# What derating should be used for NM-B in insulated ceiling?

I am building a workshop in central North Carolina. It is 1 1/2 story with the second floor unconditioned space. The ceiling joists are 2x10 with R30 fiberglass insulation. What derating, if any, do I need to use for NM-B cable runs in the ceiling?

The runs will either run lengthwise with the joists or crosswise to the joists through holes drilled about 3 inches from the bottom of the joist (ie about 1/3 of the way into the 10" of insulation).

The building is in a shaded area, but being in NC, worst case temps in the second story might reach 115 degrees (46C). Can I use a gradient between the conditioned space and the unconditioned space (46-25=21C, 21/3=7C, 25C+7=32C) and use the 31-35C derating factor?

Tom

 tkrussell Posts: 9,673, Reputation: 3698 Senior Electrical & Lighting Expert #2 Jun 16, 2011, 07:31 AM

NM cable is rated 90 degrees centigrade, in your case derating is not needed.

If you did derate, or want to, how would would you do it?
 TomWS Posts: 26, Reputation: 1 New Member #3 Jun 16, 2011, 05:32 PM
I guess I was confused by the Southern Wire web site. They said that while NM-B has a 90C rating on the outer jacket, the internal insulation is only rated to 60C.

I guess I'll have to find that link, but, can I infer from your answer that I shouldn't have an issue if I use 100% rating in this application?

Tom
 donf Posts: 4,786, Reputation: 2723 Printers & Electronics Expert #4 Jun 16, 2011, 06:58 PM
Tom,

You are mixing terms.

Article 334.80 covers the ampacity of NM cables.

For example, the 90(C) column allows for a 14 AWG NM cable to carry 25 amps.

However, because of Article 334.80, you must follow the ampacity of the 60 (C) column. This means that the max allowable current on a 14 AWG 90 (C) conductor is limited to 15 Amp.

What you are allowed to do is if circumstances require the derating or adjustment of the conductors, you can start at the 25 amps allowed by the 90 (C) column and reduce the amperage from there.
 TomWS Posts: 26, Reputation: 1 New Member #5 Jun 16, 2011, 07:13 PM
Great! Thanks TK & Don! I'm sure that's why you guys get paid the BIG bucks! :-)

Tom

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