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Joined: Dec 2004
Posts: 78
Cinner Offline OP
We are temporarily feeding a 3 phase panel with 10 awg wire to power some lighting circuits. This is a temorary lighting solution for some offices while under reno. All lighting circuits will be single phase. Obviously the 10 awg can feed maximum 30 AMPS...but would you calculate (estimate)the total wattage for lighting allowed based on 3 phase P=E*I*1.73 or single phase P=E*I?

Joined: Jan 2005
Posts: 5,316
Cat Servant
Single phase ... that's what you have. Just be sure to use the lower voltage of 208.

Joined: May 2005
Posts: 706
I hope you mean 208V on a 240V two-pole breaker.


Joined: Oct 2007
Posts: 56
I think the answer depends on whether you are supplying the panel with single, or 3-phase power. Are all 3 phases being fed from a 3-pole breaker? Or are you feeding one or two phases from a single or double-pole breaker?

Joined: Jan 2003
Posts: 174
I don't think it matters if you look at it this way and assuming the feed is 3 phase 208v. The three phase temp wiring supplying the panel is good for 30x208x1.73= 10795 watts.

Again making assumptions that the lighting is 120v.
Phase A to lighting is good for 120x30=3600w
Same for phase B and C
Total of A.B& C is 3 x 3600=10800
Basically the same thing as above.

Keep in mind this does not take into account any other code rules such as 8-104, 30-712(2) or 76-014(4)
Alot of assumptions made here but not alot of detailed info supplied. It's just a way of illustrating how to make the calculation.
You will still have to take applicable code rules into consideration.

Joined: Jul 2007
Posts: 1,335
Cinner, in the manner you are talking use E x I X 1.732. You can do it the hard why by doing each phase then adding them together but you will get the same result.

"Live Awesome!" - Kevin Carosa
Joined: Mar 2005
Posts: 1,213
Bear in mind that E x I X 1.732 is using line-to-line (delta) voltage. If you're using line-neutral (wye) voltage, the 1.73 factor as already been applied, and you can treat them like individual 1-phase circuits.

You didn't say what voltage; but for 208/120, ex:
Delta: 208 x 30A x 1.73 = 10.8kVA
Wye: 120 x 30A x 3 = 10.8kVA

Joined: Jan 2005
Posts: 5,316
Cat Servant
Absolutely ... Tiger, I meant to use 208 for his calculations, and not the 240 he would see from a single phase service.

That is, I am assuming that the three phase power is 208v leg to leg ... and 120 leg to ground.

Joined: Jul 2007
Posts: 1,335
My apologies. I realized i errored after I read renosteinke's message. I failed to mentioned the voltage was 208. I run numbers all the time and in my mind (a scary place to be) 208 Volts is 120 volts used in a three phase circuit. What was posted in a sense was a three phase circuit. I simplified the problem in my mind without explaining it to every one. I am sorry for that and I hope I did not cofused any one. People come here for direction, not confusion. If a person wants confusion, read the code. smile

"Live Awesome!" - Kevin Carosa

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