Motors 60C/75C

[boku0003]

If the motor current is less than 100A; so say you look up the FLC in 430.250 and it's 85A. Also, it is NOT marked with design B, C or D. Do you use the 60C or 75C column? According to 110.14(C)(1)(a)(4) "For motors marked with design letters B, C or D, conductors having an insulation rating of 75C or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75C ampacity."

So typically, if the motor is rated less than 100 A, I use the 75C column. But I just wanted to make sure that it does not necassarily need to be a design B, C or D.

Also, what column do you use if you apply continuous load (125%) and the current goes above 100A? So say you have a 95A continuous load. Would you look up the 60 or 75C column (since after adjustment, it's 95x1.25=119A). Same if it is more than 3 conductors or elevated temp...

Thanks

 

[sparks2000]

if its not marked you have to use 60 degresss

 

[boku0003]

That's what I thought. What is the difference between Design, B, C, D motors and others that use the 60C column? Are they rated at higher temps, and therefore have higher temp terminals?

How about a motor that is not marked but is over 100A? I assume that would use the 75C column?

 

[erickench]

I believe you would always have to assume 75' C. That rule is outlined in Mike Holts book "NEC Exam Preparation".

 

[boku0003]

According to 110.14(C)(1)(4) "For motors marked with design letters B, C, or D, conductors having an insulation rating of 75C or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75 ampacity".

So, if I have a test question that asks me to calculate the motor conductors for a motor that is rated say 85A, do I use the 60 or 75C column? It does not say that it is Design B or C. It just says that it is a motor. I am really wanting to know because this happened to me on an exam.

 

[erickench]

Since this is a test question I would assume 75' C. That's what Mike Holt's recommendation is. Most terminals in the real world are rated at 75' C.