neutral sizing

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rszimm

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Something I've always wondered a bit about:
So I'm feeding a 240V 100A subpanel with #3 THHN. Two hots and a neutral (plus a ground). Every circuit on that panel is a 120V circuit. So there's no current from one hot to the other. All current goes through one of the hots and into the neutral. Isn't the neutral now seeing double the current? Is this a phase thing that I'm missing (i.e. if there's 100A on phase 1, 100A on phase 2, is neutral current actually 0 or is it 200A)?
 
david luchini

david luchini

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rszimm said:
Something I've always wondered a bit about:
So I'm feeding a 240V 100A subpanel with #3 THHN. Two hots and a neutral (plus a ground). Every circuit on that panel is a 120V circuit. So there's no current from one hot to the other. All current goes through one of the hots and into the neutral. Isn't the neutral now seeing double the current? Is this a phase thing that I'm missing (i.e. if there's 100A on phase 1, 100A on phase 2, is neutral current actually 0 or is it 200A)?
Click to expand...

It's 0, or close to zero, depending on power factor.
 
charlie b

charlie b

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Assuming loads are balanced, two things are happening. One is that current leaves Phase A and returns on the neutral. The other is that current leaves Phase B, and returns on the neutral, but this is happening on the other half of the sine wave. Put another way, when current is positive and is leaving Phase A, current leaving Phase B is negative. That means that at that moment, current is really "leaving" neutral and returning on Phase B. That is why the neutral current is zero; the two neutral currents cancel each other out. So what is actually taking place is that current leaves Phase A and returns on Phase B.

So, does that clear things up, or does it make things more confusing?
 
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rszimm

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Makes total sense now. Thanks guys. So it would seem that it's actually pretty rare that neutral current ends up as high as the phase current. In fact, i believe it would only happen if one phase had a load and the other phase had zero load.

Let me ask a followup. Assuming one phase was pure resistive and was trucking along at 100A and the other phase had all low power factor loads also at 100A is there a critical power factor where the current in the neutral actually exceeds the current in either of the phases?
 
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