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Old 12-08-2010, 03:25 PM   #1
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Grounding requirement for 100A subpanel


Hi:

I live in Michigan

I'm installing a 100A subpanel for a kitchen remodel fed off a 100A breaker in the main panel.

I understand that the ground and nuetral should NOT be bonded in the subpanel - only in the main panel.

I'm using #2 copper THHN cable to feed the subpanel (2 hots and one neutral)

I have a couple questions:

1) What size ground wire is required for a 100A subpanel (from main panel ground to subpanel ground)? I know #2 THHN is likely overkill for the ground and I'd like to save some money if it is safe to do so.

2) If I use uninsulated ground wire, will I have problems with corrosion against the rigid conduit, or is insulated ground cable preferred?

Any advice is appreciated

Thanks

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Old 12-08-2010, 04:12 PM   #2
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Grounding requirement for 100A subpanel


Your post indicates you are using metallic raceway between panels. This would suffice as your equipment grounding. Personally, I would add a #8 copper equipment grounding conductor, either bare or insulated.

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Old 12-08-2010, 05:59 PM   #3
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Grounding requirement for 100A subpanel


How far away from the main panel is the subpanel?

To directly answer your downsizing question, you are allowed to down size your ground to 70% ampacity rating of the Current Carrying Conductor Rating (Hots) as long as it is larger than 10 AWG. Yes, #2 IS DEFINITELY OVERKILL

Assuming what I do know, if you use the metal conduit, THAT is your ground, however I would use grounding insulating bushings at both ends if you decide on this route. They are tied into your ground bar with a piece of bare copper at both ends.

I would avoid rigid metal conduit in residential rework, but thats me.

Also, you can get by with #3 THHN-2 for your Hots and Neutral. If you dont mind downsizing the panel to 90 amps, #4 will work for that.

From my understanding if you run any subpanel, it MUST have separate Ground and neutrals.....(Isolated)

I would go with 6 or 8 Insulated THHN if you decide to run a wire. (This is what I personally would do at my house..) In all honesty, the cost savings on a short run will be extremely negligible.

FYI If you are pulling a permit on this, get with your Inspector and check to see if he allows using Phase Tape on your wires or if you have to have a colored jacket.
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