Wiring Branch Circuit and Box Fill

mark_anderson_usDecember 1, 2010

Hi All

About to embark on rewire of first floor of my house and much of second floor while I can EMT between floors (don't ask: chicago).

I'll be running 3/4 EMT in basement with stub ups to outlets. There are some locations where I could feed a good lenght of EMT through stud wall (near door openings) and I wanted to check that the porposed wiring plan was OK and if there are any special calculations of the proposed plan. Below is a link to the plan.

http://www.anderson-mark.com/Photos/images/OutletWiring.jpg

Is there any issue branching off to 3 receptacles and then branching off to another (or another run of 3)?

Box fill

I want to check I'm interpreting the NEC box fill correctly. If I have 12/2 that passes through unbroken it counts as two. If another 12/2 comes in, is pigtailed to an outlet and then exits too (i.e. it branches like the two in the basement in above diagram), it counts as 6. All grounds count as 1. Do the cable clamps where the EMT enters the box count as 1 as well?

Thanks

mark

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brickeyee

"Do the cable clamps where the EMT enters the box count as 1 as well?"

Only internal box cable clamps count (used for AC and NM) and if you are using EMT you should not have any.

The EMT to box fastener is external (the lock nut and small protrusion into the box do NOT count).

    Bookmark   December 1, 2010 at 9:37AM
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terribletom

Just curious...

To follow up on Brick's comment: Since you mention "12/2" and cable clamps, are you pulling cables in the EMT or single conductors (e.g., THHN)? If you're using NM (a.k.a. "Romex") or MC/AC (a.k.a. "BX"), why?

    Bookmark   December 1, 2010 at 12:14PM
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mark_anderson_us

Thanks for the replies guys. I'm pulling single THHN in EMT. Has to be rigid for local code.

Regards

Mark

    Bookmark   December 1, 2010 at 4:20PM
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joed

Your using EMT there will be no cable clamps.
Yes it counts as 6.

If you pull more than 9 wires in any conduit you need to derate them.

    Bookmark   December 2, 2010 at 8:48AM
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groundrod

joed
It is simply not true that: If you pull more than 9 wires in any conduit you need to derate them. there is way too much electrical misinformation out there already, so please stop saying things that are just not true.

    Bookmark   December 2, 2010 at 9:21AM
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ecranny

I have an additional question about box fill calculations. In the OP's example - 12/2 passing through unbroken, 12/2 entering, pigtailed to outlet and then exiting - would the count be 9 in this case? (A single yoke receptacle counting as 2, ground as 1)

Would this be the maximum for a 4 x 1.5 square box?

    Bookmark   December 2, 2010 at 11:59AM
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btharmy

"joed
It is simply not true that: If you pull more than 9 wires in any conduit you need to derate them. there is way too much electrical misinformation out there already, so please stop saying things that are just not true."

That's a whole other post for another day.

    Bookmark   December 2, 2010 at 4:36PM
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terribletom

"Would this be the maximum for a 4 x 1.5 square box?"

Referring to NEC Table 314.16(A), the volume of a trade size 4 x 4 x 1.5 box is listed as being only 21 cubic inches. 9 units (wires + fixtures) at #12AWG requires 20.25 ci so, yeah, it's basically full.

    Bookmark   December 2, 2010 at 5:44PM
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DavidR

Mark, this question and many more are answered in the classic Wiring Simplified. If you don't have a copy, I suggest getting one right away. It helped me get started many years ago, and I still refer to it today.

As usual, the link is for reference and not an endorsement. Home Depot used to carry this book and may still.

Here is a link that might be useful: Wiring Simplified

    Bookmark   December 2, 2010 at 8:27PM
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