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Old 02-22-2019, 12:39 PM   #1
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What Size Wire for my Neutral and Ground?


I am running 100 amp line from my service panel to my subpanel through 100' of underground PVC. I have to use copper per local code. I am going to use #3 Cu for my two hot wires.

1) What AWG size does my neutral wire have to be?

I found this: Per NEC 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). You calculate the first 200A of neutral current at 100%.

Not sure exactly what that means. I suppose I could use a #3 Cu for my neutral just to be safe?

2) What AWG size does my ground wire have to be?

I found these two tables 250.66 and 250.122 (attached) that suggest #8 is the smallest Cu ground I can use for 100 amps. That seems small so just want to make sure that #8 is a smart decision.
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Old 02-22-2019, 12:50 PM   #2
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Re: What Size Wire for my Neutral and Ground?


Basically it means the neutral must be sized to carry the current of all your 120. If youíve done some load calculations (you should have) then just subtract all 240 loads from the total. But the easiest/ best way is just to make it the same size as the others. And do some digging into just what kind of wire you should use (THHN, THWN, etc.) for your specific application.
Youíre good with #8 for your protective ground according to those charts
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Old 02-23-2019, 06:40 AM   #3
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I would go ahead with #3 for the neutral just because it is not all that much more expensive than #4 and your loads could be mostly 120 volts neutral connected. #8 is ok. Remember this is the wire run from panel to panel for equipment grounding (250.66) not connected to the neutral at the garage (all grounds isolated with a grounding bar). You also need a ground rod connected to the same bar as well as any other existing grounds such as metal water pipes or metal framing if it is a metal shed style building.
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Old 02-23-2019, 06:42 AM   #4
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Correction:. I meant to say #8 for the ground.
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Old 02-23-2019, 08:36 AM   #5
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Re: What Size Wire for my Neutral and Ground?


Thanks all, I will go with #3 for neutral and #8 for my ground.
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Old 02-23-2019, 11:07 AM   #6
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Re: What Size Wire for my Neutral and Ground?


In most cases the grounding electrode conductors (ground wires to ground rods etc.) are #6 copper to direct buried or driven rods and #4 copper to water pipe or concrete encased rods. There is a separate size table in the NEC.

If the choice of current carrying conductor size includes some upsizing for voltage drop over distance then the equipment grounding conductor must be upsized comparably
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Last edited by AllanJ; 02-23-2019 at 11:10 AM.
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Old 02-23-2019, 11:45 AM   #7
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My garage is classified as attached-detached which means it’s a separate garage from main house but they share a roof.

So technically, it’s an attached building according to NEC so I don’t need to install a grounding rod for the garage ground. I was just going to connect the ground in the subpanel in the garage to my main panel ground via underground PVC conduit. I will use #8 for this unless this is wrong
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