310-15 for NM in residental

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steelbuster

Member
Location
mn
I'm doing a service panel upgrade with about 15ft between the old panel ,which will be retained as a juction box, and the new panel. The NM cable will be pulled thru several holes in the joists to the new panel and I'm trying to decipher the meaning of 310-15 as to the adjustment factors. It says where multiconductor cables are stacked or bundled longer than 24",ampacity's shall be reduced.
1. How does this apply to NM cable run loosely thru bored holes?

2. could this apply to NM cable stabled on top of one another everywhere? :eek:
 
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steelbuster

Member
Location
mn
Re: 310-15 for NM in residental

Retired Lineman from NSP... in 1968...you do the math...ya I'm older than hell,can't type worth a crap, but still find life interesting and manage to helpout the kids sometimes.
 
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tom25

Member
Re: 310-15 for NM in residental

If you are in Wisconsin, and in a single family dwelling, no need for de-rating the branch circuits.
 
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steelbuster

Member
Location
mn
Re: 310-15 for NM in residental

Thanks Tom, This is single family in Twin Cities MN. I know this is common practice here from looking at new residential work being done, but I also know the contractors are cut a lot of slack by the local inspector and was wondering what your opions were.
 
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tom25

Member
Re: 310-15 for NM in residental

My opinion is that it should not apply for 1 & 2 family dwellings based on the loads being so small
and the diversity of the branch circuits.
I have also seen several temperature tests with NM cable installed in bored holes.
 
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bphgravity

Senior Member
Location
Florida
Re: 310-15 for NM in residental

I would not considered NM to be bundled unless several cables are strapped or tapped together essentially forming a large single cable.

Then again, you have that new 334.80 requirement that indicates NM through bored holes having more of a temperature rise than I would have expected when draft or fired stopped.

In the time I have been an inspector, there have only been 2 times I have cited a job for lack of derating due to ambient temperature or number of conductors in a raceway. The previous 10 years I worked as an electrician, I was never cited, nor did I have hear or no of a jobed cited for deratng issues. And this is south Florida.
 
jwelectric

jwelectric

Senior Member
Location
North Carolina
Re: 310-15 for NM in residental

2005 cycle

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?C (140?F) conductor temperature rating. The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
Where more than two NM cables containing two or more current-carrying conductors are bundled together and pass through wood framing that is to be fire- or draft-stopped using thermal insulation or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a).

:)
 
ryan_618

ryan_618

Senior Member
Location
Salt Lake City, Utah
Re: 310-15 for NM in residental

This requirement is 100% moot, because of this exception.
310.15 Ampacities for Conductors Rated 0?2000 Volts.
(A) General.
(2) Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.
Click to expand...
This would mean that your circuit would have to be amazingly short before the new rule of 334.80 (last paragraph) would ever apply. I have a proposal drafted and ready to submit to delete this silly requirement. I have discussed this with a few different panel members (unfortunatley, not from panel 7) and they all agree that this exception nullifies the new rule. :)

Edited for typoooos

[ August 25, 2005, 09:29 PM: Message edited by: ryan_618 ]
 
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steelbuster

Member
Location
mn
Re: 310-15 for NM in residental

Originally posted by jwelectric:
2005 cycle

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?C (140?F) conductor temperature rating. The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
Where more than two NM cables containing two or more current-carrying conductors are bundled together and pass through wood framing that is to be fire- or draft-stopped using thermal insulation or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a).

:)
Click to expand...
 
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macmikeman

Senior Member
Location
Planet macmikeman
Re: 310-15 for NM in residental

Wayne is my hero for making the change proposal, and Ryan is my bigger hero for pointing out why we needn't worry about it anyway. Now if I can just get all the local inspectors around here to read Ryan's posts.
 
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tom25

Member
Re: 310-15 for NM in residental

Hurk 27...I agree with you 100%, and have seen so many temperature tests performed by UL with NM cable.
The state of Wisconsin has an exception to the NEC
that allows no de-rating for single family dwelling branch circuit conductors.
De-rating should only be used for continuous loads.
 
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steelbuster

Member
Location
mn
Re: 310-15 for NM in residental

oopps, pushed the wrong button... This run will be thru open joists only in the basement. Reading some old post on this... I have 6 or7 14/2,12/2,and12/3 runs which I think can go in the same hole and then pull the range and dryer runs thru their own individual holes. The rest of the circuits can be swung over direct to the new panel.
time to rest my brain
(Life was a lot simplier standing in your hooks)
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Re: 310-15 for NM in residental

This would mean that your circuit would have to be amazingly short before the new rule of 334.80 (last paragraph) would ever apply. I have a proposal drafted and ready to submit to delete this silly requirement. I have discussed this with a few different panel members (unfortunatley, not from panel 7) and they all agree that this exception nullifies the new rule
Click to expand...
Ryan,

Which silly requirement are you proposing to get rid of? The new rule or 310.15(A)(2)?
 
jwelectric

jwelectric

Senior Member
Location
North Carolina
Re: 310-15 for NM in residental

Originally posted by ryan_618:
This requirement is 100% moot, because of this exception.
This would mean that your circuit would have to be amazingly short before the new rule of 334.80 (last paragraph) would ever apply. I have a proposal drafted and ready to submit to delete this silly requirement. I have discussed this with a few different panel members (unfortunatley, not from panel 7) and they all agree that this exception nullifies the new rule. :)

Edited for typoooos
Click to expand...
I also agree that this is a silly rule for many reasons. One you have posted and is the strongest and another that I have seen in the field is the complete destruction of the top plate above a panel in order to comply.
Ever seen a truss being held in place by the sheeting on the roof?
:)
 
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