NM-B and sec 334

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spectra05

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Piedmont, NC
in section 334 of the nec code it lists type nm, nmc, and nms cable. nowhere in the section it talks about NM-b untill you get to 334.112 where it states that "the insulated power conductors shall be one of the types listed in Table 310.13 that are suitible for branch circuit wiring or one that is identified for use in these cables. Conductor rating shall be rated at 90 degrees C (194F)".
then says right under it FPN: types NM, NMC, and NMS cable identified by the markings NM-B, NMC-B, NMS-B. now for the purpose of de-rating type nm-b would i need to follow in accordance with 334.112 Conductor rating shall be rated at 90 degrees C (194F).

Or do i refer to 334.80 where it states that "the ampacity of types NM, NMC, NMS cable shall be determined in accordiance with 310.15. THe ampacity shall be in accordance with th 60 degree C conductor rating. The 90 degree C rating shall be used for derating purposes, provided the final de-rating ampacity does not exceed that for a 60degrees C conductor."

So if hypotheotically i had 1 14-2 type NM-B cable for a lighting circuit that passed through an attic above a range hood with an ambient temp of 42 degrees C. i would obviously de-rate 25A @ 90 degrees C. with a correction factor of .87 to get 21.75A. now would this be in compliance with 334.12. Or not in compliance with 334.80 in which i would need to at or less than 20A.

any help would be awesome thanx
 

spectra05

Member
Location
Piedmont, NC
tom baker
Derating of NM-B cable is done from the 90 Deg ampacity rating but the derated ampacity can exceed the limits in 240.4(D).

yes, but for the sole purpose of detaring the ampacity of the cable, do i follow 334.80 where the final derating ampacity cannot exceed that of the 60 degree C. rating
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
spectra05 said:
So if hypotheotically i had 1 14-2 type NM-B cable for a lighting circuit that passed through an attic above a range hood with an ambient temp of 42 degrees C. i would obviously de-rate 25A @ 90 degrees C. with a correction factor of .87 to get 21.75A. now would this be in compliance with 334.12. Or not in compliance with 334.80 in which i would need to at or less than 20A.
The 20A is coming from the 60?C column. You don't have to think about that number. You may be overlooking, in Table 310.16, the asterisk ( * ) by the 14 in the left hand column, under "Size AWG or kcmil". The asterisk ( * ) refers to the note (not a Fine Print Note) at the bottom of Table 310.16 that says "See 240.4(D)"

If the Exceptions of 240.4(E) or 240.4(G) don't apply to your installation, then the Table 310.16 rating for 14 AWG 60?C is 15A.

You use the smaller value of 15A or the calculation.
 

spectra05

Member
Location
Piedmont, NC
yes the overcurrent protection shall not exceed 15A for 14AWG, but to be clear, i am trying to find the allowable ampacity for this NM-b cable not the size of the overcurrent device. the reason i am asking is that i am confused to which rule to use either it be 334.118 or 334.80. 334.80 dosent actually list type NM-B it says types NM, NMC, and NMS
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Take a look at the Article 100 Defintion. I find the note in the Handbook particularly instructive:
Ampacity. The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

The definition of the term ampacity states that the maximum current a conductor carries continuously varies with the conditions of use as well as with the temperature rating of the conductor insulation. For example, ambient temperature is a condition of use. A conductor with insulation rated at 60?C and installed near a furnace where the ambient temperature is continuously maintained at 60?C has no current-carrying capacity. Any current flowing through the conductor will raise its temperature above the 60?C insulation rating. Therefore, the ampacity of this conductor, regardless of its size, is zero. See the ampacity correction factors for temperature at the bottom of Table 310.16 through Table 310.20, or see Annex B. The temperature limitations on conductors is further explained and examples given in 310.10 and in the commentary following that section.
Another condition of use is the number of conductors in a raceway or cable. {See 310.15(B)(2)}.
My emphasis is in red.

Your 90?C derating calculation yields a number higher than 15A. Therefore, the derating is not made smaller than 15A. 240.4(D) tells you the ampacity can't be higher than 15A.

