ECN Forum Forums General Topics of the Trade General Discussion Area Foam insulation cable derating

I did find "some" guidance here, but it still does not seem clear to me.

2-122 Use of thermal insulation

(1) Where the hollow spaces between studding, joists, or rafters of buildings are to be filled with thermal insulation, the following restrictions, as applicable, shall apply to the installation of electrical wiring in such spaces:

(a) special care shall be taken to ensure that conductor insulation temperatures are not exceeded due either to mutual heating of adjacent conductors or cables or to reduced heat dissipation through the thermal insulation;

( b) if the space is to be filled with a loose or free-flowing material that is non-corrosive, fire-resisting, and non-conductive and that is in compliance with the National Building Code of Canada, any type of wiring system recognized by this Code shall be permitted to be used, but special care shall be taken to ensure that there will be no strain on the conductors due to the weight or pressure of the insulating material;

(c) if the thermal insulation material, in the form of batts or rigid sheets, is installed prior to the installation of the wiring and secured in place so that there will be no undue pressure on the conductors, no special precaution need be observed;

(d) if thermal insulation made of or faced with metal is installed, the wiring shall conform to the following:

(i) a 25 mm separation shall be provided between the thermal insulation and the knob-and-tube wiring; and

(ii) non-metallic-sheathed cable may be in contact with the insulation; and

Δ (e) mineral-insulated cable, aluminum-sheathed cable, or copper-sheathed cable shall not be used with types of thermal insulation that are liable to have a corrosive action on the sheath.


It appears that 2-122 (1)(a) might apply here. Unfortunately the requirement to derate the cables is still a complete mystery to me. Unless I'm a complete idiot at looking at the Canadian Electrical Code, the term "special care" comes up quite often, but I have never seen a definition of that.

I'm going cross-eyed at reading the Canadian Electrical Code over and over again.

Maybe common sense would take care of those words "special care" but lately "common sense" is not an acceptable way of thinking according to most lawyers.

Anyone have any further thoughts here?

spacing would be a AHJ call

I would like to agree with you, but where is that written in NEC 334.80?
To be honest, I don't even see any "spacing" requirements listed in 310.15 for raceways, single or multiconductor cables, AC or MC, even where bundled or installed in lengths over 24-inches.
All I see is that the spacing, of whatever amount you begin with, must be maintained throughout the run, even when making bends.
On the upside though, for the purpose of derating in Table 310.15[B]2,a, the neutral conductors of 3-wire multiwire branch circuits are not counted as a current carrying conductors, which may help keep the wire count down in some instances.

The inspector I spoke with said its not the spacing distance that he is concerned with but the method of maintaining that distance.

If you're interested in an outside view... most European codes require derating for bundling when the cables are spaced closer than their own diameter (i.e. if you leave once the cable diameter in between two cables it's not bundling).
For wiring run in insulated walls there are no derating factors but in fact whole different ampacity tables, called installation methods. These cover wiring in conduit in non-insulated walls (either masonry or stud walls), insulated walls (type of insulation not specified), direct burial of cables in masonry walls, direct burial underground and wiring in free air (overhead lines), giving different ampacities for each method and cross section.

Usually the difference is pretty small, but when said difference is hovering around a breaker size it can be important (like if you have an ampacity rating of 15.7A you can't use a 16A breaker any more, but with 16.3 you can).

Since the maximum allowed fuse/breaker rating under normal conditions is specified a lot more conservative by the NEC than by any other code I know derating might not be as much of an issue anyway, but that's just an educated guess.

I think that some of you have gone off on a tangent.
The OP wanted to know about derating cables running through spray foan insulation. Not derating cables when bundled.
The CEC has clear rules for derating cables when bundled.

As for derating when run through spray foam insulation, I have worked on houses where the walls were going to be spray foamed and I didn't worry about the derating because I never load my circuits up to the max. For example the CEC allows 12 receptacles per circuit, but I never put more than 10 on a circuit. So in effect I guess you could say that I automatically derate by 16%.
I don't believe that a rule exists in Canada at this time concerning running conductors through spray foam insulation.

I was hoping that mikesh would have contributed by now. It would be interesting to hear an inspectors opinion on this.

Of course if I'm wrong and the OP was concerned about derating when bundling, then just ignore this whole post.