Spray foam and running cables?


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Old 11-18-07, 02:20 PM
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Spray foam and running cables?

I know I'm supposed to seal any holes around plumbing and water pipes, with that spray foam stuff, where they pass through the wall plates. What about one or two Romex cables running through a hole. Will sealing here cause the cables to overheat?

And are there situations where I would want to seal holes where cables pass through the other framing members?
 
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Old 11-18-07, 02:32 PM
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The sealant won't cause the cables to overheat. Whenever you pass vertically between framing members, you are required to seal the holes with proper sealant to keep down the spread of fire between floors, so the sealant on a horizontal member won't hurt.
 
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Old 11-18-07, 06:44 PM
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Thanks, I was reading up on bundling cables, de-rating, and got concerned about heat. The stuff I'm using is rated as a fire block, not stop.
 
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Old 11-19-07, 08:10 AM
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Derating only applies when the bundling is more than 24" in length.
 
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Old 11-19-07, 11:48 AM
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A Related Foam Question

Will the insulation foams from the big box stores harm the sheathing on Romex cable if used as fire block in holes where cables pass through top or bottom plates?
 
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Old 11-19-07, 12:01 PM
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Great-stuff foam is okay for contact with NM-B Romex. Silicone and acrylic caulks are okay too.
 
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Old 11-19-07, 03:00 PM
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We have to use the red fire block on vertical holes, but for the horizontal ones, the stuff Ben mentions would suffice and not harm the cover.
 
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Old 11-30-07, 12:04 AM
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Told the inspector I was going to fill holes with foam where cable passes vertically through the wall plates, while he doing my electrical rough inspection. He said not to, and that I should use caulk. Go figure.
 
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Old 11-30-07, 04:18 AM
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Originally Posted by ibpooks View Post
Derating only applies when the bundling is more than 24" in length.
Would This section apply?

NEC 334.80 Ampacity

Where more than two NM cables containing two or more current carrying conductors are bundled together and pass through wood framing that is to be fire or draft-stopped using thermal insulation sealing foam, the allowable ampacity of each conductor shall be adujsted in accordance with table 310.15(B)(2)(a).

Thanks.
 
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Old 11-30-07, 07:16 AM
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Yes, it does apply but it does not restrict the overall ampacity of the circuit until you have more than 9 current carrying conductors assuming we're only dealing with 15A and 20A circuit.

For 9 conductors, the derating factor is 70%.

The maximum (90C) ampacity for #12 is 30A. 30A * 70% = 21A. Still adequate for a 20A circuit.

Similar calculation for a #14 wire: 25A * 70% = 17.5A which is still good for a 15A circuit.

^ Please note that these maximum ampacities for #12 and #14 wire may never be used in practice in a residence -- they are defined in the code for calculation purposes only.
 
 

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