NM cable derated?

[djtazjr]

I have had discussion with my supervisor saying that NM cable doesn't have to be derated, or rather you can run as many cables through any giving raceway as you want. Ok, like going through holes in metal or wooden studs. or even through a 2" conduit into the Electric Panel.

?????????Anything further????????

 

[don_resqcapt19]

Re: NM cable derated?

If the cables are bundled for more than 24" you must derate.
Don

 

[djtazjr]

Re: NM cable derated?

Yeah, thats what I thought. My super seems to think differently.

Thanks Don

 

[tonyi]

Re: NM cable derated?

NM gets no special allowances - its just ordinary conductors as far as the derating calcs/rules are concerned.

Note: the new SouthWire 12-2-2 and 14-2-2 stuff is required to be derated up front when you read the fine print on their packaging.

 

[iwire]

Re: NM cable derated?

Originally posted by tonyi:
Note: the new SouthWire 12-2-2 and 14-2-2 stuff is required to be derated up front when you read the fine print on their packaging.

How would this change any thing?

12-2-2 would derate to 24 amps.

14-2-2 would derate to 20 amps.

 

[tom baker]

Re: NM cable derated?

NM is not different than any other wiring method in chapter 3. Derating is required.
Ask your super what the ampacity of 12-2 NM is,
20A, 25, or 30A.
Hint: NM cable conductors are 90 deg C rated.
What is the final ampacity after derating to be?
Hint: 230.3(D) and 334.80

 

[djtazjr]

Re: NM cable derated?

12-2 nm cable is rated 25amp but 80% derate factor and that 20 amps right?

Thanks Tom for the help, I really appreciate this, my Super doesn't really practice the trade much anymore so he can be misleading.

 

[iwire]

Re: NM cable derated?

12 awg NM is rated 30 amps.

14 awg NM is rated 25 amps.

 

[tom baker]

Re: NM cable derated?

The advantage of 12-2 NM Cable being rated 30 amperes is for derating, say in a hot attic. By starting at 90 Deg C-30 amp when derated to 20 amps, you avoid having to start with 10 AWG NM. NM-B was developed due to overheating of THN NM conductors in light fixtures.
We don't see overheated NM here in the cool NW, but in some areas it was a big problem, esp when the NM is covered in 10 inches of insulation. One of the reasons why Knob & Tube was such a good wiring method was it ran very cool in free air, until we started insulating buildings.

 

[Guest]

Re: NM cable derated?

It's interesting to note that covering cables with attic insulation will increase the effects from internal heating (from the cable itself), but covering with attic insulation will reduce the effects on the cabling from outside ambient temperature effects. The insulation captures the heat from the cabling itself, but creates a barrier from the attic heat to the cabling. A foot of insulation will create an ambient capillary break, but it will create a barrier to the free circulation of "cooling" air. I believe in a CA attic in mid-summer the cabling would run cooler under the insulation, than the cabling temp would run on top of the insulation. It's a Catch-22

 

[tonyi]

Re: NM cable derated?

Originally posted by iwire:

Originally posted by tonyi:
Note: the new SouthWire 12-2-2 and 14-2-2 stuff is required to be derated up front when you read the fine print on their packaging.

How would this change any thing?

12-2-2 would derate to 24 amps.

14-2-2 would derate to 20 amps.

Run the 14-2-2 in a 132F+ ambient and the significance of this upfront derating for the SouthWire product will become obvious - i.e. it becomes unusable.

 

[roger]

Re: NM cable derated?

Tony, if they were dedicated single receptacle or light fixture circuits, why would the 14 be unusable?

Roger

 

[tonyi]

Re: NM cable derated?

With Southwire's mandatory derating factor the #14 is 20A in the 90 degree column. Now apply the .71 correction for 132F.

20A * .71 = 14.2A

A 15A breaker won't protect that wire when run in a 132F+ environment. What its connected to really doesn't matter. Breakers protect wire, not loads.

Its not just the #14 that can be affected by the upfront derating. Consider what happens when you wanted to sleeve two of the #12's into panel in a 2X4 interior wall to avoid 300.4 issues. A Stak-It doesn't list 12-2-2 (or even 12-3), so stud space can become a very scarce commodity when using that particular Southwire product.

