

I run my phase converter, mig welder, compressor, and rockwell belt sander off the same outlet. Yes switching plugs gets annoying but it is not a big deal.