Sizing NM (inc NM-B) history

[steven765]

Does anyone have the history or backstory if any? On why NM-B is still beholden to the 60degC column even when landing both ends on 75degC equipment. I mean we haven't used NM in forever. NM-B came out in what 1985?!

Is it just an oversight that 334.80 doesn't provide the same exception that 110.14(C)(1)(b) allows for other wire types. Are the panels just not talking or is there a specific reason they don't want NM-B using the 75degC rating on rated equipment? Is this worth a public comment before the window closes in 2 weeks?




ref 110.14(C)(1)(b) conductors with higher than 75°C ratings provided the conductor ampacity does not exceed the 75°C ampacity of the conductor size used. This condition also permits the conductors to be used at ampacities higher than 75°C if the equipment is listed and identified for the higher rating.

 

[Dennis Alwon]

I have said this before but my take on the 60C rule is that in a home there are too many areas where the cables get close and run thru attics etc. I am guessing that no one has ever made a point of ambient temp being an issue because the nm is rated at 60C with 90C wire in the cable.

 

[augie47]

In the smaller sizes it is somewhat a moot point as the limitations in 240.4(D) and the fact that most devices 30 amps or less are 60° rated.

 

[Dennis Alwon]

Does anyone have the history or backstory if any? On why NM-B is still beholden to the 60degC column even when landing both ends on 75degC equipment. I mean we haven't used NM in forever. NM-B came out in what 1985?!

Is it just an oversight that 334.80 doesn't provide the same exception that 110.14(C)(1)(b) allows for other wire types. Are the panels just not talking or is there a specific reason they don't want NM-B using the 75degC rating on rated equipment? Is this worth a public comment before the window closes in 2 weeks?




ref 110.14(C)(1)(b) conductors with higher than 75°C ratings provided the conductor ampacity does not exceed the 75°C ampacity of the conductor size used. This condition also permits the conductors to be used at ampacities higher than 75°C if the equipment is listed and identified for the higher rating.

This is not an oversight as it has been around forever while SE cable has moved up and down a few times from 60C to 75C back to 60C and now up to 75C. NM has always remained at 60C.

 

[kwired]

This is not an oversight as it has been around forever while SE cable has moved up and down a few times from 60C to 75C back to 60C and now up to 75C. NM has always remained at 60C.

SE cable not normally (maybe never found) in sizes effected by the small conductor rule in 240.4(D) either.

 

[Dennis Alwon]

SE cable not normally (maybe never found) in sizes effected by the small conductor rule in 240.4(D) either.

Yeah that is why I don't ever bring it up. Years ago SER with #10 was around but I doubt anyone stocks it or can get it today. I guess it could be made to order but who is going to do that.

 

[kwired]

Yeah that is why I don't ever bring it up. Years ago SER with #10 was around but I doubt anyone stocks it or can get it today. I guess it could be made to order but who is going to do that.

Don't know I have ever seen #10, I have seen #8 many times. Particularly a single insulated conductor with a bare concentric neutral used (under overall clothlike covering) for service entry on old outbuildings on farms that only had 120 volt supplied to them.

 

[steven765]

I should have clarified that i was thinking about 6ga and 8ga nm-b wire. Especially as we start to spec out more car chargers for home use. Being able to use an 8 vs 6 would save over the total cost of a job.

 

[Dennis Alwon]

I should have clarified that i was thinking about 6ga and 8ga nm-b wire. Especially as we start to spec out more car chargers for home use. Being able to use an 8 vs 6 would save over the total cost of a job.

They do make se cable in those sizes so that would be at 75C