24A * .7 = 16.8A

Suddenly the #12 product can require 15A breakers. There may be an argument here that this is double derating for the same purpose, but SouthWire hadn't addressed this point when I looked up all the technical specs on this stuff some time ago. As far as the code is currently concerned it would be 24A starting point conductors for piped derating purposes. Even if someday that gets clarified and you didn't have to double derate, just 3 of these 12-2-2's sleeved would become 15A wire, and 4 would be flat out unpipeable.

In a lot of places these issues would never arise, but derating in general seems to not be something well understood or often considered in residential work . Really, I should say roped residential work - the guys who have to pipe everything should be more up on derating issues (or one would hope that's the case)

[ November 23, 2003, 11:56 PM: Message edited by: tonyi ]

 

[iwire]

Re: NM cable derated?

Tony I think you have now changed the issue we were taking about.

We where talking about derating for the number of conductors and now you throw in high temps.

We can make regular 12/2 unusable for 20 amp circuits if we raise the ambient temp enough.

I live in New England the chances of reaching 132F in the attic where the wires are located is slim.

Most of a dwelling units wiring is on the interior side of the insulation (between the strapping about 1/2" from the drywall) The only place I can think of that might reach 132F would be the wire to the attic lights and heat detectors.

And for that as Roger has pointed out 240.4(B) could apply.

So I stick with my first statement 12-2-2 is no different from 12-2 in most applications.

Bob

 

[iwire]

Re: NM cable derated?

Originally posted by tonyi:
In a lot of places these issues would never arise, but derating in general seems to not be something well understood or often considered in residential work

Tony first off I am not trying to bust your chops here, just have a discussion.

Lets think on this for a little bit.

Do you find failures do to this lack of understanding? Come on be honest.

I am talking about modern homes wired with 90C NM.

How many residential circuit do you find loaded up to the max bundled together in a way that would need derating.

I agree with you, derating is forgotten about many times in residential work.

I do not agree it is something to worry about.

How many loads in a dwelling unit would you consider continuous?

IMO dwelling units are a much different environment than commercial work.

IMO Table 310.15(B)(6) confirms that the NEC considers dwelling units different too.

Not saying I would ignore derating I just do not look at it as a significant issue in a building with modern NM.

Take a look at all the exceptions for derating specifically 310.15(B)(2)exception 5 yes I know it is for MC and AC my point is the NEC obviously thinks THHN in a cable assembly is a tolerant product.

I might add in my State the adjustment table is much different than in the NEC and we do not seem to have any problems.

Just something to talk about Tony, I would never fault a guy for running larger wire it can never hurt if the budget allows.

JMO, Bob

 

[roger]

Re: NM cable derated?

Bob,

And for that as Roger has pointed out 240.4(B) could apply.

I knew you would nail that one.

Roger

 

[tonyi]

Re: NM cable derated?

Originally posted by iwire:
We where talking about derating for the number of conductors and now you throw in high temps.

Which is relevant because the SouthWire products put you in a position where you may need to consider the effects of ambient derating where maybe you wouldn't have if a 14/12-3 were used.

I would also argue that ambient issues can be more common than expected in the north depending on the style of construction and plumbing systems used. Its not just attics where localized high temps can occur. I've seen instances of Romex near hot water and steam pipe where the sheath was distorted from the heat. If there's a rocked over chase where heating pipe and wire go from floor to floor its real easy to get a high temp area nobody pays much attention to.

 

[earlydean]

Re: NM cable derated?

When de-rating a conductor's ampacity because a portion of it is run through an area of higher temperature, keep in mind the exception to 310.15(A)(20). If the portion of the cable run through that hot attic is only a few feet, the entire run need not be de-rated. Only if the run exceed 10 feet or 10% of the entire length does de-rating need to be done. The electrician might want to keep most of his wiring below the insulation, and only come up where necessary to make a connection to an attic fan or light.
What is a problem is where wiring is done in attics spaces of large commercial buildings and the electrician chooses to run the cable or EMT runs by stuffing the runs in the space between the joists and the roofing metal. No supports are necessary, but the temperatures in the summer soar. Even in Maine, severe derating is required.