Ampacity

Ampacity

Ampacity of NM cable in thermal insulation.

The following commentary is concerning 2008 NEC 334.80 and is based on my understanding of Article  334.80. If there are questions please contact the authority having jurisdiction in your area.

Article  334.80 in the 2008 NEC requires ampacity reduction of NM cables when more than two (2) NM cables containing two (2) or more current conductors are installed without maintaining spacing and in contact with thermal insulation. Ampacity for NM cable shall be from the 60 degree column (table 310.16) as far as overcurrent protection and allowable load is concerned.  However 334.80 allows the 90 column (table 310.16) to be used for ampacity reduction.  By allowing the 90 degree column to be used for ampacity reduction the effect of this change are not as severe as one might at first conclude.  

Example: You are allowed to de-rate from 90 degrees which would allow nine #12 current carrying conductors without maintaining spacing with over current protection at 20 amps before de-rating. (30 amp from table 310.16, 90 degree column: 30amps X 70% = 21 amps) This would be a total of four (4) 12/2 NM.

Number 12 NM and 14 NM are the sizes most affected by this change. However other sizes NM are also affected. 

Example: Number 8 NM has an ampacity of 40 amps from the 60 degree column. However the 90 degree column allows 55 amps. So if you feed three loads, each with their own 8-2 NM branch circuit protected by a 40 amp breaker, they would not need separation. (55 amp from table 310.16, 90 degree column:  55 X 80% = 44 amps).

As shown, this new requirement should be relatively easy to comply with if the electrician is aware of the change in 2008 NEC 334.80 before rough-in.  In Teton county we consider NM to be separated if installed as per manufactures recommendation on multi cable staples (stack its) or stapled 1/4” apart.   

In areas where maintain spacing is difficult because of the number of cables in one stud space (panel and dimming enclosures locations for example) some contractors are furring out the wall to the depth of the enclosure so the conductors are not in contact with thermal insulation.