NM-B and allowable ampacity
- Thread starter splinetto
- Start date
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stickboy1375
Senior Member
- Location
- Litchfield, CT
splinetto said:
The 1984 NEC required the temperature rating of Type NM Cable to be increased from 60?C to 90?C, to take into account the increased use of thermal insulation in dwellings. The 90?C cable is identified as Type NM-B. NM cable manufactured prior to the 1984 NEC is rated at 60?C and is identified as Type NM.
PDF file on the subject... http://www.nema.org/stds/eng-bulletins/upload/Bull92.pdf
- Location
- Logan, Utah
Yes the suffix B identifys that the conductors within the cable are rated at 90 degrees C
Take a look at 334.80 this section requires that the ampacity of NM cable be taken from the 60 degree column.
Since the terminals that you connect the #10 THHN to are more than likely not rated at 90 degrees you would not be able to use the 90 degree rating of the #10 THHN for the conductor ampacity.
Chris
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
splinetto said:
Sure you could but look at the asterik in T310.16. See 240.4(D). Other than for a few exception you would have to fuse at 30 amp.
You would use 90C for derating.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
No-- and I doubt you would find 90 C terminalssplinetto said:
- Location
- Massachusetts
Dennis Alwon said:
On equipment I agree 100% :smile:
For splicing there are a number of options rated at least 90C.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
iwire said:
Yeah but somewhere you will be connected to a 75C terminal.
- Location
- Massachusetts
Dennis Alwon said:
I think what he was suggesting was using smaller conductors in the middle of a run. Lets say 6 AWG at breaker, change to 8 AWG for the run and then back to 6 AWG at the equipment.
That would be legal, the biggest problem is once the run gets long enough to make doing this worth while the run is also long enough that voltage drop will become a problem.
- Location
- Massachusetts
splinetto said:
Yes, that works assuming the equipment at the far end is rated 75C.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
iwire said:
I gotcha
The op says #10- so I don't believe you can do that since 240.4(D0 plays a hand here. In your scenario I agree.
- Location
- Massachusetts
Dennis Alwon said:
I agree 240.(D) does through a wrench in it.
jwelectric
Senior Member
- Location
- North Carolina
Well lets say that using the 90 degree column to derate and the math came to 12 amps. Could a 15 amp overcurrent device then be used on a #14 conductor using 240.4(B)?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
jwelectric said:
A very good question. I would say it depends on the loads being served. If it were serving receptacle loads I would say no however if it were serving a fixed piece of equip. as a dw. I would say yes.
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