2002 article in "Fire & Arson Investigator" about thermally insulated NM cable

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2002 article in "Fire & Arson Investigator" about thermally insulated NM cable

According to authors Goodson, Perryman, and Colwell, they investigated a house fire and found thermally damaged NM cable embedded in polyurethane foam insulation. They then set up a test rig and reported alarmingly high temperatures, enough to degrade electrical insulation, on foam-embedded cables carrying a prolonged 100% of maximum rated load. That's not a realistic test, of course, but if the circuit breaker permits it then it needs to be provably safe.

Fire & Arson Investigator magazine, July 2002, online at http://goodsonengineering.com/wp-co...yurethane-Foam-Systems-on-Wiring-Ampacity.pdf

Is anyone seeing problems in the field with NM cable in older houses that's been buried under retrofitted insulation, also including blown-in cellulose? It's a pretty common situation and if there's a real issue there should be lots of incident reports by now.

There have been some related discussions on this forum:
http://forums.mikeholt.com/showthread.php?t=120134
http://forums.mikeholt.com/showthread.php?t=121136

The NEC requires derating if multiple cables are bundled together or crammed through an insulated hole together. If the 2002 article is right, then that's not adequate.

It may not be right. There's an NEMA publication, based on a University of Toronto study, which reports utterly reasonable temperatures for heavily loaded conductors in thermal insulation:
https://www.nema.org/Technical/Docu...NM Cable encased in spray-foam insulation.pdf

Besides, the reported temperatures in their control group of conductors in free air seem strange.

On the other hand, I can't find the University of Toronto study and it's hard to argue with a building that's caught fire.

What are people's professional opinions about whether there's a real issue? Has there been any additional lab work since? Do tests of suitability for "insulation contact" already include completely surrounding the cable by deep thermal insulation?
 
Here in the UK our equivalent of your NEC code, namely BS7671, does demand that twin & earth cables (equivalent to your romex) are de-rated when they pass through a thermally insulating material.

For us this is mainly in the loft space (attic) when covered with fibreglass matting type insulation. This de-rating can be down to as much 40% of its free air ampacity in the 'wrong' conditions.

I'm sure that the NEC must have a similar requirement?

Adrian
 
Two great countries divided

Two great countries divided

The Commonwealth has a fire investigation backing up their requirement:
www.resolvematters.ca/09/images/file/Cable_Ampacity.pdf
That was a case of continuous operation near rated limits if I've read it correctly.

The US NEC has provisions for derating in insulation when multiple conductors are too close.

It's imaginable that the situation has simply never come up in the US. A US attic is likely to be in a house where the only continuous load is a few amps running the lights, with the large loads like the microwave oven only lasting a few minutes at a time. There's also the difference in heat output between a 15 amp line, even at capacity, and line carrying 200 amps.

Frederic Hartwell talked about experimental data in a submission:
http://www.nema.org/Standards/Docum...or-the-2005-massachusetts-electrical-code.pdf, skip down to 05-36 about halfway down. Sadly, my Google skills were inadequate to find the test reports, even with the numbers he gave.
 
I have never seen issues in the attic except where squirrels have gotten in an gnawed on the cable. I only do residential and I have been in thousands of home.
 
Like Dennis, I have been in a thousand or more attics. The only time I've seen heat damaged romex was from a lightning strike or a loose connection. I've never found any cable insulation that appeared heat damaged because it was covered in building insulation. And yes I have been on my belly crawling under insulation with a flashlight.

Yes a fire investigator is going to find heat damaged cables in a house that caught fire. They are probably going to find some charred wood too...
 
More information about large conductor testing

More information about large conductor testing

Thanks to all for supplying the field reports, from expert observers with large sample sizes.

Switching subjects from residential-duty branch circuits to large conductors carrying rated current, here's a quote from American Electrician's Handbook:
"The present rule for 60?C works well for branch circuits, but not as well for large feeders. In the 1987 NEC cycle, this was confirmed by actual NEMA testing, with dramatic results. The test used 2 AWG aluminum Type SE cable of the ?SEU? style (two insulated conductors and the grounded conductor configured as a spirally-wrapped conductor around the ungrounded conductors). This cable is used routinely for 100 A residential services [due to the special ampacity accorded for this purpose by 310/15(B)(6)]. It was run through thermal insulation under controlled test conditions. Specifically, the cable was run embedded in cellulose insulation with 7 in above it. Thermocouples were placed on the cable, and the various loads under test were maintained for long periods. This cable has a Table 310.16 ampacity (terminations not considered for this purpose) of 100 A. When the cable was loaded to 100 A, the cable jacket was ?completely charred? as well as adjacent ?charred wood members,? all while the cable was operating within its table ampacity limitations. In fact, the testing showed (65 A caused 96?C operation) the cable exceeded its rated operating temperature at any time the continuous current exceeded about two-thirds of the table ampacity."
Click to expand...

Annoyingly, even that has no citation to a test report.
 
Dennis Alwon said:
I have never seen issues in the attic except where squirrels have gotten in an gnawed on the cable. I only do residential and I have been in thousands of home.
Click to expand...

Here in the UK we seen to like electric showers. These tend to be high power (approx. 8 to 12kW @ 230V) instantaneous electric water heaters which are supplied on their own dedicated circuit from a 32 - 45A MCB. These are often run in 6 or 10mm2 cable which will often pass through the loft space on its route to a ceiling mounted cord operated isolator switch.

Our BS7671 demands de-rating of a cable if more than 100mm of it is run through loft insulation. Shower circuits can be one of the most challenging ones to route if the de-rating factor is not to demand an unrealistic cable size. There have been reported issues in the UK of damaged cable insulation due to overheating caused by running either shower/cooker (range)/or ring final circuit cables through loft insulation and not ensuring that they are clipped clear.

We tend to use 1 or 1.5mm2 cable for lighting circuits. 1mm2 has a rating of around 11A & 1.5mm2 around 18A in free air, but because we fuse a lighting circuit at 6A these cables don't tend to suffer damage if they are covered in fibreglass - only the 'power' cables suffer.
 
He's not talking about roll insulation or blown in fluff, he's taking about the spray in foam type insulation. I would believe that if the NMC was embedded in it there would be no way to dissipate the heat.

Yet I've seen them use it over and over on Holm's On Homes. So I'd be curious to see the actual test that was done.
 
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