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NM-B Wire

Posted By: Curtis Wilson

NM-B Wire - 08/02/02 02:36 AM

Question?
Article 336.26 states that type "NM" cable must be amperage rated using the 60c column on table 310.16
Article 336-30.(b) states that NM cable having the "-B" extension have their conductors insulation rated at 90c.
If I have a 200 amp main panel feeding a subpanel and both terminal ratings (on the breaker and lugs on the subpanel)are rated @ 75c, can I use 6-3 NM-B romex as the subfeeder conductors using a 70 amp breaker in the main panel?
Thanx..
Curtis
Posted By: caselec

Re: NM-B Wire - 08/02/02 03:13 AM

You may use the 90C column for applying correction factors but the maximum ampacity of NM-B cable after any correction factors are applied can not exceed what is listed in the 60C column. 6-3 is good for 55 amps max.

Curt
Posted By: 3 Way

Re: NM-B Wire - 08/02/02 01:58 PM

You might consider 110.14(C)(1)(a)...(2),(3) covering equipment terminations generally.

(a) Termination provisions for circuits rated <100 amps...
(2)Conductors w/ higher temp ratings are allowed, provided the ampacity of such conductors is based on 60C
(3)Conductors of higher temp are allowed if the equipment is listed for use with such conductors

Provision (3) suggests 75C may be possible since the equipment lugs are rated 75C, however...

I would defer to NEC 2002 334.80 which specifically restricts NM to 60C. Section 334.112 requires NM cables be constructed w/ 90C insulation.

With all that, I see it like "caselec". 90C sets the starting ampacity for derating purposes, but in no case shall the conductor be operated above it's 60C limit.

3way
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