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Discussion Starter #1
I'm installing a sub panel in a new "barn", from the main panel in the pump house, about 100 feet away. The sub panel will be 100A to support a 60A circuit for a demand water heater.

Because of the distance from the main panel to the sub panel, I'm running #2 AWG wire to reduce the voltage drop, which is larger than what the code requires, which I understand is #3.

My question is about the neutral and ground wires. I understand there is table 250.122 in the NEC that allows a #6 ground wire for up to 200A. What about the neutral? The biggest load in the sub panel will be the water heater that doesn't draw any current through the neutral. Everything else is lighting and small loads to 15A outlets. Does that mean I can use #3 for the neutral from the main panel to the sub panel?

The path from the pump house to the barn is 2-inch PVC, buried. It is located in Northern California, in rural Sonoma County.
 

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Very Stable Genius
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Won't comment on the wire sizes since CEC ins't exactly the same, except
to say #2 Cu seems right.
Want to ask though, have you considered running AL? Lot cheaper, easier to
work with, and just as good IMO.
 

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A "Handy Husband"
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For a branch circuit the neutral is the same size as the ungrounded conductors.
 
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Discussion Starter #4
I will investigate aluminum, which may make my question moot.

For a branch circuit the neutral is the same size as the ungrounded conductors.
I know this is standard practice, but is it a code violation to use heavier gauge wire than is required, but only for L1 and L2?
 

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A "Handy Husband"
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If you upsize the ungrounded conductors you must do the same for the grounded conductor.
 
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The grounded conductor does need to be larger when upsizing the ungrounded conductors but only the bare minimum size is affected. The minimum grounded conductor size for a feeder is either the same as the equipment grounding conductor or sized for the calculated load for the grounded conductor, whichever is larger.

Since you are upsizing for voltage drop, the equipment grounding conductor size (and thus the minimum grounded conductor size) must be adjusted proportionally. #8 copper is the minimum size for a 100A feeder. Using the circular mil rule for upsizing from #3 to #2, #8 is proportionally upsized:

(66360/52620)*16510 = 20821 mils

#6 is 26240 circular mils which is more than sufficient. So #6 is the absolute minimum size. #3 is fine. Or anything in between as long as it supports your expected load.
 
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