why won't nm-b satisfy NEC?

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onlyinamerica

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it's new construction and I added a 10kW electric heat package onto the HVAC install. The total current pull during operation is 41.5amps. That's the motor and the coils. One unit is in the basement and the other is in the attic. I will only be using this heating, obviously, in the winter where the temp in the attic is not 75 or 90 degrees C. The NEC code will not allow me to run 8awg, NM-B to these units but rather it needs to be of high temp insulation, such as THHN. I would need to run #6 that is rated for 55amps at 60C rather than the #8 at 40amps. Is this an application where I can since the temps won't get high? Besides, if the insulation is rated for 90C and the copper wires are still copper, why do they derate the NM-B for 60C? I don't see why I would need to go to #6.

Thanks.
onlyinamerica
 
Re: why won't nm-b satisfy NEC?

110.14(C) 05' NEC temperature limitations at terminations use lowest rating which normally is 60c the higher temp ratings 90c on conductors is used for ampacity derating correction factors. "Other replies will provide you with a great deal of late night reading".
 
Re: why won't nm-b satisfy NEC?

If you're adding up the nameplate values and getting 41 amps, you may need a branch circuit rated for more. Electric resistance heating branch circuits need to be sized at 125%. Your fan motor also requires a 125% circuit. Your heater should have an ampacity value listed on its nameplate specific for the heaters that are installed. This value is probably around 51A after all the 125%'s are taken into account.

So if its just over 50A, even #8 THHN wouldn't be enough because the furnace terminals are probably not rated for more than 75C. You may be able to squeak by in a cold attic heater install, but why push it.

I don't know why NM-B is limited to 60C. You may end up having to run #6 anyway.
 
Re: why won't nm-b satisfy NEC?

OK, so if I have a 50A range receptacle with 75 C terminals, I can use 8/3 NM-B on a 50A breaker to feed it?
 
Re: why won't nm-b satisfy NEC?

Originally posted by m73214:
OK, so if I have a 50A range receptacle with 75 C terminals, I can use 8/3 NM-B on a 50A breaker to feed it?
Click to expand...
Nope. But you could use #8 MC cable with the 75 degree device. NM cable is limited to 60 degrees. Look at 334.80.
 
Re: why won't nm-b satisfy NEC?

If NM-B has conductors of THHN, rated at 90 degrees, why do we go to the 60 degree chart?
 
Re: why won't nm-b satisfy NEC?

I understand your point but sometimes I just try to figure out the logic behind some of the articles that seem to be questionable. If I stripped the outer jacket off the NM-B, put the #8's in a raceway, I could connect the 75 degree device to a 50A circuit.

[ January 18, 2006, 04:22 PM: Message edited by: m73214 ]
 
Re: why won't nm-b satisfy NEC?

Originally posted by triphase:
I thought I gave the code section?110.14(C)
Click to expand...
110.14 has nothing to do with this.


334.80 is the section that requires NM to used at 60 C regardless of the terminal ratings.
 
Re: why won't nm-b satisfy NEC?

Originally posted by m73214:
If NM-B has conductors of THHN, rated at 90 degrees, why do we go to the 60 degree chart?
Click to expand...
I am not sure, but I have heard that it has to do with installing NM cable in thermal insulation will cause the conductors to overheat if they are not derated to a 60 C ampacity.

I am pretty sure I heard this from Don G (aka resqcapt).

[ January 18, 2006, 04:50 PM: Message edited by: peter d ]
 
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