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Old 03-14-2014, 06:44 PM   #1
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Default Conductor derating

I'm trying to understand the following concept:


Example let's suppose I have a 2" raceway from a panel to a trough with hrs branching out of the trough. The 2" conduit is 3' long. Can I fill the 2" conduit to it max conductor capacity according to nec tables. Or does derating apply in this situation. If so why even have a conduit fill chart if you can only have max 3 ckts without derating. Someone please clarify.

Thanks
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Old 03-14-2014, 06:59 PM   #2
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Quote:
Originally Posted by jayare813
I'm trying to understand the following concept: Example let's suppose I have a 2" raceway from a panel to a trough with hrs branching out of the trough. The 2" conduit is 3' long. Can I fill the 2" conduit to it max conductor capacity according to nec tables. Or does derating apply in this situation. If so why even have a conduit fill chart if you can only have max 3 ckts without derating. Someone please clarify. Thanks
Derating doesn't apply under 24".
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Old 03-14-2014, 07:01 PM   #3
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Raceway fill is merely a calculation of how many little circles you can put into a big circle.

Derating goes to how much heat an excessive number of little circles inside said big circle create, and now well the colored coating on the little circles can handle it.
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Old 03-14-2014, 07:05 PM   #4
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Originally Posted by 480sparky View Post
Raceway fill is merely a calculation of how many little circles you can put into a big circle.

Derating goes to how much heat an excessive number of little circles inside said big circle create, and now well the colored coating on the little circles can handle it.

Nice!

A little too technical, but, nice.
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Old 03-15-2014, 01:49 AM   #5
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I'll give you an example that has meaning for me and maybe you will understand.

According to Table C.1 (9) 12 ga wires is max fill for 1/2" EMT. According to Table 310.15 90 degree 12 ga THHN/THHW max amp is 30A (not allowed to fuse over 20A). By Table 310.15(B)(2)(a) adjustment for more than 3 CCCs you multiply that 30A X .7 for the 9 CCCs which equal 21A and still good on a 20A fuse.

However if you go to 3/4" EMT the fill is (16) 12 ga wires (same insulation) which must be derated to 50% which equals 15A. No longer good on a 20A fuse.

With this 12 ga wire and 9 CCCs in 1/2" EMT six circuits can be run with single phase if all are shared neutrals (3X3).
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Old 03-15-2014, 09:23 AM   #6
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The derating does not affect this install but the fill is still pertinent. Look at Table 9 Note 4. You can only fill the nipple 60%
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