14. 310.15 Conductor Ampacity - Is this as bad as I think

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Bob Sisson

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Boyds, MD
I see what I consider "problem" bundling all the time during new Home construction. The local electrician or AHJ routinely overrules me saying "no problem." In this last instance the insulation was going to be FOAM, not fiberglass so no air circulation. So these cables were going to be encased in FOAM insulation for 10' (1 floor) and half of them for ANOTHER 10' as they continued into the attic....

On the right are 2x12s, 4x14s, 1X10. On the Left are 5 x 14's, 1X12 and and extra 14 for the outlet

I flagged it, the the AHJ and local electrician said it was OK...

My question is: Who is right, are these "Bundles" and thus need to be derated or spread out more?

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Dennis Alwon

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
We can argue this all day and we won't know until 10 years from now when these walls get opened up. By the nec I would have to say it is compliant.
 
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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
NEC aside, with closed cell foam becoming so popular I think these kind of things are going to become serious problems that may take years to manifest themselves. I think you are correct to be concerned about this. I would not allow it in my house. There have already been problems with can lights.
 
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dana1028

Senior Member
Location
San Francisco Bay area
From the photo - looks like 'stackers' are being used to keep the NM separated in some areas, NM stapled to 1x board separates cables in other areas....as others have said, it appears these cables meet the requirements for being separated.

There have been discussions about the foam insulation however mfr's and UL have commented that the foam does not create a problem.

So - I too agree with Dennis.
 
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gndrod

Senior Member
Location
Ca and Wa
Bob Sisson said:
I see what I consider "problem" bundling all the time during new Home construction. The local electrician or AHJ routinely overrules me saying "no problem." In this last instance the insulation was going to be FOAM, not fiberglass so no air circulation. So these cables were going to be encased in FOAM insulation for 10' (1 floor) and half of them for ANOTHER 10' as they continued into the attic....

On the right are 2x12s, 4x14s, 1X10. On the Left are 5 x 14's, 1X12 and and extra 14 for the outlet

I flagged it, the the AHJ and local electrician said it was OK...

My question is: Who is right, are these "Bundles" and thus need to be derated or spread out more?

View attachment 6283
Click to expand...

Bob,

Very astute concern for a home inspector. Check out [300.4(D)] Edge clearance...definitely a violation if bundles are less than 1 1/4" of stud edge surface paneling.
 
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steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Refering to 334.80 in the 2008 handbook, it clearly looks like it could be considered a code issue. (This section changed in 2005, and 2008 again, so the 2011 may be different).

(Side note: I tried to scan and upload the handbook commentary, but this forums file size limit is iinsane. I can't get a single page PDF compressed enough to upload it.)

However, I doubt the grouped wires will ever be a real issue. The derating starts at the 90 degree ampacity. So the example in the handbook is for (4) 2 conductor #12 wires, and the wire derates to 21 amps. So its still good for a 20 amp circuit.

And odds are, all those wires will never be fully loaded at the same time.
 
steve66

steve66

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Location
Illinois
Occupation
Engineer
donf said:
Steve,

Did you mean, "derating starts at 90(C)" instead of "derating starts at 90 degrees ampacity"?
Click to expand...

I'm not quite following the difference. It sounds more like you are talking about derating for ambient temperature.

To elaborate a little, the code section says you can use the 90(C) rating of the wire, as long as the final derated ampacity doesn't exceed the 60(C) rating.

So in the handbook example, they started with the 90(C) rating of #12 wire which is 30 amps (table 316). They derated it by 70% for 8 current carrying conductors (table 315.10(B)(a)), which gives a total rating of 21 amps.

Then you use the lesser of the two - the derated ampacity of 21 amps, or the 60(C) ampacity of 20 amps.

In this case, 20 amps is less, so I'm #12 wire is still good for a 20A circuit, and nothing is any different than if we hadn't bundled the cables together.

Again, all this is the 2008 code. The 2011 may be different.
 
iMuse97

iMuse97

Senior Member
Location
Chicagoland
Installed it like that before the housing market tanked. I don't do houses anymore.

First, there is no problem with code. Dennis called it.

Second, I don't see a problem with overheating in a foam insulation setting.

Third, those wires are all by the book as to the 1 1/4" depth from the stud surface.

Fourth, I agree with steve66's logic if not with his math or semantics. What with only some of the wires ever loaded to ampacity, and with continuous loads of high ampacities very unlikely, I don't think there will ever be a problem caused by overload on these thhn/twhn-2 rated wires for their expected life.
 
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Bob Sisson

Member
Location
Boyds, MD
Only being a home inspector, NOT a full Electrical Inspector, I have only my paranoia and limited training to go on...

Some of the circuits can be full loaded (clothes dryer, Heat pump Aux Heat) while others rarely are (general lighting) and still others are only occasionally loaded (bathroom GFCI's -hairdryers)

The way I look at it is that "I don't know" so I must assume worst case, where the cicuits are all at capacity. Individually they would be fine, but if they are "Bundled" toggether, they heat each other and could be in need of derating. That the point of that section of the code (I think)

I understand where most AHJs have no problem with Fiberglass insulation, as there is still considerable air-flow within the cavity. My concearn is when we use an "Encasulating" type insulation such as high-density blown in celulose or foam. These products are specifically engineered to prevent air-flow. The cances of a "group" of conductors ending up in a common void in the insulation is not zero, and I invisioned that "void" that the conductors ran inside from floor to ceiling (10') being similar to a conduit or raceway. The way I read the intent of the code was when I had a group of more than X conductors running inside a conduit/raceway for more than 24" you had to look at the derating tables... I put in a "x" as the number of conductors seems to varry depending on the code revision.

Sounds like this one is going to take time to see if it is a problem...
 
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