As for 334.118, you must mean 334.112, right? Are you looking at a pre-1985 length of NM cable in that attic by the hood? Or is the nonmetallic sheathed cable a more modern length of NMB?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
spectra05 said:
yes the overcurrent protection shall not exceed 15A for 14AWG, but to be clear, i am trying to find the allowable ampacity for this NM-b cable not the size of the overcurrent device. the reason i am asking is that i am confused to which rule to use either it be 334.118 or 334.80. 334.80 dosent actually list type NM-B it says types NM, NMC, and NMS

Nm-B although not specifically mentioned is bound by 334.80-- it would be considered NM cable.

I don't know what art. 334.118 is since it does not exist in 2005
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
al hildenbrand said:
Are you looking at a pre-1985 length of NM cable in that attic by the hood?
Belay that question. I just refreshed my view of your opening post "hypothetical NM-B". :rolleyes:
 

spectra05

Member
Location
Piedmont, NC
As for 334.118, you must mean 334.112, right?
yes, my typo error 334.112 not 334.118.

Are you looking at a pre-1985 length of NM cable in that attic by the hood? Or is the nonmetallic sheathed cable a more modern length of NMB?
This is not necessarily a real world senario, its a question from class. i was just checkin the de-rating part was correct because there was a discrepancy about the exact wording of 334.80 with using the 60 degree chart after de-rating off the 90 degree chart on table 310.16 and as it is exactly worded as NM cable not NM-B cable for the total ampacity of the cable.
 

jwelectric

Senior Member
Location
North Carolina
I am not sure but I think that the letter ?B? is confusing him.

334.2 Definitions.
Nonmetallic-Sheathed Cable. A factory assembly of two or more insulated conductors enclosed within an overall nonmetallic jacket.
Type NM. Insulated conductors enclosed within an overall nonmetallic jacket.
Type NMC. Insulated conductors enclosed within an overall, corrosion resistant, nonmetallic jacket.
Type NMS. Insulated power or control conductors with signaling, data, and communications conductors within an overall nonmetallic jacket.


334.80 Ampacity.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15.

So far there has been no mention of the ?B?

334.112 Insulation.
The insulated power conductors shall be one of the types listed in Table 310.13 that are suitable for branch circuit wiring or one that is identified for use in these cables. Conductor insulation shall be rated at 90?C (194?F).

FPN: Types NM, NMC, and NMS cable identified by the markings NM-B, NMC-B, and NMS-B meet this requirement.

At last we find the ?B?. What this ?B? means is this cable now adheres to the requirement found in 334.112, the conductors inside the cable are rated at 90 degree C.
This rule plays no part in the method used in the derating of the conductor.
As outlined in 334.80 we must always choose our conductors from the 60 degree column.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
spectra05 said:
its a question from class.
The bark on the tree may look real interesting. . .but let's look at the forest for a moment.

Have you ever seen a length of NM-B that had the insulation on the individual conductors labeled with one of the types of cables listed in Table 310.13 (referenced from 334.112)?

I haven't, but that's not saying much.

The FPN at the end of 334.112 is telling us that the unlabeled insulated conductors inside the sheath of an NM-B cable can be included with the labeled 90?C conductors in Table 310.13 as satisfying the requirement of 334.112 that the insulated conductors inside NM have a 90?C rating.

The implication is, if the cable is rated something other than NM-B, then the FPN doesn't come into play, and the unlabeled insulated conductors are NOT 90?C (which is true).

The requirement that changed the temperature rating of the insulated conductor was introduced in the 1984 NEC with this passage:
1984 NEC
Article 336 - Nonmetallic-Sheathed Cable
Types NM and NMC

336-2. Construction. . . . .Conductors shall be rated at 90?C (194?F). The ampacity of Types NM and NMC cable shall be that of 60?C (140?F) conductors in Table 310-16.
NM cable has been with us for a long time. I have some I took out of a house that was first installed in the 1930s. The piece of 14/3 without ground that I have is an inch in diameter, bituminous compound impregnated clothe and paper. The conductors are rubber insulated and tinned.

NM was first introduced by manufacturers in the early twenties or late teens.

The only 90? conductor insulation in NM is inside sheaths labled NM-B.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
al hildenbrand said:
The only 90? conductor insulation in NM is inside sheaths labled NM-B.
Okay, so do we get to take advantage of this in any way other than a starting point for derating, or does the 60-deg. final rating still overrule?

Simply put, is the type NM all-encompassing, including NM-B? Let's include wire larger than #10 to exclude the 15a,20a,30a limitations.
 
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