240VAC Neutral Wire Gauge?

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A transformer supplies 240VAC to a 20A circuit breaker as shown in the attached image. I believe I can use 12AWG wire for wires X1, X4, 2 & 3. However, I'm not sure what gauge wire the neutral should be. Maybe 12AWG or something larger?
 

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  • #2
berkeman
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A transformer supplies 240VAC to a 20A circuit breaker as shown in the attached image. I believe I can use 12AWG wire for wires X1, X4, 2 & 3. However, I'm not sure what gauge wire the neutral should be. Maybe 12AWG or something larger?
Welcome to PhysicsForums.

What does the NEC say? If you don't know what the NEC is, what does your electrician say? :wink:
 
  • #3
Baluncore
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However, I'm not sure what gauge wire the neutral should be. Maybe 12AWG or something larger?
A 240V device drawing current from the two outer terminals will not have a neutral current.

However, when both 120V circuits are being operated with the shared neutral, since they are of opposite phase, the currents returning from the two actives will not sum but will partly cancel in the neutral.

For that reason I would expect all conductors to be of the same gauge appropriate to the 20 amp breaker.
 
  • #4
hutchphd
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Yes. Absolute worst case is simply using one side.
 
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  • #5
Averagesupernova
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Even if you have a 240 volt only load with no 120 volt loads, you have to size what you are calling the neutral large enough to carry maximum fault current. You have the center tap grounded at the transformer, so you have to have a safety ground and this is what you are calling the neutral. If you have 120 volt loads as well, you need a fourth conductor.
 
  • #6
Baluncore
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240 volt at 20 amp is 4800 VA.
So why is the transformer only rated for 3000 VA ?
 
  • #7
Tom.G
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The NEC says the Neutral must be not smaller than the supply conductors.
(I don't have my copy handy at the moment, somone borrowed it and it is overdue. :cry:)

Cheers,
Tom
 
  • #8
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Even if you have a 240 volt only load with no 120 volt loads, you have to size what you are calling the neutral large enough to carry maximum fault current. You have the center tap grounded at the transformer, so you have to have a safety ground and this is what you are calling the neutral. If you have 120 volt loads as well, you need a fourth conductor.
At least in the US, conductor #1 is indeed referred to as the neutral. The safety ground is a fourth conductor, not shown, connected to the case or frame of the equipment being powered but not intended to supply power.
 
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I did find a website that apparently references the NEC:

“Sizing the neutral: Sec. 220-22. You must size the neutral conductor to carry the maximum unbalanced current in the circuit (i.e. the largest load between the neutral and any one ungrounded phase conductor)...”

The website also says:

“You must use a multiplier of 140% when calculating the neutral current for a 3-wire, 2-phase or 5-wire, 2-phase system.”

So, for this scenario it would appear that the maximum load between the neutral and a phase conductor is limited to 20A by the circuit breaker. Also, this appears to be a 3-wire, 2-phase system, so my guess is that 20A x 140% = 28A so 10AWG wire for the neutral is probably the best bet.

Baluncore, you are correct, it does appear that either the Transformer is too small or the Circuit Breaker needs to be reduced.
 
  • #10
Averagesupernova
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Also, this appears to be a 3-wire, 2-phase system, so my guess is that 20A x 140% = 28A so 10AWG wire for the neutral is probably the best bet.
No. It is NOT 2-phase.
 
  • #11
Averagesupernova
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At least in the US, conductor #1 is indeed referred to as the neutral. The safety ground is a fourth conductor, not shown,
Based on the diagram I don't know how you can say it is one or the other. They are not the same by definition but the diagram makes no attempt to label.
 
  • #13
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Based on the diagram I don't know how you can say it is one or the other. They are not the same by definition but the diagram makes no attempt to label.
I see your point. From a US-centric perspective, that is a diagram of a residential supply, and I thought there could have been safety issues regarding nomenclature.
 
  • #14
hutchphd
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Nomenclature: Power in the US is typically called split phase power which comes from a center tapped transformer with local ground. Two phase power has one quadrature (##\pi/2## out) signal and so is not symmetric under time reversal.
 
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