de-rating conductors in conduit

13828 Views 13 Replies 9 Participants Last post by localunionman, Dec 15, 2013

Paul S.

Discussion Starter · Dec 14, 2013

Hi All,
I have a customer who wants a transfer panel put in and we plan on moving about 16 circuits to the new transfer panel, (which will include two or three 240V circuits.) These circuits will be traveling through conduit for just over 24 inches and I am thinking I will have to de-rate each of the conductors per table 310.15(B)(2)(a) [2008 NEC]. I would guess that there will be approx. 36 or so wires passing through conduit from the existing panel to the generator panel, (which I will have to locate outside due to clearance issues). If my thinking is correct, I will have to de-rate each of these wires to about 40 percent of their allowable ampacity per table 310.16. That will mean I will have to run a number 6 for each 20A circuit. Is that right?....Whoa! I am thinking this will be waaay too much! Is there something I might be missing here? A little confirmation or help here will be appreciated. - If I am correct, I am thinking it will be better to open up the wall and avoid using the conduit all together.

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Giorgio.g

· #2 · Dec 14, 2013

Who derates? Barely any contractor ever debates, never done it in the last 12 Years of my career.

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Ultrafault

· #3 · Dec 14, 2013

310.15(b)(3)(a)(2) derating due to conductor fill only applies to conduits longer than 24 inches.

You are also nit derating correctly. If you had to derate in this case you would see there are 32 current carrying conductors. Whitch leads you to 40% derate. So for instance you had a 20 amp rated circuit. Your math would be 20/.40=50 your conductor must be rated at 50 amps. A number 8 at 90 degrees is rated at 55 amps so it would be acceptable.

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Ultrafault

· #4 · Dec 14, 2013

Giorgio.g said:
Who derates? Barely any contractor ever debates, never done it in the last 12 Years of my career.

Professionals who care about doing things up to code and not cutting corners that an inspector may or may not see. If we can't take the code seriously how can we be expected to be taken seriously.

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Ultrafault

· #5 · Dec 14, 2013

Ultrafault said:
310.15(b)(3)(a)(2) derating due to conductor fill only applies to conduits longer than 24 inches.

You are also nit derating correctly. If you had to derate in this case you would see there are 32 current carrying conductors. Whitch leads you to 40% derate. So for instance you had a 20 amp rated circuit. Your math would be 20/.40=50 your conductor must be rated at 50 amps. A number 8 at 90 degrees is rated at 55 amps so it would be acceptable.

Edit: I see you said just over 24 inches. Cut the conduit shorter.

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buddhakii

· #6 · Dec 14, 2013

If you are just over 24 inches like say 26 inches I would not worry too much about derating. If it is getting inspected talk to the inspector and see if he will let it fly.

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SteveBayshore

· #7 · Dec 14, 2013

Ultrafault said:
310.15(b)(3)(a)(2) You are also not derating correctly.

I would derate a little differently than you suggest. Without pulling out a codebook, and using your 40% as the factor, #10 THHN is rated at 40 amps at 90c. 40% of 40 amps is 16 amps. You can use the next size breaker up, 20 amps, as long as you don't exceed 800 amps. You can use the actual temperature rating of the conductor (90c) to start derating from and as long as you don't exceed the lower temperature rating when sizing the breaker.:thumbsup:
Any thoughts, or have I been doing it wrong all along?

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di11igaf

· #8 · Dec 14, 2013

SteveBayshore said:
I would derate a little differently than you suggest. Without pulling out a codebook, and using your 40% as the factor, #10 THHN is rated at 40 amps at 90c. 40% of 40 amps is 16 amps. You can use the next size breaker up, 20 amps, as long as you don't exceed 800 amps. You can use the actual temperature rating of the conductor (90c) to start derating from and as long as you don't exceed the lower temperature rating when sizing the breaker.:thumbsup:
Any thoughts, or have I been doing it wrong all along?

Only if the load is less than 16 amps could you use next size up rule.

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Ultrafault

· #9 · Dec 15, 2013

SteveBayshore said:
I would derate a little differently than you suggest. Without pulling out a codebook, and using your 40% as the factor, #10 THHN is rated at 40 amps at 90c. 40% of 40 amps is 16 amps. You can use the next size breaker up, 20 amps, as long as you don't exceed 800 amps. You can use the actual temperature rating of the conductor (90c) to start derating from and as long as you don't exceed the lower temperature rating when sizing the breaker.:thumbsup:
Any thoughts, or have I been doing it wrong all along?

Keep reading the same article you will see it does not apply to rec circuits for cord and plug equip. Rules out most residential applications.

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Jlarson

· #10 · Dec 15, 2013

Yeah, I wouldn't even bother.

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SteveBayshore

· #11 · Dec 15, 2013

Ultrafault said:
Keep reading the same article you will see it does not apply to rec circuits for cord and plug equip. Rules out most residential applications.

Thanks, must have hi-litghted that section in my code book with the black magic marker.

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pete87

· #12 · Dec 15, 2013

Paul S. said:
Hi All,
... These circuits will be traveling through conduit for just over 24 inches and I am thinking I will have to de-rate each of the conductors per table 310.15(B)(2)(a) [2008 NEC].... . If my thinking is correct, I will have to de-rate each of these wires to about 40 percent of their allowable ampacity per table 310.16. That will mean I will have to run a number 6 for each 20A circuit. Is that right?....Whoa! .

Put a Jct. Box In the pipe run that is Over 24 inches .

Then you have 2 nipples , each under 24 inches .

Pete

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Ultrafault

· #13 · Dec 15, 2013

pete87 said:
Put a Jct. Box In the pipe run that is Over 24 inches .

Then you have 2 nipples , each under 24 inches .

Pete

Freaking brilliant.

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localunionman

· #14 · Dec 15, 2013

or

Open up a bit of drywall, take the co2 tank and regulator, use your multitool to adjust opening and swap the cans. Guarantee it will be easier and faster than conduit, splicing pigtails on in the old can after gutting it and routing to a new location. Buy a sub where you can install a transfer kit (Sq D has this available for both QO and Homeline). Leave the jury in the courtroom. But if you must, mount your new panel outside, inside chase a 4x8 or 10 and use a couple or more smaller conduits and limit your ampacity reduction with less fill per. Or--------

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