Using a 200 amp main with 100 amp disconnect upstream?

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Old 02-10-09, 07:59 PM
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Using a 200 amp main with 100 amp disconnect upstream?

Hello,

I have a question that I think I know the answer to already, but I want to ask people here first.

I'm working in a house that currently has a 100 amp service coming from the street into the meter on the side of the house. Just inside the house (about 2 feet from the meter) is a load center with only a 100 amp breaker set up as a disconnect. From there, a #2 copper service cable runs over to the main panel about 15 feet away. The panel that is currently in service has a 100 amp main, with 20 slots. It's a Square D HOM type panel that is jammed full, and has some modified tandems in there, obviously not good.

The plan is to replace this panel with one that has more slots. Square D makes a 100 amp QO panel with 32 slots that would do the job. The problem is, in the next few years the plan is to add a woodworkers shop to the back of the house. We'd hate to replace the 100 amp panel with another 100 amp panel because of the potential of doubling the amperage in the not too distant future.

Square D makes a 40 slot load center that would be ideal, other than the fact that it comes stock with a 200 amp breaker. To purchase a 100 amp breaker for this panel, it would be between 100-150 bucks. Ideally, we'd like to not have to spend this.

The question here is, since the panel is fed with a 100 amp breaker serving as a disconnect right inside the house, can we use the new panel with the 200 amp breaker in it and not buy a 100 amp main for it? At that point the 200 amp breaker I guess would just be a second disconnect at the panel.

I guess another option would be to get a retaining kit for a 100 amp breaker and backfeed the panel that way. Is that legal as well?

Am I thinking incorrectly here? Thanks in advance for any assistance.
 
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Old 02-10-09, 08:05 PM
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as long as the conductors are rated at least for the 100 amp disco, the 200 amp main becomes just a disco, not overload protection. That is fine.

don't forget, the 100 amp disco is your service disco and all ground/egc/ gec etc. is bonded there, not at the panel. You would need 4 conductors to the panel.
 
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Old 02-10-09, 08:37 PM
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Thanks. Thats what I thought.

From what I can tell, there are three wires coming into the house, two hots and a neutral. Those run into the disconnect box. A fourth wire that goes into that disconnect comes from the water pipe where it enters the house, and shares the same lug as the neutral in the disconnect.

From the disconnect to the "main" panel is a 3 wire #2 conductor. In addition to that, the same ground wire that goes from the water pipe to the disconnect continues to the 20 slot panel. Inside that panel, all of the neutrals and bare wires on the panel go to the same place on the neutral bar.

With the new 40 slot panel, a separate ground bar was purchased. Are you saying it's best practice to keep the neutral and ground bonded at the disconnect, and then rather than leave them bonded at the main panel, separate them into a neutral bar and separate ground bar? I assume that it would also be good practice to replace the ground from the water pipe to a couple 8ft ground rods 6 ft apart connected to the initial disconnect.


Thanks again for the advice.
 
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Old 02-11-09, 04:47 AM
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Yes, separate the neutral and grounds at your new panel.

Leave the cold water bond connected and then add 2 ground rods and hook to your disconnect also.
 
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Old 02-11-09, 07:57 PM
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Am I correct in that the wire from the disconnect to the panel should be a AL 4/0-4/0-4/0-2?, or half that for copper?

Its probably best for us to replace the copper service cable that is already there so the walls won't have to be ripped apart again when the service is upgraded (they are currently unfinished).

Thanks again.
 
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Old 02-11-09, 09:38 PM
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Originally Posted by eaglek96 View Post
Am I correct in that the wire from the disconnect to the panel should be a AL 4/0-4/0-4/0-2?, or half that for copper?
Residential 200A service is done with the aluminum size you mentioned or #2/0-2/0-2/0-4 copper. The usual cable type is SER for indoor or above ground outdoor use. You could also install individual conductors in 2" conduit.
 
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