hde_electric
Member
What does NM(Romex) fall under when identifying it (exmample:THHN,THWN)in the codebook? Do you have to derate when you bundle them up in a dwelling?
NM Derating
- Thread starter hde_electric
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pierre
Senior Member
- Location
- Westchester County, New York
Re: NM Derating
hde
if the NM cable is bundled for two feet or more, then there is a requirement for 'ampacity adjustment'.
334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60 degree celcius conductor temperature rating. The 90 degree celcius rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60 degree celcius rated conductor. (quote from the 220 NEC)
So you can use the 90 degree column for derating, but you have to use the 60 degree column for installation purposes, regardless of the terminal rating.
Pierre
hde
if the NM cable is bundled for two feet or more, then there is a requirement for 'ampacity adjustment'.
334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60 degree celcius conductor temperature rating. The 90 degree celcius rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60 degree celcius rated conductor. (quote from the 220 NEC)
So you can use the 90 degree column for derating, but you have to use the 60 degree column for installation purposes, regardless of the terminal rating.
Pierre
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: NM Derating
The NM conductors are rated 90 deg c, but they are not identified as THHN or similar. the cable jacket limits the ampacity to 60 Deg C
The NM conductors are rated 90 deg c, but they are not identified as THHN or similar. the cable jacket limits the ampacity to 60 Deg C
G
Guest
Guest
Re: NM Derating
NM=THN=75-degrees
NM-B=THHN=90-degrees.
The conductors are not marked but with a micrometer you can tell if it's THN or THHN. Additionally, the NM or NM-B marking will tell you.
Don't you only have to derate NM-B to 75 degrees, and NM to 60 degrees?
Not to split hairs here but I want you to help me by clarifying my understanding of NM.
NM=THN=75-degrees
NM-B=THHN=90-degrees.
The conductors are not marked but with a micrometer you can tell if it's THN or THHN. Additionally, the NM or NM-B marking will tell you.
Don't you only have to derate NM-B to 75 degrees, and NM to 60 degrees?
- Location
- Massachusetts
Re: NM Derating
2002 NEC
[ September 20, 2003, 11:42 PM: Message edited by: iwire ]
2002 NEC
I do not realy find NM-B listed in the book
[ September 20, 2003, 11:42 PM: Message edited by: iwire ]
pierre
Senior Member
- Location
- Westchester County, New York
Re: NM Derating
Wayne
Where did you find THN? I have never myself seen or heard of it. NM-B is about all you will see in the supply houses for sale today.
The NEC requires manufacturers to have 90 degree insulation, and the installer to use the 60 degree column. Not too hard to follow, especially since we are limited by 240.4(D) for allowable ampacity of 14, 12, and 10 AWG conductors anyway.
Pierre
Wayne
Where did you find THN? I have never myself seen or heard of it. NM-B is about all you will see in the supply houses for sale today.
The NEC requires manufacturers to have 90 degree insulation, and the installer to use the 60 degree column. Not too hard to follow, especially since we are limited by 240.4(D) for allowable ampacity of 14, 12, and 10 AWG conductors anyway.
Pierre
- Location
- Illinois
- Occupation
- retired electrician
Re: NM Derating
The code requires that NM have 90? conductors. I believe that this first appeared in the 1897 code. NM-B is the marking that indicates that the conductors are 90?, but unless you are working on an older installation there is no need to look for this marking as all listed NM after 1987 is NM-B.
Don
The code requires that NM have 90? conductors. I believe that this first appeared in the 1897 code. NM-B is the marking that indicates that the conductors are 90?, but unless you are working on an older installation there is no need to look for this marking as all listed NM after 1987 is NM-B.
Don
G
Guest
Guest
Re: NM Derating
This is my understanding:
TW=60-degrees.
NM=THW=75-degrees.
MN-B=THHN=90-degrees.
I only use NM-B and would not know where to get NM. Actually, I just thought of a place!
Another belief of mine: As the insulation gets better, it gets thinner, and therefore the rating goes up. Smaller can mean better! Size does matter! I will take my answer on the air.
THANKS!
[ September 22, 2003, 02:12 AM: Message edited by: awwt ]
Typo. Meant THW. Good catch. Thanks!
This is my understanding:
TW=60-degrees.
