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Old 10-01-2010, 05:28 PM   #1
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recommended ground size for 100A sub panel


I am planning on running a 100A sub pannel about 35' from the main. I have a bunch of 4AWG THHN that I was going to use for the hot legs and neutral.
Would I be OK with 10AWG for the ground, or would it be best to step up to 8AWG?

A conduit calculator is listing 1" PVC as minimum, for either 4 wire combination.
thanks for any insight.
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Old 10-01-2010, 07:56 PM   #2
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recommended ground size for 100A sub panel


According to table 310.16, that #4 THHN is only good for 85 Amps. You can upsize the breaker feeding it to a 90 if you like, provided your connected load does not exceed 85 Amps.

The Equipment Grounding Conductor will need to be a minimum #8 on any feeder exceeding 60 Amps, up to 100 Amps.
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Old 10-01-2010, 11:30 PM   #3
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recommended ground size for 100A sub panel


According to my chart the 25mm˛ { #4 AWG } is only good for 85 amp with THHN/THWN conductors.

For grounding conductor you will need 10mm˛(#8AWG) That will be fine for this breaker size { It is good up to 100 amp breaker size }

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Old 10-02-2010, 06:53 AM   #4
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recommended ground size for 100A sub panel


Thanks guys, I appreciate the help.
I will step down to a 90Amp breaker. Funny thing here is I just pulled out 3-150' 4AWG conductors that were feeding a sub to a pole barn at our home that we are selling. The home was originally built by an electrician, and he had set it up with a 100A 11,000AIC breaker in the main service panel. Then the sub panel was a main breaker style that had a 100A 22,000AIC breaker.

I was hoping to use this sub panel but to do so the main breaker in the sub panel should be rated at 90A with a 11,000AIC or lower. Now the main breaker in the sub is totally different from a standard double pole branch circuit breaker right? I'm working with square D qo. If I do need different "main breaker" for the sub, can I find these at home centers, or should I check out Grainger or Graybar? Is it going to be cheaper to abandon the main breaker sub panel and just go buy a new main lug sub panel fed with a 90A DP at my main service panel?

Man, how come the pros sometimes cut corners if it's their own stuff, Makes me think of seeing the auto mechanics drivin' around with the 'ol rustbucket smoking and squeeling & held together with some duct tape and a few coat hangers.
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Old 10-02-2010, 07:01 AM   #5
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recommended ground size for 100A sub panel


You will need a means of disconnect at the remote building.

Just leave the 100 amp main breaker and install the 90 in the main panel.
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Old 10-02-2010, 07:26 AM   #6
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recommended ground size for 100A sub panel


This sub is going to be located in the attached garage (same structure) with reguards to the disconnect. But I don't care as I already have in hand.
But I do want to pay attention to my new 90A DP feeder breaker and make sure it's at least 22,000AIC (same or higher rating than my 100 main sub disconnect right.
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