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#46219 - 12/16/04 04:48 AM Question about deration...
dmattox Offline
Member

Registered: 10/20/04
Posts: 267
Loc: Anaheim, CA
I keep getting different answers from different people so I thought I would ask you guys to get even more different answers

Going to try to keep the numbers easy in my example, so lets say we have 10 #12 THHN current carrying conductors in a conduit 25 inches long all protected by single pole 20 amp breakers.

So we start out in table 310.16 90 degree column since we are doing deration. #12 THHN at 90 degrees is 30 Amps.

Now from table 310.15(B)(2)(a), we must apply a 50% deration, so that drops us down to 15 Amps allowed.

Now for my question, does this mean that we need to have an over current protection not exceeding that, or only load that is not to exceed that?

Say for example we have a lighting circuit with a load of 5 amps, is this ok with at 20 amp breaker?

Another case, say we have 2 15A convenience outlets on one circuit, this one to me seems that it shouldn't be ok since someone could exceed the 15 amps for deration, but it would not trip the breaker.

The case that seems pretty clear to me that’s not allowed is a dedicated 20A outlet on a 20A circuit. Since it would be easy to exceed the deration?

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#46220 - 12/16/04 05:40 AM Re: Question about deration...
Steve Miller Offline
Member

Registered: 08/30/01
Posts: 322
Loc: Loudoun Cty, VA
Q: "does this mean that we need to have an over current protection not exceeding that, or only load that is not to exceed that?"
A: Since the wire is now only rated for 15A, the OCPD cannot exceed 15A. (that's why we try not to put the 10th current carrier in the pipe)
Q: "Say for example we have a lighting circuit with a load of 5 amps, is this ok with at 20 amp breaker?"
A: Sure, as long as the conductors are good for 20. In this case you have some extra capacity on that ckt for future use.
Q: "say we have 2 15A convenience outlets on one circuit, this one to me seems that it shouldn't be ok since someone could exceed the 15 amps for deration, but it would not trip the breaker."
A: If the derating lowered our ampacity to 15 and we put a 15A breaker on the ckt we're fine. If the derating lowered our ampacity to 15 and we put a 20 on it ... we have a prob. The 15A breaker will prevent the wires from carrying too much amperage and becoming overheated.
Q: "The case that seems pretty clear to me that’s not allowed is a dedicated 20A outlet on a 20A circuit. Since it would be easy to exceed the deration?"
A: If it is a true 20A ckt(meaning the conductors and breaker are rated 20 amps after all derating) you're ok with a single or duplex 20A receptacle. If it should be a 15A ckt after derating (even though it has #12 THHN) then a 20A receptacle is not allowed (but a 15A single or duplex is).

Hope this helps

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#46221 - 12/16/04 05:45 AM Re: Question about deration...
dmattox Offline
Member

Registered: 10/20/04
Posts: 267
Loc: Anaheim, CA
Thanks for the reply Steve.

Can you provide me with a code section that says a OCPD must match the derated value of the wire so I can finally put this to a rest?

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#46222 - 12/16/04 06:00 AM Re: Question about deration...
Steve Miller Offline
Member

Registered: 08/30/01
Posts: 322
Loc: Loudoun Cty, VA
310.15 B2a
"(2) Adjustment Factors.
(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where
single conductors or multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways, the allowable
ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a)."

"240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities"

Sorry if the cut & paste produced a lousy format job but these are quotes from the 2002 NEC.

The important part of the first one is "the allowable ampacity of each conductor". That means the ampacity after all derating factors are applied. That's just like trying to put a 60A breaker on a #14 THHN. The allowable ampacity of #14 is only 15A.

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#46223 - 12/16/04 06:01 AM Re: Question about deration...
Steve Miller Offline
Member

Registered: 08/30/01
Posts: 322
Loc: Loudoun Cty, VA
Good luck ... I gotta go to work now

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#46224 - 12/16/04 05:52 PM Re: Question about deration...
Peter Offline
Member

Registered: 06/08/04
Posts: 93
Loc: San Diego
You need a new tape measure.
"...a conduit 25 inches long..." surely you mean a 24" long conduit. Ever see an inspector with a measuring tape? Does 1" really matter?
Perhaps you should get an engineer to do the Neher-McGrath calculation for you.
Best regards,
~Peter

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#46225 - 12/16/04 06:43 PM Re: Question about deration...
dmattox Offline
Member

Registered: 10/20/04
Posts: 267
Loc: Anaheim, CA
I was making it clear it met the requirements of that section and wasnt a nipple, but I didnt want it to be so long that distance deration would become an issue

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#46226 - 12/16/04 06:46 PM Re: Question about deration...
dmattox Offline
Member

Registered: 10/20/04
Posts: 267
Loc: Anaheim, CA
Ah, ok, that makes sense. I think I now have the ammo to win a battle we have been having at the shop.

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#46227 - 12/20/04 10:26 PM Re: Question about deration...
Norstarr Offline
Member

Registered: 09/25/04
Posts: 89
Loc: Wi
Peter, Most of us knew what dmattox meant with the 25" length of conduit. In fact, I thought it was very clever and would be a neat trick question in a code quiz. And in some cities around me one inch does make a difference between a nipple and a run of conduit as silly as it seems. On a lighting canopy in a store the local inspector made us run another 20 amp circuit to the canopy because one of the lighting circuits listed 16.5 amps. Wouldn't think .5 amps would be such a deal either, especially on a lighting canopy that will continuosly be changed as fixtures are sold.
Ron

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