NM=THW=75-degrees.
MN-B=THHN=90-degrees.
I only use NM-B and would not know where to get NM. Actually, I just thought of a place!
Another belief of mine: As the insulation gets better, it gets thinner, and therefore the rating goes up. Smaller can mean better! Size does matter! I will take my answer on the air.
THANKS!
[ September 22, 2003, 02:12 AM: Message edited by: awwt ]
charlie
Senior Member
- Location
- Indianapolis
Re: NM Derating
I think type NM-B started long before 1987. The -B was to indicate the internal wires were insulated for 90?C and could be used in attic spaces, NM was permitted to be used everywhere else. Since NM was still used in attics and were not being caught, the Code was changed to require all cable to be NM-B. After the NM was completely phased out, the explanation of the -B was removed from the product standard but the cable is still required to carry the marking.
From the UL White Book
This category covers Types NM-B and NMC-B nonmetallic-sheathed cable, rated 600 V, intended for use in accordance with Article 334 of NFPA 70, ??National Electrical Code?? (NEC), and Listed in copper sizes 14 to 2 AWG inclusive and aluminum or copper-clad aluminum sizes 12 to 2 AWG inclusive.
This cable contains conductors rated 90?C; however, the ampacities of the cable are those of 60?C conductors as specified in Article 334 and Table 310.16 of the NEC.
I think type NM-B started long before 1987. The -B was to indicate the internal wires were insulated for 90?C and could be used in attic spaces, NM was permitted to be used everywhere else. Since NM was still used in attics and were not being caught, the Code was changed to require all cable to be NM-B. After the NM was completely phased out, the explanation of the -B was removed from the product standard but the cable is still required to carry the marking.
From the UL White Book
This category covers Types NM-B and NMC-B nonmetallic-sheathed cable, rated 600 V, intended for use in accordance with Article 334 of NFPA 70, ??National Electrical Code?? (NEC), and Listed in copper sizes 14 to 2 AWG inclusive and aluminum or copper-clad aluminum sizes 12 to 2 AWG inclusive.
This cable contains conductors rated 90?C; however, the ampacities of the cable are those of 60?C conductors as specified in Article 334 and Table 310.16 of the NEC.
bjp_ne_elec
Senior Member
- Location
- Southern NH
Re: NM Derating
Pierre - I know this is an old post, but I'm interested in where you got the reference that NM has to be derated if it's bundled for 2' or more? I would like to run a few NM cables from panel in to a trough - then branch out of the trough. A conduit would be used - so they would be considered bundled - between the panel and the trough.
Thanks
Brett
Pierre - I know this is an old post, but I'm interested in where you got the reference that NM has to be derated if it's bundled for 2' or more? I would like to run a few NM cables from panel in to a trough - then branch out of the trough. A conduit would be used - so they would be considered bundled - between the panel and the trough.
Thanks
Brett
bphgravity
Senior Member
- Location
- Florida
Re: NM Derating
See 310.15(B)(2)(a).
See 310.15(B)(2)(a).
bjp_ne_elec
Senior Member
- Location
- Southern NH
Re: NM Derating
Bryan - thanks for that. I knew I had heard the 24" rule - but glanced right over it in 310.15
Now - let me ask another question - and this is mainly to try to firm up my understanding of derating. Say I have eight (8) 12/2 NM cables run in a three foot (3') long nipple. Now I've gone over the 24", and I also have eight NM cables. Now, how does Table 315.10 (B)(2)(a) impact my derating.
Again, one thing I'm really trying to get clear, is how the neutrals impact the derating when it comes to the Table 310.16 (B)(2)(a). Now let's specify this is a situation in a residential dwelling - so single phase 120/240.
Thanks
Brett
[ January 22, 2006, 11:07 AM: Message edited by: bjp_ne_elec ]
Bryan - thanks for that. I knew I had heard the 24" rule - but glanced right over it in 310.15
Now - let me ask another question - and this is mainly to try to firm up my understanding of derating. Say I have eight (8) 12/2 NM cables run in a three foot (3') long nipple. Now I've gone over the 24", and I also have eight NM cables. Now, how does Table 315.10 (B)(2)(a) impact my derating.
