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#36189 - 03/30/04 09:25 PM when do we need to derate conductor ampacity?
Edward Offline
Member

Registered: 12/14/02
Posts: 309
Loc: California
NEC states if you have more than three current carrying conductors in a raceway you must derate the allowable ampacity of the conductors.
Does this apply to current carrying conductors for continuous loads?

Edward
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#36190 - 03/31/04 12:17 AM Re: when do we need to derate conductor ampacity?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
It applies to all conductors.

And not just in raceways, cables too.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#36191 - 03/31/04 03:53 AM Re: when do we need to derate conductor ampacity?
electure Offline

Member

Registered: 12/24/00
Posts: 4226
Loc: Fullerton, CA USA
 Quote:
It applies to all conductors


Almost.

Exceptions to this would be in auxiliary metallic gutters 366.6(A) or in metallic wireways 376.22, where up to 30 conductors in a cross section do not need to be derated.

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#36192 - 03/31/04 12:56 PM Re: when do we need to derate conductor ampacity?
CDN_ELECTRICIAN Offline
Member

Registered: 01/22/03
Posts: 75
Loc: Concord,Ontario,Canada
..... and control wiring.

had to throw that in. Had a somebody once tell me my 2" conduit full of #14 control wires would not pass code requirements.

Out came the "BOOK"!!!!

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#36193 - 03/31/04 02:56 PM Re: when do we need to derate conductor ampacity?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
OK Scott caught me, kind of, derating still applies to these conductors at 30 instead of 3 , but CDN can you show a code reference for your statement?

Embarrassed Bob.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#36194 - 03/31/04 07:20 PM Re: when do we need to derate conductor ampacity?
Attic Rat Offline
Member

Registered: 12/14/03
Posts: 530
Loc: Bergen Co.,N.J. USA
..Also no derating for conductors in pipe nipples not EXCEEDING 24"
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#36195 - 04/01/04 01:49 AM Re: when do we need to derate conductor ampacity?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
When will I learn to not use terms like "never" and "always" when talking about the NEC.

And CDN I see the exception you must have been talking about.

OK

 Quote:
Exception No. 1: Where conductors of different systems, as provided in 300.3, are installed in a common raceway or cable, the derating factors shown in Table 310.15(B)(2)(a) shall apply to the number of power and lighting conductors only (Articles 210, 215, 220, and 230).

Exception No. 2: For conductors installed in cable trays, the provisions of 392.11 shall apply.

Exception No. 3: Derating factors shall not apply to conductors in nipples having a length not exceeding 600 mm (24 in.).

Exception No. 4: Derating factors shall not apply to underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of rigid metal conduit, intermediate metal conduit, or rigid nonmetallic conduit having a length not exceeding 3.05 m (10 ft) and if the number of conductors does not exceed four.

Exception No. 5: Adjustment factors shall not apply to Type AC cable or to Type MC cable without an overall outer jacket under the following conditions:

(a)Each cable has not more than three current-carrying conductors.

(b)The conductors are 12 AWG copper.

(c)Not more than 20 current-carrying conductors are bundled, stacked, or supported on “bridle rings.”

A 60 percent adjustment factor shall be applied where the current-carrying conductors in these cables that are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing exceeds 20.


You guys are tough!

Thats good, keep it up.

Still Embarrassed, Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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