NM wire and 60 degree ampacity limit

[dhowie]

I've seen a number of comments similar to the one that follows relative to the usage of non-metallic sheathed wire (a.k.a. Romex).

The ampacity of Types NM & NMC cables shall be that of 60ºC(140ºF) conductors.

Do I take the "NM" in the comment above literally, meaning NM-B is excluded from the 60ºC ampacity limit?

Thanks

 

[Little Bill]

I'm allowing this since there is no "how to" question, only information regarding code and requirements.

 

[suemarkp]

I would say no, but they should make this more clear. NM, with or without a dash is type NM cable so all NM rules apply. I don't believe you can buy NM anymore, it is all NM-B. The older NM cable was 60C ampacity and 60C (or maybe 75C) for derating. Since they are allowing derating from the 90C column, NM-B is implied.

I think the code book used to mention that NM-B could be derated from the 90C column, but they took that out eventually when NM-B became the normal cable for many years. They should probably clarify for older installations that NM without the -B suffix is limited to the 60C or 75C derating column.

 

[Little Bill]

The conductors have a 90 deg rating but the cable has to be used under the 60 deg column. NM-B is not exempt, it is the norm. It is true you can derate using the 90 deg column but the results must be what the 60 deg column says after the derating. Example: After derating, using the 90 deg column, you come up with a #12 conductor, but the same ampacity in the 60 deg column says #10. You have to use the #10 due to having to use the 60 deg column.

 

[dhowie]

Thank you for the replies...this is where the confusion comes in for me relative to using #6 NM-B on a 60 amp circuit. If the 60 degree column is to be used, then #6 is limited to 55 amps (50 amp circuit).

My electric inspector is telling me I'm good to use #6 NM-B on a 60 amp circuit. I'm a little reluctant to go back to him a second time on this topic for fear of sounding like I'm insulting his intelligence.

The other thing that seems odd about having to use the 60 degree column is that you then must go down to #4 to support a 60 amp circuit. Yet #4 NM-B seems hard to come by. Neither the box stores (Home Depot / Lowes) or my local electric supply carry #4. The electric supply store told me everyone uses #6, so either no one in my area is putting in 60 amp circuits, or they're not meeting code, or the 60 degree column does not apply to modern day NM (NM-B).

If you're wondering why I want to use NM-B on a 60 amp circuit, it is because I have a 50 ft indoor run from the service panel to a junction box where the NM-B will be spliced with #6 THHN that runs outside through conduit.

Thanks

 

[electrofelon]

Thank you for the replies...this is where the confusion comes in for me relative to using #6 NM-B on a 60 amp circuit. If the 60 degree column is to be used, then #6 is limited to 55 amps (50 amp circuit).

My electric inspector is telling me I'm good to use #6 NM-B on a 60 amp circuit. I'm a little reluctant to go back to him a second time on this topic for fear of sounding like I'm insulting his intelligence.

The other thing that seems odd about having to use the 60 degree column is that you then must go down to #4 to support a 60 amp circuit. Yet #4 NM-B seems hard to come by. Neither the box stores (Home Depot / Lowes) or my local electric supply carry #4. The electric supply store told me everyone uses #6, so either no one in my area is putting in 60 amp circuits, or they're not meeting code, or the 60 degree column does not apply to modern day NM (NM-B).

If you're wondering why I want to use NM-B on a 60 amp circuit, it is because I have a 50 ft indoor run from the service panel to a junction box where the NM-B will be spliced with #6 THHN that runs outside through conduit.

Thanks

See 240.4(B)

 

[winnie]

#6 NM-B has an ampacity of 55A, and cannot be used on a circuit which requires a higher ampacity.

The 'trip rating' of the breaker supplying the circuit may be higher than the required circuit ampacity, within limits as specified by code. 240.4(B) is probably the most common situation where the breaker trip rating is greater than the circuit ampacity.

-Jon

 

[infinity]

Thank you for the replies...this is where the confusion comes in for me relative to using #6 NM-B on a 60 amp circuit. If the 60 degree column is to be used, then #6 is limited to 55 amps (50 amp circuit).

My electric inspector is telling me I'm good to use #6 NM-B on a 60 amp circuit. I'm a little reluctant to go back to him a second time on this topic for fear of sounding like I'm insulting his intelligence.

The inspector is correct if your load is 55 amps or less. This is permitted by the "next size up" rule that EF posted.

 

[kwired]

Old NM cable without the -B suffix is truly 60C designed AFAIK.

Once they created NM-B you still had to base your absolute minimum ampacity from 60 C ampacity table, but could and still can make any adjustments starting with 90C ampacity values.

We do similar thing all the time with THHN/THWN-2, the terminations are usually only rated for 75C so we must use 75C ampacity table to determine absolute minimum conductor size allowed, but we then can start with 90C values on portions of the circuit where we must make ampacity adjustments. If adjustments result in a smaller conductor than 75C table allows you must still use the 75C table conductor size.

 

[dhowie]

Thank you everyone for the responses.