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Sizing wire for 3 phase current
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made2hack:
Hi all,

I need help trying to size solid core conductor wiring for 3 phase current.

Let us assume the following:
1 - A resistive load with a power factor of 1
2 - 10.4 kW of power
3 - P/P Voltage of 380V, P/N voltage of 220V.

My kVA in this case is 10.4 since pf is 1. As a result, current per phase is  I =  Power / (sq. root of 3 x pf x P/P V), so I = 10400 / (1.732 x 1 x 380) or approximately 15.8 Amps, say 16 Amps.

Do I size my wire according to the current per 1 phase? Ie 16 Amps? Or do I size it according to all 3 phases, ie 48 Amps? I guess since not all the current flows at the same time through the Neutral, since they are 120 deg out of phase, I would be sizing for something closer to 16 Amps.

Thanks,
soldar:
3.5 KW per phase = 16 A per phase.

In theory neutral will carry little or no current but you need to size it at least for 16A in case one phase or load fails.
made2hack:
Ok, thanks
bdunham7:
You need to specify whether the resistive loads are across the phases or from phase-to-neutral.  If across phases, no neutral is needed, balanced load or not.  If from phase to neutral, then if the loads are perfectly balanced the neutral current will be zero, if not the neutral current will be no more than the largest phase current--so I'll leave it to you to guess what size wire you'd need.

Be careful--if your resistive load assumption isn't true, bad things can happen.  Non-PFC SMPS systems can have a neutral current that is 3X the phase current.
soldar:
Always follow local codes and regulations.
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