Again, one thing I'm really trying to get clear, is how the neutrals impact the derating when it comes to the Table 310.16 (B)(2)(a). Now let's specify this is a situation in a residential dwelling - so single phase 120/240.
Thanks
Brett
[ January 22, 2006, 11:07 AM: Message edited by: bjp_ne_elec ]
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Re: NM Derating
Brett, welcome to the forum!
Since the requirements of 210.11 & 210.52 call for 15 and 20 amp circuits and receptacles for certain areas, if those circuits are a part of this bundled NM, you would have to upsize the conductors to maintain that rating on the handle of the circuit breaker.
(As in, you couldn't simply put a 10 amp breaker on dining room wall space receptacles, because 20 amp circuits are called for.)
You would derate by 70%.
Also, by 334.80, you can use the 90?C rating for derating purposes. By 310.16, #12 CU is good for 30 amps @ 90?C.
30 amps x 70% = 21 amps.
You can still use a 20 amp breaker for these circuits.
Brett, welcome to the forum!
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- <font size="2" face="Verdana, Helvetica, sans-serif">They are bundled over three feet.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Being two-wire, the neutral is carrying as much current as the ungrounded conductor</font>
Since the requirements of 210.11 & 210.52 call for 15 and 20 amp circuits and receptacles for certain areas, if those circuits are a part of this bundled NM, you would have to upsize the conductors to maintain that rating on the handle of the circuit breaker.
(As in, you couldn't simply put a 10 amp breaker on dining room wall space receptacles, because 20 amp circuits are called for.)
So if instead of 8 12-2 cables, you had 4 12-3 cables in the same conduit, you'd have 8 CCC's. The neutrals are only conducting unbalanced current (310.15(B)(4)(a)).
You would derate by 70%.
Also, by 334.80, you can use the 90?C rating for derating purposes. By 310.16, #12 CU is good for 30 amps @ 90?C.
30 amps x 70% = 21 amps.
You can still use a 20 amp breaker for these circuits.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: NM Derating
Pierre:
"Not too hard to follow, especially since we are limited by 240.4(D) for allowable ampacity of 14, 12, and 10 AWG conductors anyway."
Correct for most applications, there are some exceptions, such as HVAC and motors
Pierre:
"Not too hard to follow, especially since we are limited by 240.4(D) for allowable ampacity of 14, 12, and 10 AWG conductors anyway."
Correct for most applications, there are some exceptions, such as HVAC and motors
pierre
Senior Member
- Location
- Westchester County, New York
Re: NM Derating
You are correct Tom, sometimes I forget about 240.4(G), it is good to be reminded from time to time.
You are correct Tom, sometimes I forget about 240.4(G), it is good to be reminded from time to time.
Re: NM Derating
Gerns
[ January 23, 2006, 12:03 AM: Message edited by: gerns ]
Not sure but since you are in resi wouldn't you use Annex B Table B.310.11 for your percent of derate, since you have divers loads. So in my understanding I have 8 12/2 so 16ccc's, I could use 90degree and the above table says 70 percent.Is this correct or not? They are rated 30amp 90degree 10-24ccc's 70 percent =21amps.Originally posted by georgestolz:
So, you have 16 CCC's. You would have to derate them by 50%.
Since the requirements of 210.11 & 210.52 call for 15 and 20 amp circuits and receptacles for certain areas, if those circuits are a part of this bundled NM, you would have to upsize the conductors to maintain that rating on the handle of the circuit breaker.
(As in, you couldn't simply put a 10 amp breaker on dining room wall space receptacles, because 20 amp circuits are called for.)
So if instead of 8 12-2 cables, you had 4 12-3 cables in the same conduit, you'd have 8 CCC's. The neutrals are only conducting unbalanced current (310.15(B)(4)(a)).
You would derate by 70%.
Also, by 334.80, you can use the 90?C rating for derating purposes. By 310.16, #12 CU is good for 30 amps @ 90?C.
30 amps x 70% = 21 amps.
You can still use a 20 amp breaker for these circuits.
Gerns
[ January 23, 2006, 12:03 AM: Message edited by: gerns ]
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Re: NM Derating
Gern, how do you know for sure we have load diversity? Isn't that presumptive?
Gern, how do you know for sure we have load diversity? Isn't that presumptive?
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