Phase VA contribution
Phase VA contribution
(OP)
I'm configuring a 4 wire 120/208v panel and have this question. I have a 1581VA load, that I'd like to supply with a 2-pole 208v supply (Vac). I understand that a current of 7.6A would be seen by phases A and C. What does that mean VA wise to each phase? In other words, would each phase's load be 1581/2? Or that should follow the phase power equation: S=Vpahse*Iphase = 120*7.6?





RE: Phase VA contribution
The load current is not in phase with the voltages from A to the neutral or from B to the neutral.
In the case of a 120/208 Volt panel the phase current is not equal to the arithmetic sum of the line to line load currents and the line to neutral load currents.
Bill
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"Why not the best?"
Jimmy Carter
RE: Phase VA contribution
You can't put a load "on a phase". The load must be connected phase to phase or phase to neutral so the load is across the source. You would be correct trying to determine this if considering the load on the source and trying to determine the capability of the source to supply the load. For example, if the source was 3 single phase transformers wye connected in a bank then you should make sure the load doesn't exceed the capacity of any single transformer. Using your example numbers, the load on a single transformer in this bank would be 120V*7.6A.
RE: Phase VA contribution
RE: Phase VA contribution
RE: Phase VA contribution
Assume Vca = 208∠0° V and 1581∠0° VA (angles assumed for ease of calculation), then
Ic = S/Vca = 7.6∠0° A and
Ia = -Ic = 7.6∠180° A
With Vca defined above, then
Vc = 120∠-30° V and Va = 120∠-150° V
Finally,
S = Sa + Sc = VaIa* + VcIc* = 912∠30° VA + 912∠-30° ≈ 1581 VA.
xnuke
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RE: Phase VA contribution