NM-B and table 320.15(16)B as well as temperature derating

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Mustwin351

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Why is it that NM cable is not listed in ampacity tables? Also what is the "B" in the NM-B stand for?


I've also never understood the two temperature derating tables 310.15(B)(2)(a) and 310.15(B)(2)(b)....why is there a need for two tables? They both cover temperature ranges other than 30 and 40 degrees celsius...why not just one table?
 
My guess without looking into the UL white book (B) means bare copper. I could be wrong.

The conductor in the NM cable is rated at 90 C and you can use that column for derating purposes. However, the final OCPD can not be higher than 60C. Some exceptions do apply.
 
Mustwin351 said:
Why is it that NM cable is not listed in ampacity tables? Also what is the "B" in the NM-B stand for?


I've also never understood the two temperature derating tables 310.15(B)(2)(a) and 310.15(B)(2)(b)....why is there a need for two tables? They both cover temperature ranges other than 30 and 40 degrees celsius...why not just one table?
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The reason for the 2 two tables is located deeper in 310.15-

The ampacity tables in 310.15(B)(18,19,&20) are based on 40c, whereas 310.15(B)(16&17) would be based on 30c... you would then refer back to 310.15(B)(2)(a) or 310.15(B)(2)(B) for your corrections: Ex- 310.15(B)(18) would correspond to 310.15(B)(2)(B).

As for the NM-B, AFAIK the "B" simply denotes that the wire is rated for 90c- old nm prior to 1984 didn't have that designation, and was only rated 60c.
 
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user 100 said:
The reason for the 2 two tables is located deeper in 310.15-

The ampacity tables in 310.15(B)(18,19,&20) are based on 40c, whereas 310.15(B)(16&17) would be based on 30c... you would then refer back to 310.15(B)(2)(a) or 310.15(B)(2)(B) for your corrections: Ex- 310.15(B)(18) would correspond to 310.15(B)(2)(B).

As for the NM-B, AFAIK the "B" simply denotes that the wire is rated for 90c- old nm prior to 1984 didn't have that designation, and was only rated 60c.
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There we go. thank you! Didn't realize 310.15(B)(2)(B) was for higher temperature conductors. So unless i'm using what i would guess are very expensive conductors not sold at big box stores I have no need to worry about 310.15(B)(2)(B).
 
user 100 said:
As for the NM-B, AFAIK the "B" simply denotes that the wire is rated for 90c- old nm prior to 1984 didn't have that designation, and was only rated 60c.
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Little Bill said:
The "B" means new version of NM when they changed the rating to 90°
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Thanks gents. :thumbsup::thumbsup:


My guess without looking into the UL white book (B) means bare copper. I could be wrong.
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:thumbsdown::thumbsdown:
 
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