# Thread: Wiring rotary phase converter- wire gauge / amperage

1. Senior Member
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## Wiring rotary phase converter- wire gauge / amperage

I've just bought a 5 hp rotary converter off eBay. When I opened the box I found out that the guy used 18 ga wire for input and output. To me it sounds a way too thin, but I might be wrong.

At this point I'm going to run only 1 1/2 hp Bridgeport out of this converter. However, later I may need to add more loads.

Here are a few questions:

1. What gauge wire should I use to connect the converter to my 50 amp wall outlet (about 30 ft. run)?

2. What gauge should be used to connect the output of the converter to 3-phase outlets that I'll use to connect my machines to?

3. What internal wiring gauge should the converter have to be able to withstand a full load? It is a simple self starting type of RPC with two capacitors (no balancing capacitors)

I'm totally confused because I cannot figure out expected amperage running though each wire. The motor plate on my Baldor idler mentions 12 amps at 230V.The main Bridgeport 1 1/2 hp motor plate shows 4.4 Amp at 230 V which is 1.5 Amp per leg. If I connect my Bridgeport to the converter, the combo will draw 16.4 Amp from my single phase outlet. Correct so far?

What happens when a 5 hp converter feeds 5 hp load and the second load, while running, helps connect an additional 10 hp load to the system? Will the whole system draw an equivalent of a 20 hp load?

And now, if my assumptions were correct so far, let's try to figure out reasonable wire gauges for a situation when a 5 hp idler runs a 5 hp load. Based on the plate rating of my Baldor (12 Amp) and adding a safety margin, I assume that the combo will draw, approximately, 30 amp out of my single phase outlet. It means I'll need 10ga cable for the input. As for the output, since the collective 3-phase load applied to the converter will be 5 hp, the draw from the converter will be around 15 amp. So for the converter output I can use 14 ga wire.

Am I correct that the 18 ga used to wire input and output inside the RPC is insufficient?

Last edited by MichaelP; 05-01-2009 at 04:48 PM.

2. SGW
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I'll leave the details to the electricians among us who know a lot more than I do, but you're certainly correct about one thing: 18 gauge is way too small.

3. Senior Member
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Yes, 18 is way to small. You should protect your converter at the right ampacity. Putting a sub panel at the outlet so you could put a 30 amp breaker to power your converter would be a good idea. Plugging it into a 50 amp outlet would work but not offer much protection to your converter. If you do use the 50 amp outlet # 6 wire or cord would be fine. To make sure you don't have a problem # 12 on the output should be ok.

The more motors you run the better your converter works.

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Use #10 gage to feed in to converter and 14are 12 gage out to machine and at least 14 in box.

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Thank you, guys!

I replaced the inside wires with 10 ga already and will use 10-3 cable for the input and 14-4 one for the machines.

Another problem is that, based on the schematics, it needs the neutral on the input side. The neutral is required for the relay that switches the starting capacitor on and off. My 220V outlets don't have neutral wires, and I'm not yet ready to hardwire the unit.

One solution that I was able to find is to use a transformer with two primaries (110V + 110V). Any potential problems or shortcomings of this?

Maybe somebody has a better solution? A button instead of the relay contact to press and hold until the idler gets to full speed? Is there a chance for the idler to stop unexpectedly when I'm in the middle of work? How about power outage? Any other possible problems?

Maybe I should just disconnect the relay and the capacitor and hand wind the idler till I'm ready to hardwire the unit?

P.S. I was going to post the schematics, but the option to manage attachments doesn't exist (for me, at least). I don't know why...
Last edited by MichaelP; 05-02-2009 at 06:38 AM.

6. SGW
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There is no "option to manage attachments." That's why you don't have it.

You have to host images yourself, somewhere, and link to them. See the sticky note at the top of the General section, "How to post pictures with Photobucket," for an explanation of one way to do it.

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Thank you!

8. Senior Member
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Michael,

Motors are not "protected" with circuit breakers. Circuit breakers provide overcurrent protection for the conductors supplied. Motor overload protection is accomplished via motor starter (manual or magnetic) or definite purpose contactor with overload heaters/eutectic alloy etc..
Don't forget the inrush current required to get your idler spinning. That can be 5-10 times nameplate FLA . Again, a properly sized motor starter will handle this. For a 5 hp idler, I would start here http://www.grainger.com/Grainger/items/4A698
You can buy the on/off push button assembly http://www.grainger.com/Grainger/wwg...ryString=3kl81
and N.O. contact http://www.grainger.com/Grainger/wwg...ryString=4b972

This will cost you almost as much or more than the converter but it's the way to go. Troll ebay and you may get lucky. In any case, I highly recommend further investigation/study of electrical installations prior to proceeding. The NEC NFPA code for one. A really good site for electrical info is here http://forums.mikeholt.com/search.php?searchid=1768281

Homeowners policys are usually rendered invalid if a fire is caused by bogus electrical installs.

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I'll certainly look into this when I'm ready for hardwiring the unit.

As for the motor overload protection, do you really feel it's pertinent with an idling motor?

10. gsp
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I have # 10 wire from the breaker, to a 30 amp fused switch, to the converter. #12 wire going out.

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