Table 310.15

[Howard Burger]

Anybody know why the NEC in Table 310.15 allows smaller wire for a feeder that would be used in a non-residential situation?

We just ran #2 Cu to a home workshop building through conduit that had been put in the ground when the house was built. We pulled #2 Cu because the owner wanted 100amps (and the parts stored didn't stock #3 Cu). Only afterwards did I recall that T310.15 permits using #4 Cu for this in a residential installataion. The smaller wire would have made an easier pull (there were also standby generator wires from the shop to the transfer switch in the pipe) and would have been cheaper. But why should residential applications get smaller wire for the same amperage? Thanks for any help with this.

 

[Pierre C Belarge]

Actually, Table 310.15(B)(6) cannot be used for the workshop. Take a closer look at the wording in that Section.

 

[480sparky]

Anybody know why the NEC in Table 310.15 allows smaller wire for a feeder that would be used in a non-residential situation?..........

Part of the title for that table is "Dwellings". Therefore, it cannot be used for non-residential situations.

 

[Sierrasparky]

It also must supply the entire load of the dwelling./

 

[Howard Burger]

thanks

thanks

Thank you. I missed the 'dwelling', thinking that the shop, which is adjacent to but not part of the dwelling (I think 'curtilage' is the legal term) would be included. My bad.

 

[jeffhornsby]

Studies show dwellings load are diverse and rarley get close to max operating value and if they do it is not for very long especially not for 3 hours so they let fudge a little bit

 

[charlie b]

And that makes sense, as many people are not at home during a large part of the day, and even homes with a stay-at-home parent are not likely to see all of their electrical equipment in use at the same time.

 

[Howard Burger]

Thanks, Jeff and Charlie, after I got my original question answered it occurred to me to wonder why the code allowed resi service/feeders to carry more amps than non-res. I was thinking probably lack of overall volume, lack of heavy continuous loads and also there wasn't danger of non-linear loads on the neutral that you could get with 3ph. Appreciate your input.

 

[charlie b]

. . . and also there wasn't danger of non-linear loads . . . .

That had not occurred to me. Another reason a dwelling unit's actual loads are never as high as we are required to calculate them to be. That's one "Tip-O-the-Hat" for you.