View Full Version : 208 3 Phase

11-17-2007, 09:16 PM
At work we just moved in to a new (for us) building.
The power coming in is marked "208 3 Phase/120v AC" if I understand correctly 1 208 line and neutral gives 120 VAC.
I need to wire up a couple of 220V single phase motors and was wondering if this is possable ? 2 electricians say no, 2 say yes but don't remember how to do it.
All help is thanked,


J Tiers
11-17-2007, 09:23 PM
It is 208 phase-to-phase, and 120 phase to neutral.

The 208 is only about 5% low for 220, so a 220 motor should be OK wired from phase-to-phase. Isolation and insulation should be OK for that usage, it really isn't much different from 220.

11-17-2007, 09:34 PM
208 single phase is what it is called

Just use any 2 of the hot lines (120v ea.) and connect as normal

11-17-2007, 09:37 PM
OK sounds pretty much like I have here at my shop. Building is supplied with 120/208 VAC. Lighting and plug-in loads are supplied between any phase conductor and neutral. Try to balance the current draw per phase....

As for heavier loads, eg. motors, try to use 3 phase if you can. Motors are cheap, easily reversed, run smoother etc. All has been said before on here.
To directly answer your question, heavier single phase loads like water heaters, larger motors etc. are powered by any 2 phases. (208 VAC line to line). If the motor was nominally rated as a 240 V motor it will draw a bit more current at the lower voltage to produce the same amount of horsepower, but it shouldn't matter much.

If you have some sort of motor starter with overload heaters, you may have to change the heaters to prevent annoying tripping if you're operating near the high end of the motors rating.

My guess is that you have some sort of machine that currently has single phase motor(s) and controls & you don't want to rewire it. If you have any 120 Volt controls, lights etc. on that machine, you'll still have to run a neutral as well to provide 120 V.

Hope that helps;

11-17-2007, 09:43 PM
One word of caution that I was given by some electricians when I was performing a similar task was the following:

If you load two of the 3 legs heavily, the 3rd can increase in voltage -- substantially in some cases. My situation allowed me to distribute my "208 volt, single-phase" loads across the 3 legs (i.e. connect one load to A&B, one to B&C, and one to A&C). That worked very well and I had no problem with voltage spikes.

I do not know if there was any truth in the advise that I received but I figure that I'd pass it along. I'm not professionally trained as an electrician either.

J Tiers
11-17-2007, 09:53 PM
That is perfectly correct for some instances, and good to do in all cases.

Try to distribute the single phase loads around the phases equally. Of course 3phase loads are balanced by nature.

11-17-2007, 11:22 PM
Thanks for the help. My concern now is a 220V motor @ 15A and a 220V motor @ 28A (flywheel grinder).

Thanks again,