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Discussion Starter · #1 ·
I've read the NEC requirement to reduce the ampacity of a NM (i.e. romex) electrical cable when encased in spray foam insulation (because a NM wire heating up due to a significant current draw cannot dissipate the genereated heat quickly enough due to the spray foam insulation, and it may cause a short or a fire).

But I'm not clear on whether the requirement for ampacity reduction is the same for Type AC cable. Can anyone clarify what the code says?
Thanks
Theodore2.
 

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Use 60C to size and use 90C conductor if AC is in contact with thermal insulation. Adjustments and corrections are to the 90C ampacity.
 

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Discussion Starter · #3 ·
Thanks. This is for installing spray foam over existing AC wire, against basement wall. House was built in 1956. I'm not familiar with how to tell the existing AC wire's temperature rating. Can you offer any additional advice?
 

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Are you sure it's type AC and not flexible conduit?
60C is basically the gauge of wire. I.E. #12 is rated at least 20 amps and won't reach 60C no matter the wiring method. It's easy to tell if a new wire is rated 90C because it's labeled. My guess is that if the wiring is as old as the house, the conductor isn't rated 90C. Hopefully one of the more seasoned guys will help out with this.
 

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Discussion Starter · #5 ·
To me, it's what I call "BX cable." But I'm told that it more proper to refer to it as AC. Regardless, I apologize for my confusing nomenclature.

And it's only either 12gauge or 14gauge... I have to check back at the panel.
 

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60C is basically the gauge of wire.
60C is the temperature rating of the insulation or cable sheathing. The insulation rating limits the ampacity rating for a given gauge of wire.

Personally, I would have someone check that wiring for general condition and specifically for a bonding strip, before I buried it.
 

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60C is basically the gauge of wire.
60C is the temperature rating of the insulation or cable sheathing. The insulation rating limits the ampacity rating for a given gauge of wire.

Personally, I would have someone check that wiring for general condition and specifically for a bonding strip, before I buried it.
This is correct. I wasn't very clear. AFAIK, 60C is the lowest temp rating of a current carrying conductor. So all you need to know is the gauge of a wire to know how many amps it can handle at 60C. Thanks for clarification.
 

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That's the easy part. If you can't find a 90C marking on the insulation than it probably isn't suited to be in thermal insulation. Not sure but I think 90C conductor insulation started in the 80's.
 

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To me, it's what I call "BX cable." But I'm told that it more proper to refer to it as AC. Regardless, I apologize for my confusing nomenclature.

And it's only either 12gauge or 14gauge... I have to check back at the panel.

there are two very similar cable types: AC and MC.

BX is/was never an actual cable type. It was always AC in the code; Armor Clad. BX never was an actual NEC listed cable type. BX was originally designed in 1899 by Sprague Electric. The made two types AX and BX with the X standing for experimental. So, type A and type B experimental cable. The name stuck so now a lot of people call AC cable BX. It had a steel sheath. Newer AC cable is available in steel or aluminum sheath

MC; Metal clad. Very similar to AC in outside look but different internally.

AC has paper wrapped insulators. MC has plastic wrapped.

AC has a small gauge bare steel bonding wire inside of the sheath. That is how the sheath is acceptable as a grounding conductor

the types of conductors you can get in each of them vary as well.

MC has no bonding wire and the sheath is not acceptable as a grounding conductor although a few companies have made this MC cable with a full sized bare egc (in addition to a green insulated egc) in trying to mimic the uses where AC is acceptable and MC is not.


In my area, the most commonly used cable is MC. AC is usually reserved for medical treatment facilities where MC is not acceptable.

there are other differences but that is what generally allows one to be able to identify the type of cable they have.
 

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I don't remember MC cable being around since the 50s.
 

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I've read the NEC requirement to reduce the ampacity of a NM (i.e. romex) electrical cable when encased in spray foam insulation (because a NM wire heating up due to a significant current draw cannot dissipate the genereated heat quickly enough due to the spray foam insulation, and it may cause a short or a fire).
Can you share this article with us? I think you read it wrong.
 

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Discussion Starter · #14 ·
Three references on NM cable, although none of them mention armored cable.

This one says it's ok: https://www.nema.org/Technical/Docu...NM Cable encased in spray-foam insulation.pdf

This scary one shows that it's not ok:
http://goodsonengineering.com/wp-co...yurethane-Foam-Systems-on-Wiring-Ampacity.pdf

And this reference from the NEC suggests it's not ok:
NEC 334.80

Where more than two NM cables containing two or
more current-carrying conductors are installed, without
maintaining spacing between the cables, through the same
opening in wood framing that is to be fire- or draft-stopped
using thermal insulation, caulk, or sealing foam, the allowable
ampacity of each conductor shall be adjusted in accordance
with Table 310.15(B)(2)(a) and the provisions of
310.15(A)(2), Exception, shall not apply.
Where more than two NM cables containing two or
more current-carrying conductors are installed in contact
with thermal insulation without maintaining spacing between
cables, the allowable ampacity of each conductor
shall be adjusted in accordance with Table 310.15(B)(2)(a).
 

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Three references on NM cable, although none of them mention armored cable.

This one says it's ok: https://www.nema.org/Technical/Docu...NM Cable encased in spray-foam insulation.pdf

This scary one shows that it's not ok:
http://goodsonengineering.com/wp-co...yurethane-Foam-Systems-on-Wiring-Ampacity.pdf

And this reference from the NEC suggests it's not ok:
NEC 334.80

Where more than two NM cables containing two or
more current-carrying conductors are installed, without
maintaining spacing between the cables, through the same
opening in wood framing that is to be fire- or draft-stopped
using thermal insulation, caulk, or sealing foam, the allowable
ampacity of each conductor shall be adjusted in accordance
with Table 310.15(B)(2)(a) and the provisions of
310.15(A)(2), Exception, shall not apply.
Where more than two NM cables containing two or
more current-carrying conductors are installed in contact
with thermal insulation without maintaining spacing between
cables, the allowable ampacity of each conductor
shall be adjusted in accordance with Table 310.15(B)(2)(a).
I'll believe NEMA before that other no-name link.

Also, read that code section again. It references cables through opening in wood. And also, keeping mind the derating rules for smaller cables.
 

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The code article quoted is referring to multiple cables being in close contact in thermal insulation. Because of the heat from neighboring conductors you need to derate. But a single cable isn't referenced.
 
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