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Old 01-30-2013, 12:23 PM   #16
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You have to derate whenever you have more than three CCC in a raceway. You can use reducing washers to fix the knockout that is to big, but it's gonna be work getting one behind the panel. All unused openings need to be closed. You won't get a KO seal in there, but a blank box cover could be screwed over it.

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Old 01-30-2013, 12:24 PM   #17
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I think a couple lb's would have worked nicer outside on the bottom of the panel.
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Old 01-30-2013, 01:41 PM   #18
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Originally Posted by busman View Post
You have to derate whenever you have more than three CCC in a raceway. You can use reducing washers to fix the knockout that is to big, but it's gonna be work getting one behind the panel. All unused openings need to be closed. You won't get a KO seal in there, but a blank box cover could be screwed over it.

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Is conduit considered raceway? I thought the derating would only apply if I was using NM....

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I think a couple lb's would have worked nicer outside on the bottom of the panel.
I thought about that....but....I want to be able to pull wire through there at a later date...on the backside, I'll end up having a sheet of OSB covering the wall cavity....so no access from there....and on the front side, it's going to be covered as well...so I will not be able to access the cover on an LB.
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Old 01-30-2013, 02:07 PM   #19
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I'll only say it SO many times. It's a raceway. Unless it is shorter than 24 inches you have to derate.

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Old 01-30-2013, 02:11 PM   #20
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Is conduit considered raceway? I thought the derating would only apply if I was using NM....
You will need to derate in your situation. I've also heard the misconception that conduit size affects derating; it does not. Derating is the same for 6 conductors in 1.5" pipe as it is in 4" pipe.
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Old 01-30-2013, 03:44 PM   #21
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Thanks for the input guys....

For the record...I've never seen the derating tables before...even though I design a lot of control systems using conduit, I've never had to worry about it...my only concerns are conduit fill. All of my wires are basically low current....for the most part, I'll have 50-300 THHN wires in multiple conduits....and all of that is protected with a couple 10 or 15A breakers....

Raceway vs conduit....I use both....they are two different animals...though I can see treating them the same as far as derating goes.

I've been looking at the NEC tables....seeing how I'm using THWN/THHN 90 deg C wire....I think I'm ok on the derating....
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Old 01-30-2013, 05:00 PM   #22
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You may be less than 15 amps for the #12s after derating. Not good especially if these are required 20 amp circuits.
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Old 01-30-2013, 05:04 PM   #23
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You may be less than 15 amps for the #12s after derating. Not good especially if these are required 20 amp circuits.
I couldn't find the actual derating chart....just the one that shows max amps for each size conductor. But based on some of the examples I saw...assuming a 50% derating....I would still be good to over 15A. Most of those ckts are for existing wiring in the house. It would not hurt my feelings to drop them down to 15A breakers. As it is, I've shed a bunch of loads off the existing.
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Old 01-30-2013, 05:56 PM   #24
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The derating chart is in
Table 310.15(B)(3)(a)
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