NM cable. WHY 60 degree.

[kbsparky]

My understanding of the 60 degree rating was due to the construction spec of the conductors lack of heat dissapation thru the sheathing and in the 1970's they went to the 90 degree insulation on the conductors to reduce to adverse affects of the excessive heat on the 60 degree conductor insulation when out of the jacket in outlet boxes for lighting applications.

It was circa 1984 that the NM-B was implemented. Way past the 1970's ....

 

[Dennis Alwon]

My understanding is that nm cable has 90C wire strictly for the purpose of connecting to light fixtures and for bundling derating.I can only imagine that if nm cable was loaded to it's 90C rating their would be some real heat problems. Look at SE cable and the 2011 statement. If the wire runs thru insulation then the wire must be rated 60C. Although nm can be installed in areas without insulation it is less common then when it is in insulation. It would be difficult to write the code for all those situations esp. given 240.4(B).

I believe this as well as termination on recep. would warrant a carte blanc 60C rating.

BTW, 50 amp recep. are rated 60/75C.

 

[kwired]

I believe this as well as termination on recep. would warrant a carte blanc 60C rating...

JMO but 240.4(D) makes that happen anyway with 14 - 10 AWG conductors no matter what insulation they have.

 

[tsamples]

Didn't the old NM have 60C conductors?

The NEC changed the requirements for NM in 1984 to require 90degree wiring. This became NM-B, and all NM cable since is NM-B with 90 degree wiring, although the NEC has dropped the -B for simplicity.

 

[kwired]

The NEC changed the requirements for NM in 1984 to require 90degree wiring. This became NM-B, and all NM cable since is NM-B with 90 degree wiring, although the NEC has dropped the -B for simplicity.

Actually 334.112 says that NM cable shall have 90 degree conductor insulation. The FPN that follows recognizes that cables marked with a "-B" meet that requirement.

This makes it a violation to install old cable that is not identified as 90 degree type.

 

[badashuka]

i really dont have anything to add just trying to learn here, but I was taught in school that amps under 100 you use the 60 degree column and over a 100 you use the 75 degree column to cover the 80-125 percent rule. The 90 degree column is only used for adjustment and derating.

 

[kwired]

i really dont have anything to add just trying to learn here, but I was taught in school that amps under 100 you use the 60 degree column and over a 100 you use the 75 degree column to cover the 80-125 percent rule. The 90 degree column is only used for adjustment and derating.

That is the general rule if the termination temperature rating is not otherwise marked. {110.14 (C)(1)}
Most newer equipment is all marked 75?C anymore.

Don't know what you are getting at with the 80-125% rule. Might be a shortcut for adding 125% that works most of the time, but I wouldn't count on it.

 

[badashuka]

nice to know, the 80-125% rule I was taught is the 80% usage of the circuit and the 125% is for continuous load, is this not a good rule of thought. Just want to make sure I get better and not worse.