Ampacity of NMC "Romex" cable

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OleSparky

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City of Parkland
I need clarification between 2011 NEC 334.80 and 334.112.
NEC 334.80 says to rate conductors at 60* C and 334.112 says 90* so long as the NM type cable is identified with a -B. Am I reading that right? This has come up several times when wiring 10K heat strips for instance with #6 NMC romex. At 60*C it's only rated at 55Amp. If the nameplate is one the AC AHU is 58A they would need to run pipe with #6 THHN or use a #4 romex? Or if it has the -B (NM-B) you can use the #6 romex because the 90* column gives you 75A rating instead of 55A in the 60* column.?
 

luckylerado

Senior Member
334.112 says that the insulation must be 90° which gives you a place to start for derating and temp correction.

334.80 says that the adjusted and corrected ampacity of the conductor may not exceed that of the 60° column.

240.4 would allow you to use a 60 amp OCP device for 55 amp rated wire because 55 amps not a standard breaker size in 240.6
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
240.4 would allow you to use a 60 amp OCP device for 55 amp rated wire because 55 amps not a standard breaker size in 240.6

With a MCA of 58 amps you cannot use a 60 amp OCPD and a 55 amp conductor.

I would use MC cable which can be sized at 75 degrees C.
 

Dennis Alwon

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Location
Chapel Hill, NC
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Retired Electrical Contractor
I agree with Trevor. If the unit has a mca greater than 55 amps then 6/3 nm is not compliant. You either go with a #4, use mc, or get the hvac guy to use a slightly smaller heating element. The latter is done around here quite often
 
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