PI: 310.12(A)

[wwhitney]

This PI just changes the title of Table 310.12(A) to exclude conductors limited to 60C ampacity.

Cheers, Wayne

Table 310.12(A) Single-Phase Dwelling Services and Feeders For Conductors Not Limited to 60C Ampacity

Statement of Problem and Substantiation for Public Input

As currently written, Table 310.12 would allow #4 NM cable to be utilized for a 100A feeder "supplying the entire load associated with a one-family dwelling," since there is no restriction on the wiring method mentioned. However, the ampacity of #4 NM cable is limited to 70A per 334.80, so this would provide a ratio of ampacity to rating of 70%, much less than the 83% specified in section 310.12.

Thus it is my understanding that the use of Table 310.12 is intended to be limited to wiring methods that can make use of the 75C ampacity columns in Tables 310.16 et al. Therefore that restriction needs to be placed on Table 310.12. Since multiple paragraphs refer to Table 310.12, putting the restriction in the table heading seems simplest.

 

[letgomywago]

I don't think I've used 4awg NM unless it's already existing. I think you'd be adding something unnecessary especially since from a safety perspective it's minimal in its protection since there's 90° insulation on the wire so unless the whole thing was in spray foam or blow in insulation the slight potential over usage won't get it hot enough.

That said I do think it would be looked at seriously since it's a nit picky thing that won't cause much consequence but justify the need for an update to the NEC and they seam to every year add one or two of these odd silly things in. #jobsecurity