NM-B cable restriction

[davengr]

NM-B cable is comprised of two internal conductors with insulation rated for 90ºC. Why then is NM-B ampacity restricted only for use not to exceed 60ºC? Is it because the hot and neutral are encapsulated inside the outer sheath? Thank you.

 

[roger]

Please update your location, an email address is not a location.

 

[infinity]

IMO no the outer sheath has nothing to do with it because as you've stated the conductors are rated for 90° C. It's more by tradition than anything technical in nature.

Welcome to the Forum.

 

[Jaybone812]

IMO no the outer sheath has nothing to do with it because as you've stated the conductors are rated for 90° C. It's more by tradition than anything technical in nature.

Welcome to the Forum. being rated at 90 c ampacities has nothing to do with the

Correct me if I’m wrong , but original or older nm cable before nm-b was actually a rated as a 60c cable assembly, and when nm-b cable was introduced with conductors rated at 90c ampacities restriction was kept in place to prevent safety concerns due to overheating when run in contact with thermal insulation,
Mass amends 334.80 by only restricting nm to 60c ampacity when run in contact with thermal insulation. If its run in open space for applications not subject to the restrictions of 240.4(d) it can be sized to the rated temp of the equipment in accordance withh 110.14(c)(1)(a)(3)