wire sizing

[Matt Mckenzie]

In the last supervisor class that I took the instructor said that any branch circuit less than 100 amps must use the 60 degree column for wire sizing? This does not sound right as we all know that a #10 thhn is good for 35 amps in conduit & 30 amps in nmb romex. #8 good for 50 amps in conduit & 40 amps in nmb romex. Does anybody understand maybe what she was referring to? I have never heard of anything changing at the 100 amp mark.

 

[petersonra]

In the last supervisor class that I took the instructor said that any branch circuit less than 100 amps must use the 60 degree column for wire sizing? This does not sound right as we all know that a #10 thhn is good for 35 amps in conduit & 30 amps in nmb romex. #8 good for 50 amps in conduit & 40 amps in nmb romex. Does anybody understand maybe what she was referring to? I have never heard of anything changing at the 100 amp mark.

#10 copper of any insulation type is not allowed to have more than a 30 A OCPD on it. 240.4 (D).

 

[Dennis Alwon]

#10 copper of any insulation type is not allowed to have more than a 30 A OCPD on it. 240.4 (D).

Not quite true but yeah in general

To the OP

The instructor is not totally correct. If all the equipment and termination are label and have a 75C rating then you can use the 75C for 100 amps and less. If the labeling is not there then you must assume 60C for those ampacities. Pretty safe to say you will not see any new stuff rated 60C as they are all rated at least 75C.

 

[jumper]

To add to Dennis' post, look at 110.14(C)

 

[infinity]

1) In the last supervisor class that I took the instructor said that any branch circuit less than 100 amps must use the 60 degree column for wire sizing?

2)This does not sound right as we all know that a #10 thhn is good for 35 amps in conduit & 30 amps in nmb romex. #8 good for 50 amps in conduit & 40 amps in nmb romex.

3)Does anybody understand maybe what she was referring to? I have never heard of anything changing at the 100 amp mark.

1-They may be referring to the 60° C rating when 75° components are not used.

2-#10 THHN at 35 amps could only be used for specific loads as outlined on 240.4(G). As noted the rating for NM cable must be at the 60° C ampacity.