# De-Rating breaker vs. Upgrading &quot;Subpanel&quot; with 4/0 Aluminum

Registered Users Posts: 8
Here is a question where I can't get my math straight.

I installed a system with two (2) SMA SB7000US inverters. My home was serviced by a CH 400A service with 200A at the main and a 200A "sub-panel" on the opposite side of the home. This "sub" is protected by a 175A breaker in the main and is connected by 4/0 Aluminum. The system needed to be protected by an 80A breaker in the "sub"(we combined two 40A breakers in a combiner), so we replaced the "sub" with a 225A "sub-panel" and left the 175A in place because of the existing loads, creating enough "room" to backfeed 80A+. This was the easier method, believe it or not. This was approved by the local utility, however at the city final, the inspector is requiring us to FURTHER de-rate the 175A main to a 150A main because the Aluminum 4/0 can only handle 205A. I argued that the load would never be greater than the current load of the panel. Now, load calcs reveal I CAN further de-rate the breaker, but then I swapped the 200A for the 225A for nothing. Any insight would rock....thanks.

• Solar Expert Posts: 482 ✭✭✭
Re: De-Rating breaker vs. Upgrading "Subpanel" with 4/0 Aluminum

I think the inspector is correct.

Have a look here: http://www.mpandc.com/case_studies/Point-of-connection/point_of_connection.html

Specifically, the short in feeder diagram.

http://www.mpandc.com/case_studies/Point-of-connection/Short_in_feeder.jpg
• Solar Expert Posts: 291 ✭✭✭✭✭✭
Re: De-Rating breaker vs. Upgrading "Subpanel" with 4/0 Aluminum

I don't see how there can be more than 175 amps in any point on the conductor.

The Miller Solar short in feed diagram shows 140 amps in whatever the conductor is shorting against, not the conductor.

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Similarly, the NEC specifically allows you to do a line tap. If there were a short between the tap and the transformer, the same thing could happen.
Re: De-Rating breaker vs. Upgrading "Subpanel" with 4/0 Aluminum

lot of code being used , get rid of the /AL . and why use ALU. anyway
were in the 21st century ? more current with 4/0 thhn 90c CU?
code correct. (spend the money)
• Solar Expert Posts: 9,583 ✭✭✭✭✭
Re: De-Rating breaker vs. Upgrading "Subpanel" with 4/0 Aluminum
denziel56 wrote: »
lot of code being used , get rid of the /AL . and why use ALU. anyway
were in the 21st century ? more current with 4/0 thhn 90c CU?
code correct. (spend the money)

I've got 900' to run, x3, will you reimburse me the difference?
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• Solar Expert Posts: 593 ✭✭
Re: De-Rating breaker vs. Upgrading "Subpanel" with 4/0 Aluminum

When İ was working in İndia my electrical guys would have fainted if someone suggested using copper where aluminum was possible. The cost difference was very great.

Nothing wrong with Al cable at all as long as the person designing & installing knows their business.