Table 310.12 Single Phase Dwelling Service & Feeder vs Mike Holt's Electrical Toolbox

[Jimland58]

I used Mike Holt's Electrical Toolbox to size a Residential Circuit Feeder for Single Phase 120/240 on a 125 amp Circuit Breaker and it tells me to use 1 AWG Copper or 2/0 AWG Aluminum. I then looked at the 2020 NEC, Table 310.12 and it says for 125 amp service or feeder rating I can use a 2 AWG Copper or 1/0 AWG Aluminum. Why the difference?

 

[Jimland58]

I think I might have found the answer to my own question. Table 310.12 only applies to Feeders that supply the entire load of a dwelling. So if I am sizing a "feeder" going from a Main Panel to a Sub-Panel then Table 310.12 doesn't apply since the Sub-Panel Feeder doesn't supply the entire load of the house. Therefore, Mike Holt's Electrical Toolbox sizing is correct for feeders that don't supply the entire load of a house.

 

[LarryFine]

 

[texie]

Keep in mind 310.12(C). Let's say you have a 125 amp dwelling service using 1/0 AL as allowed by 310.12. You then run a 125 amp feeder from the service that is not serving the entire load. You can still use 1/0 AL as no feeder has to be larger than the service conductors.