100 Amp Sub-Feed

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jazer

Senior Member
Location
Gibsonia, Pa
It's been a little while since I sized a sub-feeder. I have a single phase commercial application that requires metal clad installation. I see that a common conductor size for residential sub-feed cable is #2 aluminum, but according to 310.15 (b)(16), that is only good for 75 amps on the 60 degree column. It looks like I would need a 1/0 to handle a 100 amp sub-feed? I guess I have two questions, how is #2 aluminum ok in a resi sub-feed application and is 1/0 truly what I need?

Thanks
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I think MC cable can be used at 75C which means a 1 awg

There has always been a deduction in service cables and main feeder sizes for residential because the loads are not generally continuous. All I can say is that it is an exception for a residence and commercial jobs need to use the main tables.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I see that a common conductor size for residential sub-feed cable is #2 aluminum, . . .
. . . how is #2 aluminum ok in a resi sub-feed application . . .
Not in a "sub-feed" but only in a main service feeder.
 

tom baker

First Chief Moderator
Staff member
Dwelling units can use a conductor that is larger than non-dwelling, at the same ampacity/ OCPD, as dwelling units have greater load diversity IE not all loads on at the same time. There used to be a table in 310, the table is gone, you use 83%, but the result is the same.
The table still exists in the examples at the back of the code.
 
It's been a little while since I sized a sub-feeder. I have a single phase commercial application that requires metal clad installation. I see that a common conductor size for residential sub-feed cable is #2 aluminum, but according to 310.15 (b)(16), that is only good for 75 amps on the 60 degree column. It looks like I would need a 1/0 to handle a 100 amp sub-feed? I guess I have two questions, how is #2 aluminum ok in a resi sub-feed application and is 1/0 truly what I need?

Thanks

Regarding the second question, you usually get to use the 75 degree column for wiring methods other than NM, UF, and maybe SE depending on the code cycle. Don't forget about the next size up rule too.
 
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