Quite possibly a dumb question

[ggunn]

Per 310.16, the 75 degree ampacity of #10 copper THWN is 35A, but per 240.4(D)(7) the highest rated OCPD you can use on #10 copper is 30A, so what good is that extra 5A of ampacity?

 

[wwhitney]

See 240.4(G) for applications where 240.4(D) does not apply.

Cheers, Wayne

 

[Dennis Alwon]

It is also good for de-rating

 

[ggunn]

See 240.4(G) for applications where 240.4(D) does not apply.

Cheers, Wayne

So, most of the time that 5A is useless. Thanks.

 

[Dennis Alwon]

#10 at 90C is 40 amps so you can see why this is helpful with de-rating

 

[ggunn]

It is also good for de-rating

As a starting point for derating, OK.