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NM-B temperature/amperage rating

Posted by remodeler_matt (My Page) on
Wed, Dec 29, 10 at 21:51

Hi all:
I'm looking to hook up a new induction cooktop in place of an old resistance-style electric cooktop. To my surprise, the induction top actually needs more power than the one in-place now.

The instructions for the induction cooktop specify a 40 amp breaker, but note that a 10 gauge conductor with insulation rated at 90 degrees C would suffice. I take this to mean 10-3 NM-B cable would be fine. However, when I look at the specs for the NM-B cable I have, it states that its insulation is rated at 90 degrees C, but then notes that the ampacity should be limited to the rating at 60 degrees C, which I believe is only 30 amps.

So, the question I have is, can I replace the in-place 10-3 NM cable (from 1974 or so) with 10-3 NM-b and use a 40 amp breaker, or do I need to step up to 8-3? The run to the panel is about 25 feet.

Thanks, in advance.

Follow-Up Postings:

RE: NM-B temperature/amperage rating

you would have to use 8-3.
you are correct in that 10-3 NM is limited to 30 amps

RE: NM-B temperature/amperage rating

Thanks for the reply.

Just to be clear ('cause I'm not sure I was): The NEC specifies that a 10-3 copper conductor is capable of handling 40 amps, but only if the insulation is rated at 90 degrees C (see the table at for instance).

I know the insulation in NM-B cable is rated at 90 degrees C, but the specifications of the cable I have says that the ampacity of the cable must be limited to the rating at 60 degrees C, which is indeed 30 amps (thanks for confirming; I was knee deep in a torn out kitchen at the time).

So, I guess a follow up question would be, is there any 10-3 NM-B cable that I could connect to a 40 amps breaker, one that doesn't have the ampacity limit at the 60 degree rating? Or, conversely, why does the NEC state that a 10 ga. wire can handle 40 amps when the insulation is rated at 90 degrees C, but the wire manufacturer says to limit ampacity to the 60 degree limit?

Here is a link that might be useful: Wire Chart

RE: NM-B temperature/amperage rating

You won't find any connections that are rated at 90ºC so you can't use that column.

RE: NM-B temperature/amperage rating

The 90 column in table is only useful if you need to take derating into account. As Joed points out, for most purposes you're stuck with the 60.

RE: NM-B temperature/amperage rating

Thanks, guys. I pulled out the old 10-3 NM and put in 8-3 NM-B. I always wondered about that column, as in why it was there if no one made a connector that could handle that heat.

Learn something new every day.

RE: NM-B temperature/amperage rating

A secondary (or perhaps tertiary) issue to consider is whether you really want to waste electricity heating the wires in the walls rather than the food that you're cooking. When you're on the high end of the wire's ampacity, upsizing conductors a size can be worthwhile, kind of like adding more insulation to your attic.

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