Three-Phase and Single-Phase Power

Shop owners discuss running a single-phase motor using two legs of a three-phase power source. May 28, 2006

Question
We have a 240 3 phase service. I was wondering if I can run a 240 single phase motor off of 2 legs of the three phase panel? Also, what if the motor is rated at 220 single phase - will using 2 legs of a 240 3 phase feed hurt it?

Forum Responses
(Solid Wood Machining Forum)
From contributor B:
Yes, you can run a 240v single phase motor on two legs of a 240v three phase service. You will actually be running on 208 volts, but that is within range of the motor tolerances.


From contributor L:
If he's in Europe, he may actually have a 240 VAC 3 PH system. Still using two legs and a neutral of the 3 ph system will yield 240 single phase. Most motors can tolerate a 10% voltage swing without harm. If it is rated for 220 VAC, then you can operate it in a range of 200-242 VAC without too many problems. If this motor is to be operated at full load all the time, like a compressor, I would suggest getting a buck/boost transformer to change the voltage to the proper range.

From contributor H:
I disagree. If he actually has a 240 volt three phase service (a delta bank), there will be 240 volts between any two legs, however if you check between the line and ground, one leg will be "wild," that is it will have about 175 volts on it. He would use the two stable lines and have 240 volts, not 208. A motor rated for 220 volt three phase would run on the 240 volts. Depends on your power source as to how dirty this might be. Voltage fluxes can be common, so what is 240 today might be 237 tomorrow. 208 three phase is different in that each leg is equal in voltage and usually more stable (not dirty) - between any two there is 208 volts. Usually this is used for lighting such as in a large department store but can be used for industrial, providing your motors are rated for such. This may require that you change the motor leads as per the wiring diagram in each motor. Most three phase motors can be configured to run on 208, 220/240 and 440/480. The current draw will be different at each setting, requiring different wire sizes to work properly.

From contributor B:
Thanks for the additional details. I simply based my comments on my experience. Years ago I lived in a place where we had either 220v or 240v 3-phase (way too long ago to remember which) power. When we used any 2 legs we ended up with 208v. Over the years I've seen that configuration again and so described the situation as I did.

I'm surprised that all 3 legs in a 240v service are not balanced by the transformer on the pole. I have a very good friend who is a high voltage industrial electrician. I'm going to ask him to explain to me why that would occur. Electricity can be a very basic and also an extremely complex topic. It took my friend several attempts to get me to understand shared neutrals in single phase circuitry as an example. I never would have imagined such a thing.


From contributor S:
Are you in Britain or Europe? If so, the 3-phase supply is measured phase to phase - 415 volts @ 50Hz in the UK/Ireland or 380 volts @ 50Hz in most of mainland Europe. In that case, any single phase to neutral will measure 240 volts in the UK/Ireland, which is how we wire things here.

From contributor R:
Two things come to mind in this situation: use of terminology, and Delta vs Wye configuration. I have 240v three phase Delta config at my shop, with no neutral (I cannot get 120v from it). Between any two legs, I get 208v IIRC. Re my terminology comment, it's been a while since I had the discussion with my electrician, but I *think* the two flavors of 2**v power are 208v three phase Wye, and 240v three phase Delta. A quick phone call to your power company would clarify exactly what voltage and which configuration you have.

From contributor H:
The interesting thing is that in this article, the delta system is not the norm. Around here it is what you would get unless you requested wye transformers. I run single phase 120v, 220v and three phase from the same panel. I just have to be careful not to use the wild leg in a single phase application. Therefore I have a lot of open single slots in my electric panel. Doing it over I would split my panels so that I had one for single phase in which I could use all the slots and one for three phase in which I also could use all the slots.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor J:
The voltage measured between legs is dependent on how your system is supplied and the type of transformer used. In my case where I work I have both. I have one transformer setup with a wild phase, hence 208, 240, 240. However I have a second transformer setup where I have 240 on all three phases.

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