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Old 08-23-2010, 01:25 PM   #1
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Over or under insulation in attic?


Is it better to run electrical wires over or underneath the insulation in the attic?

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Old 08-23-2010, 08:17 PM   #2
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Over or under insulation in attic?


six on one hand half dozen on the other. It doesn't matter but it looks better if it's under.

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Old 08-23-2010, 09:08 PM   #3
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Over or under insulation in attic?


Your cable may need to be derated(increased in gauge)when run on the hot side of the insulation.
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Old 08-23-2010, 09:18 PM   #4
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Over or under insulation in attic?


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Originally Posted by daveb1 View Post
Your cable may need to be derated(increased in gauge)when run on the hot side of the insulation.
Derating your wires due to the insulation in a house? Im sorry but that has to be the ??????????? i've ever heard....

Last edited by beenthere; 08-24-2010 at 03:11 PM. Reason: Rephrased teh staement to be more polite
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Old 08-23-2010, 09:47 PM   #5
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Over or under insulation in attic?


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Originally Posted by dSilanskas View Post
V
Derating your wires due to the insulation in a house? Im sorry but that has to be the most stupid thing i've ever heard....
Um, what? Ambient temperature is a necessary factor to consider for derating. If putting the wire on the outside of the insulation exposes it to very high temperatures in the attic, it may need to be derated. With that said, it will not USUALLY be necessary to derate because of this, and most people ignore it.
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Old 08-23-2010, 09:58 PM   #6
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Over or under insulation in attic?


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Originally Posted by mpoulton View Post
Um, what? Ambient temperature is a necessary factor to consider for derating. If putting the wire on the outside of the insulation exposes it to very high temperatures in the attic, it may need to be derated. With that said, it will not USUALLY be necessary to derate because of this, and most people ignore it.
Magnetic everyone ignores it because no need to derate in an attic. If it was such an issue than why isn't there ever issues with the way we (electricians) wire houses. Never ever have i had to derate while running my wires in attics. Its just not needed
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Old 08-24-2010, 09:10 AM   #7
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Over or under insulation in attic?


It seems if you put it over the insulation then it would be exposed to much higher temperatures but if you bury it under the insulation so that it's almost right against the cooled part of the house, then it wouldn't be able to disipate the heat that's generated in the wire itself due to current. So which is worse, the ambient heat of the attic or the heat from the current??
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Old 08-24-2010, 09:59 AM   #8
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Over or under insulation in attic?


At the attic floor the temperature should not be too bad. I would just go by the NEC (RE cable spacing) and not bother to derate the cables simply because of the attic temperature.
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Old 08-24-2010, 12:36 PM   #9
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Over or under insulation in attic?


"Secure within 12" of box & maximum 4-1/2' intervals --- 3702.1 '06 IRC 314.17C '08 NEC(not to the insulation) right below de-rating from the manufacturers stamp: http://books.google.com/books?id=eLo...0attic&f=false
Remember the boards within 6 &7' of the access.

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Old 08-24-2010, 03:10 PM   #10
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Over or under insulation in attic?


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Originally Posted by dSilanskas View Post
V
Derating your wires due to the insulation in a house? Im sorry but that has to be ??????????? thing i've ever heard....
Strange. Wire is rated by the ambient temp it is installed in.

While low draw circuits aren't usually effected. Higher current drawing devices are.
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Old 08-24-2010, 03:24 PM   #11
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Over or under insulation in attic?


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Originally Posted by AllanJ View Post
At the attic floor the temperature should not be too bad.
That depends on where you live.
For my house, on a 100 degree day with the A/C running
The temp at the peak can top 125.
The temp above the insulation can hit 105.
The temp under the insulation against the sheetrock is about 80.
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Old 08-24-2010, 10:36 PM   #12
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Over or under insulation in attic?


Quote:
Originally Posted by dSilanskas View Post
V
Derating your wires due to the insulation in a house? Im sorry but that has to be the ??????????? i've ever heard....
This has already been taken into account with NM cable having to be rated out of the 60 degree column. and now for se and ser when run in the interior of a house in insulation with the 2008 NEC.

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