125 subpanel feeder requirement

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sanzibar21@gmail.com

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looking for feed back, I currently did an electrical inspection of up grade 125 amp subpanel. in a 1br apartment. A 125 amp subpanel require a 1/0 feeder, the client is installing a undersized feeder to supply only the total amp he will be using, according to his calculation he will be using around 75 amp only. Is installation allowed or does he has install the one 1/0 feeder from the service to the subpanel to comply with the maximum 125 amp subpanel is rated for.

Joe P
Mountain View
 
125 amp bus in a panel is pretty common as the "low end" that is produced. If you want to supply with only say a 60 amp feeder circuit, chances are you will still only be able to find a panel with 100 or 125 amp bus. Nothing wrong with protecting it lower then it's rating but protecting 125 amp panel with 200 amp OCPD is a problem.
 
You have us assuming the wire type for amperage of your 1/0; also assuming wire size for the calculated 75 amps.

In short, calculated load must be in accordance with the NEC. We aren’t assured here that the client did such a thing.

That said, see NEC 310.15(B)(7). It has specific requirements, but generally says between 100-400A the feeder may be sized for not less than 83% of the service rating.

If all is code compliant otherwise, sizing a 125A service feeder at 60% is not.
 
sanzibar21@gmail.com said:
looking for feed back, I currently did an electrical inspection of up grade 125 amp subpanel. in a 1br apartment. A 125 amp subpanel require a 1/0 feeder, the client is installing a undersized feeder to supply only the total amp he will be using, according to his calculation he will be using around 75 amp only. Is installation allowed or does he has install the one 1/0 feeder from the service to the subpanel to comply with the maximum 125 amp subpanel is rated for.

Joe P
Mountain View
Click to expand...

If the calculated load is 75 amps using a 75 amp feeder is not undersized. The fact that the bus rating of the panel is 125 amps is irrelevant.
 
publicgood said:
I took this as the breaker is sized 125A.
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Which breaker, the one supplying the feeder or the one in the supplied panel. I can, if I want to be able to isolate individual circuits, feed a 200A 42 space panel containing (for example) 42 15A breakers with a 200A __SWITCH__ (er, breaker) from a 50A breaker ... conceptually we do this commonly in industrial panels to disconnect some loads for troubleshooting. Industrially, it also allows using 16AWG wire with 6A breakers ...
 
GeorgeB said:
Which breaker, the one supplying the feeder or the one in the supplied panel. I can, if I want to be able to isolate individual circuits, feed a 200A 42 space panel containing (for example) 42 15A breakers with a 200A __SWITCH__ (er, breaker) from a 50A breaker ... conceptually we do this commonly in industrial panels to disconnect some loads for troubleshooting. Industrially, it also allows using 16AWG wire with 6A breakers ...
Click to expand...
You could serve a 2000 amp switchboard with a 15 amp circuit if there is less then 15 amps of load connected to it. Probably going to be expensive compared to what you could have done or might just be something temporary.
 
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