I have been studying the code and watching instructional videos on sizing motor conductors. I know that for continuous duty motors you have to use values from Tables 430.247-430.250 and multiply by 125 percent for conductor sizing (unless the note applies at the bottom of table 430.250). There are some that state under 100 amps use the 60 degree column and 100 amps or over use the 75 degree column. Another video said for motors always use 75 degree column. I took a practice test and even though the question specifically stated 60 degrees the correct answer on the test applied the 75 degree rating. If there is no derating or adjustment factors, can somebody explain to me which column to use and when it is applicable. I am studying for my license and I know the word "Minimum" is used quite often. Thank you in advance!!
Correct temperature rating for sizing motor conductors
- Thread starter ENH
- Start date
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
So the correct way to size this is... under 100 amps use the 60 degree column and 100 amps or over use the 75 degree column.
IF motor and breaker are rated for 75 degrees and it draws below 100a the 60 degree column would be used for conductor selection?
IF motor and breaker are rated for 75 degrees and it draws below 100a the 60 degree column would be used for conductor selection?
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
If the motor and breaker are rated for 75 degrees, then the 75 degree column may be used even when under 100A. Someone with more hands-on experience can tell us if motors are typically rated for 75 degrees conductors (I'm guessing yes, but can't confirm). Modern breakers are typically rated 60C/75C so you can use them at 75C.
Cheers, Wayne
- Location
- Illinois
- Occupation
- retired electrician
110.14(C)(1)(a)(4) addresses this for design B, C, and D motors.
In the real world, not the testing world, you can almost always use the 75 degree C ratings.