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Adding new panel & making old panel a sub-panel. Size of wire?

Adding new panel & making old panel a sub-panel. Size of wire?


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Old 07-25-13, 11:39 AM
J
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Adding new panel & making old panel a sub-panel. Size of wire?

Here's the scenario:
My current service is overhead and feeds a 100 amp panel. I want to install a new 250 amp panel on the opposite side of the house, and (not that it matters) the service will be underground. I would then feed the existing panel by pulling a wire under the house. My subsequent room additions would be fed directly off of the new panel. The distance between the new and existing panels is about 55 feet. So my question is what size/type of copper wire is required? Alternatively, is this information defined in some code book. I live in Los Angeles county, if it matters.
Thanks, Jim
 
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Old 07-25-13, 12:28 PM
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Edit: Nevermind. I misunderstood what you were doing!

I think you need to coordinate with the local utility company to change to point of service (and the amperage delivered) to your dwelling? That is what I was told by a local electrician (if I need to increase my main service panel). I did not end up needing to do this (in my case).

Your not really changing the amperage "delivered" to your dwelling as the load is determined by your use but rather ensuring that your current service feed will support 250A.

I have no idea if this varies from state to state and county to county.

From what I understand, you will need at least #2 and more than likely #1 underground rated cable.

Also, from what I have read, you will definitely want to verify what is acceptable by your local inspector since this can vary slightly from one area to another. At those sizes and lengths, the difference will not be "chump change".
 

Last edited by gmanvbva; 07-25-13 at 12:57 PM.
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Old 07-25-13, 02:24 PM
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I would then feed the existing panel by pulling a wire under the house... I live in Los Angeles county, if it matters.
In LA County, I'm pretty sure the local code will require you to install individual conductors in conduit. The wiring in your addition will also need to be in metal, I think.

The distance between the new and existing panels is about 55 feet. So my question is what size/type of copper wire is required?
The size of the conductors is determined by the load being served. In your case, you would need to pull #1 copper for a 100A load, but only #4 copper for a 70A load. You are unlikely to need 100A, since that's the full capacity it has now while it's serving the entire house. Do a load calculation, using the loads that will be on your existing panel after you make the change, and we can go from there.

Alternatively, is this information defined in some code book.
Yes and no. The need to calculate from the load is specified, but not the method of calculation.
 
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Old 07-25-13, 06:35 PM
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My current service is overhead and feeds a 100 amp panel. I want to install a new 250 amp panel on the opposite side of the house, and (not that it matters) the service will be underground.
If you haven't talked to your power company yet, do it right away. All utility companies are not the same and some won't even consider what you want to do. Some will, but may require you foot the entire bill for the underground conduit and service wiring. Next, 250 amps is a non-standard service panel. You have 100 amps now. From there, you could probably have a 125 amp, 150 amp, 200 amp, 225 amp, 300 amp or 400 amp. Depending on your power company's rules of service. You probably don't need any more than 200 amps, but if the load warrants more than 200, I think I'd suggest a 300 amp service (2 - 150 amp panels with a 320 amp meter socket. Power company rules are vital on this part.

I would then feed the existing panel by pulling a wire under the house.
You want to refeed the existing 100 amp service panel as a subpanel, under the house. Refeeding the existing panel is a good way to go, but you don't provide enough information. Is your house on a slab, on a crawl space or do you have a basement? I have never recommended going under any existing building and won't here either, there are simply too many things you could hit such as water lines, sewer lines and natural gas lines. If the subpanel feeder cannot be fed through a crawl space, a basement or though an attic, I would go underground around the house.

So my question is what size/type of copper wire is required?
For a 100 amp feeder to the existing panel, I'd use 1 1/4" conduit with 3 - #3 THHN/THWH (Blk, Red & Wht) and 1 - #8 THHN/THWN (Grn) ground.

I seriously doubt you need more than a 200 amp service unless you are more than doubling the size of the house.
 
 

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