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Old 08-06-2008, 09:05 PM   #1
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Default # of conductors in a box

Two coworkers of mine have told me that somewhere in the code book is a maximum number of conductors permitted in any size box. I have failed to find this. Article 314.16 covers box fill and nowhere in it is there a maxim number of conductors allowed no matter how large of a box you install. Article 310.15 (B)(2)(a) lists 41 or more current carrying conductors to be derated at 35%. This is in raceways however, not boxes. Article 100 does not define a raceway as a box. Annex C lists several hundred #12 ccc's in a 3 inch conduit. Again, not a box. However, if you are allowed so many current carrying conductors in conduits, why not boxes? I think this is the reason the inspector has limited the number of wires (specifically #12 thhn wires) per box, because if they were not running enough seperate conduits to the same box in order to keep the derating factor down (no more than 9 ccc's per pipe. because 30 amps X 70%=21 amps a 20 amp breaker can still be used) then they would not be at an allowable ampacity sufficient to install a 20amp ocd on the wire, which article 240.4 D says we have to for a piece of copper 12 awg. Any thoughts? Am I wrong? Is there an article that limits the # of ccc's in a box of any size, no matter how large? They said the number was 30 to 40, they couldn't remember exactly. Heck, we can install 80 circut panels now that sq D makes. No more limit to 42 spaces. Any help here?

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Old 08-06-2008, 09:35 PM   #2
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There is NO "maximum". You are on the right track with 314.16 for conductors up to #6.
For boxes not on the list on T314.16 you have to know the volume of the box and us the "Minimum Volume" column for the conductors.

314.16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies

Boxes and conduit bodies shall be of sufficient size to provide free space for all enclosed conductors. In no case shall the volume of the box, as calculated in 314.16(A), be less than the fill calculation as calculated in 314.16(B). The minimum volume for conduit bodies shall be as calculated in 314.16(C).
The provisions of this section shall not apply to terminal housings supplied with motors.

FPN: For volume requirements of motor terminal housings, see 430.12.

Boxes and conduit bodies enclosing conductors 4 AWG or larger shall also comply with the provisions of 314.28.

For conductors #4 and larger you have to go to:

314.28 Pull and Junction Boxes and Conduit Bodies

Boxes and conduit bodies used as pull or junction boxes shall comply with 314.28(A) through (D).

Exception: Terminal housings supplied with motors shall comply with the provisions of 430.12.

(A) Minimum Size For raceways containing conductors of 4 AWG or larger, and for cables containing conductors of 4 AWG or larger, the minimum dimensions of pull or junction boxes installed in a raceway or cable run shall comply with (A)(1) through (A)(3). Where an enclosure dimension is to be calculated based on the diameter of entering raceways, the diameter shall be the metric designator (trade size) expressed in the units of measurement employed.

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Old 08-06-2008, 09:42 PM   #3
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First: Welcome to the forum.

Second: When posting, break up your post so that it's not one long paragraph. This will help others when reading your post.

See T. 314.16(B) Metal Boxes
See T.314.16(B) Volume Allowance Per Conductor

"fill" refers to what type of wire, etc. can be put into the box legally. All boxes have a cubic inch rating on then. Use the cubic inch rating of the box to determine the number of allowable conductors.
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Old 08-07-2008, 06:22 AM   #4
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Default boxes

To my knowledge we do not derate wires because of the number of them in a box. If I am mistaken, please tell me.
I guess I am putting myself in the inspector's shoes and trying to come up with a reason to limit the number of wires in a box. These are all #12 wires, some mc and some thhn.

Last edited by michaeljr7249; 08-07-2008 at 06:25 AM.
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Old 08-07-2008, 06:59 AM   #5
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No, we do not derate because of box fill, and your inspector cannot simply institue his own rules on a whim.
The ONLY limit to box fill is the volume of the box. There is no magical "maximum number" for all boxes. Each one is different.
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