Please explain how to size feeder to sub panel in commercial installation and more

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451F

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Work in Maryland, Virginia, West Virginia, DC.
I was looking at a site last week and saw a 3 phase 200A MCB Load center with no breakers in it. It was fed from another building underground by color wrapped (size and metal not visible but MCB lug filling) conductors tapped off a 400 A disconnect. Two of the stabs were bolted to a pair of lugs to 3/0 CU which fed a sub panel load center without MCB (but full of breakers) in the same building.

I'm trying to determine if:

1. Is this existing installation going to pass inspection?
2. Can I add some breakers (25A load distributed over several) to the 200A MCB panel since it appears to be used as a disconnect now?

I do this work so seldom I have forgotten more than I knew.
 
451F said:
Apparently, I'm not alone.

Thanks everyone for looking (I happened to be looking at the post when you did).

How about this question:

Can three 3/0 THHN in EMT be connected to a 200A CB used for feeding a sub panel?
Click to expand...


3/0 copper at 75C (t. 310.15...) is rated 200 amps so as long as the wire is rated 75C or better than you can use 200 amp overcurrent protective device assuming no other de-ratings.

In this situation- feeding a sub panel- it is really not any different than commercial. The table gives the ampacity but there is exception for residential where the feeder or service carries the entire load of the building. If you use T.310.15 you can't go wrong
 
451F said:
I was looking at a site last week and saw a 3 phase 200A MCB Load center with no breakers in it. It was fed from another building underground by color wrapped (size and metal not visible but MCB lug filling) conductors tapped off a 400 A disconnect. Two of the stabs were bolted to a pair of lugs to 3/0 CU which fed a sub panel load center without MCB (but full of breakers) in the same building.

I'm trying to determine if:

1. Is this existing installation going to pass inspection?
Click to expand...

Nothing you have said suggests to me anything obvious that does not meet code.

451F said:
2. Can I add some breakers (25A load distributed over several) to the 200A MCB panel since it appears to be used as a disconnect now?
Click to expand...
Only if your load calculation says you can.
 
Dennis Alwon said:
3/0 copper at 75C (t. 310.15...) is rated 200 amps so as long as the wire is rated 75C or better than you can use 200 amp overcurrent protective device assuming no other de-ratings.

In this situation- feeding a sub panel- it is really not any different than commercial. The table gives the ampacity but there is exception for residential where the feeder or service carries the entire load of the building. If you use T.310.15 you can't go wrong
Click to expand...

Thanks Dennis: I thought that it was the panel's lug temperature rating which limited the OCD? I think the panel is a SQD sub (no MCB) 38 or 40 stab panel.
 
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