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msbxxx

07:54AM | 03/27/07
Member Since: 03/26/07
1 lifetime posts
Bvelectrical
I have a 200A main panel, I'd like to add a 100A subpanel. It will be on an interior wall in an interior room of the basement.

I was planning to run the feeder wire through the floor joists, then down into the subpanel. I have a few questions:

1. Can I use a 100A main panel as a sub panel?

2. If so, do I need to do anything to it? (I've heard in a sub panel that the neutral bus and the grounding bus shouldn't be bonded?)

3. What gauge wire should I run? My guess is that the distance between the panels will be 40 feet.

4. Do I need to use conduit?

5. Does the subpanel need to have a grounding bar placed into the ground? I'm going to run a 100A breaker to this subpanel, so wouldn't the subpanel just make use of the bar in the main panel?

Thanks in advance for any help/advice...

Mike B

TimBonham

11:13PM | 03/28/07
Member Since: 01/09/07
197 lifetime posts
Here are answers to some of your questions.

"1. Can I use a 100A main panel as a sub panel?"

Don't. You'd be paying for a set of 100A circuit breakers in the subpanel that you don't need. You can get the same kind of panel, without the main breakers installed, and save money. But you may need to go to a bigger electrical supplier to find this, or have your local hardware store order one.

"3. What gauge wire should I run? My guess is that the distance between the panels will be 40 feet."

Minimum would be #3 copper or #1/0 Aluminum (rated 100A). Using #2 copper or #2/0 Aluminum (rated 115A) would give you more safety margin. This assumes that the space where you run it is inside the house, and doesn't normally get above 95ºF. And a short run like 40 feet doesn't affect the rating.

"4. Do I need to use conduit?"

Not necessarily. If you get wires in a NM cable, you can just use that inside the house. (You probably can't, in this size.) So you will likely end up using individual wires, then they do need to be protected inside conduit (plastic, metal, or flexible metal).

"5. Does the subpanel need to have a grounding bar placed into the ground?"

No! In general, there should only be one grounding point per house.

"I'm going to run a 100A breaker to this subpanel, so wouldn't the subpanel just make use of the bar in the main panel?"

Yes. But that means you need to run a separate ground wire (in addition to the neutral & hots) from the main panel.
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