Feeder wire size

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olly

Senior Member
Location
Berthoud, Colorado
Occupation
Master Electrician
They did away with table 310.15(B)(7) in 2016 NEC, that's the one that gave you an allowance for single phase residential services and feeders. I understand that you are still allowed use the same allowance through the new language in 2016 code is that correct? So does this old table/ new language allow to use say #4 CU for a 100a, to a sub panel say out to a barn?
 
There is no NEC 2016. I presume you meant 2014. Neither the old table nor the new text allowed the information to be applied to a sub panel. It could only be used to the service wires (or feeder wires) that carried 100% of the load for the entire dwelling unit.
 
A barn is not a dwelling so IMO you would not be able o use the Table. BTW, the 2014 NEC, as Charlie stated, uses an 83% rule. For instance, if a dwelling service is 200 amps then you could take 83% of 200 amp and get 166 amps--- a conductor sized 166 amps after any derating would be compliant in this case. Basically it is very much the same as he table but it clarified alot of issues.
 
Your barn is not a dwelling and was never able to apply any reduced size conductors as permitted by 310.15(B)(7)

The changes to the 83% in 2014 still yields same size conductors (with no additional adjustments) as was in the tables before.

Problem before was it wasn't clear if adjustments were required when using that table, or exactly how to apply them.
 
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