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If I plug in a moving light that uses a total of 1,000 W at 208 V into two legs of a three-phase service, how many amps will be flowing on each leg and what is the formula to calculate this? Also, will the current change for that light (flowing through the feeder) if I have it plugged in alone or if I have three lights plugged in using all three of the phase combos?

Signed,

Legs

Dear Legs,

My teacher, Swami MyMommie the Enlightened One, once told me never to measure with my head what I can feel in my heart. The corollary of that is, never let electricity pass through your heart; do your calculations in your head! 

When you connect a single-phase 208V load across two legs of a 3-phase system, say, for example, phase A and B, then current goes through those two legs. But when you connect a second load across phases B and C, then there is some phase cancellation in phase B (the common leg). If the two loads are exactly equal then the total current in that leg is 1.732 times the current drawn by one fixture. For example, if the first load draws 10 A and the second load draws 10 A, then the current going through the common leg is not 20 A but it's 10 A x 1.732 = 17.32 A.

If the current drawn by the two loads is not exactly the same, then it's much harder to calculate the current flowing through the common leg. So here's what I do. I take all the fixtures and divide them among the three legs as evenly as possible. (That's not the same as adding all the wattages and dividing by three b/c it doesn't always work that way; if fact, seldom does it ever work that way.) Then take the leg that is loaded the heaviest and pretend all three legs have the same current. Now the math is easy - just multiply by 1.732 and you have the total current on that leg. That may be a slightly high estimate depending on how unbalanced the system really is, but it's the quickest easiest way I know how to do it. 

That explains why the formula for 3-phase power is: 

P = V x I x PF x 1.732. When you use this formula, make sure you plug in the phase-to-phase voltage and not the phase-to-neutral voltage to make it work correctly.

Sincerely,

Swami Candela of the Third Millennium

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