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#97795 - 03/21/06 02:21 PM even more derating fun
earlydean Offline
Member

Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
Section 310.15(B)(20(a) requires derating of more than 3 current carrying conductors in a cable or raceway.
Section 240.4(B) allows the next higher standard overcurrent device rating above the ampacity of the conductors being protected to be used.
Does that mean I can bundle 40 14/2 NM cables together for long distances, and connect each to a 15 amp cb?
80 current carrying conductors would require a 35% derating from 25 amps to 8.75 amps. Can I then bump the cb up to 15 amps because Section 240.6 gives me the lowest standard rated breaker as 15 amps?
Or, am I required to use a 10 amp fuse?
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#97796 - 03/21/06 02:28 PM Re: even more derating fun
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
IMO you could use the next standard size listed (15 amp) on any of the circuits that are not multioutlet branch circuits supplying receptacles for cord-and-plug-connected portable loads.

Of course the ampacity of the cables would still be only 8.75 amps and the connected load could not exceed that.
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Bob Badger
Construction & Maintenance Electrician
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#97797 - 03/21/06 02:57 PM Re: even more derating fun
earlydean Offline
Member

Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
So, if we combine my two posts, using luminaires that do not draw more than 2 amps, I could get away with that scenenario?

12 current carrying conductors require a deration of 50%, 50% of 25 aamps is 12.5 amps. Dividing by six luminaires, gives me 2.08333 amps each.
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#97798 - 03/21/06 03:15 PM Re: even more derating fun
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Cool questions.

IMO yes, 240.4(B) applies to the circuit as conditions (1), (2) and (3) are met and it tells us clearly "The next higher standard overcurrent device rating" a couple of times.

240.6 Standard Ampere Ratings shows the smallest standard rating as 15.

Bob
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Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#97799 - 03/21/06 03:47 PM Re: even more derating fun
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
I'm giving you guys a red sticker. Wording in the code is the "next higher standard overcurrent device" Not the next size circuit breaker. 10a. is the next size overcurrent device for earlydean's post. Guess someone is installing fuses on these circuits.
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#97800 - 03/21/06 04:27 PM Re: even more derating fun
Tom Offline
Member

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
I think that sticker should be pink instead of red, thereby giving partial credit. After all, Iwire did point out that going up to the next OCPD does not allow you to increase the ampacity (or load) of the conductors and he did correctly make the call about multioutlet circuits. These points seem to be lost on many, at least here in the Mountain State.
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#97801 - 03/21/06 04:44 PM Re: even more derating fun
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Interesting, I have to agree with you George, that is what it says.

That aside I am not convinced the NEC intends to force the use of a fuse over a breaker.

Bob

BTW I will happily take a RED tag over a pink tag.

Pink is the color of the spare hardhats for those that always misplace their own.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#97802 - 03/25/06 08:09 AM Re: even more derating fun
earlydean Offline
Member

Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
It takes a real manly man to wear a pink hardhat on a construction job!
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Earl

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