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[romex jockey101]

so when finding ampacity of insulated conductors in a dwelling unit for branch circuits what colum would you use for nm.. nm insulation is thhn with is rated at 90* i guess im confused now can you please explain 310.16 to me thank you for your time.i thought each insulation is rated at a certain degree depending on whats the temp in the install

 

[Dennis Alwon]

so when finding ampacity of insulated conductors in a dwelling unit for branch circuits what colum would you use for nm.. nm insulation is thhn with is rated at 90* i guess im confused now can you please explain 310.16 to me thank you for your time.i thought each insulation is rated at a certain degree depending on whats the temp in the install

When using NM you must use 60C for your ampacity. You can use 90C when derating is involved but the final ampacity still cannot be higher than the 60C.

Think of 8 CCC- 4 nm cables run thru a hole that is being fire caulked. That make us have to derate based on 334.80. Going to 310.15(B)(2) I see that 8 CCC must be de-rated at 70%. The #12 wire in NM is 90C so I use 30 amps and multiply by 70% and I get 21 amps. This means I can still use a 20 amp breaker.

Now if we use 90C and we only have to de-rate at 80% we would have 30*.8= 24 amps. We could not use the next size up because the 60C column restricts us to 20 amps for #12 NM