Temperature termination

[Jpflex]

This area still confuses me and doesn’t make much sense. NEC says that you must use the lowest ampacity when rating and terminating a conductor onto a component for what it’s temperature limit is rated.

However, the NEC says that a conductor’s ampacity is rated by it’s overcurrent protection (limits amperes on conductor).

Therefore, if you derating a conductor starting with its 90 degree rating but final ampacity barely exceeds the conductors ampacity under the 60 degree column of NEC table, then you would think that placing a breaker ahead of the conductor rated at the 60 degree ampacity would limit the conductor to 60 degree ampacity (excluding possible breaker going over its rating). From this the conductor would be appropriately rated and limited to required 60 degree column.

Apparently you can’t do this nor use a wire with ampacity going over 60 degree rating. More often then not you get final results with ampacity that is just over 60 degree column or under the target amperes.

Should you use a derated 90 degree conductor that is just under the target 60 degree ampacity and bump up breaker beyond ampacity of this conductor since it does not equate to a standard size breaker such as 17.5 going up to 20 amperes? How does this all work

 

[Dennis Alwon]

If you de-rate a 90C conductor and it's value is rated 38 amps then you are allowed to go to 40 amps as long as the load on the conductors is not greater than 38 amps.

Article 240.4(B) lets us go to the next size breaker but the load cannot be greater than the conductors calculated ampacity.

 

[Jpflex]

Indeed this is what has been bothersome to me is that derating yields a conductor too small for original target. It makes more sense to me to limit conductor ampacity with breaker

 

[Rick 0920]

Refer to Art. 110.14 (C), Art. 240.4 (B), and if using NM cable, Art. 334.80 110.14 (C) is pretty clear and concise about the temperature limitations of conductors.