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So I am coming out of my subpanel with 1/2 in metal conduit and using metal handy boxes with NEMA 5-20 for all the receptacles on my two indoor branch circuits. One is dedicated for a window unit AC. All wires are 12AWG stranded copper THHN/THWN.

The ac receptacle is protected with AFCI and GFCI because part of the unit is outdoors. The other receptacles are protected with AFCI only.

So six conductors in all come out. At a t-type conduit body we split off L1/N/G and go up to a handy box near the window. L2/N/G continue on to the wall receptacles.

A separate N ang G run to each circuit. I chose to use a separate G for the two circuits in case the G to one circuit somehow comes loose. No splices are planned anywhere in this setup. All receptacles use the better clamp-type screw connectors the hold the wire captive between two plates.

Is this safe? Am I violating the fill calculation for the conduit body? I used an online calculator assuming 6 conductors pass thru box, and it yielded 13.5 cubic inches ... really? Is that much room really needed with no splices?

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You said metal conduit--1/2" EMT & rigid can have nine #12 thhn/thwn wires max fill per NEC annex C conduit fill.

The area of the conduit body would only be needed only if a splice is made in that location, so the derate for more than 3 current-carrying conductors would be the next concern. 4-6 conductors require a 80% derating but the higher ampacity 90 degree table can be used for derating and you still meet code because the derated value is 24 amps.

So this is safe for a 20 amp circuit(s).

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You can't splice in a conduit body. At all.

Other than that, conduit bodies don't require thinking about. If the number of wires coming in on each of the three inlets is within limits, then mathematically I don't see how you could overfill the conduit body.

Splicing is a good application for handy-boxes (the 2"x4" types) but watch your box fill. Handy-boxes fill very quickly especially if a yoke is involved.

What do I mean by "at all"? A conduit body is treated as conduit (with conduit fill rules per 314.16C1) if there are no splices. 314.16C2 says putting a splice in it makes it a box (with box-fill maths) and you're instantly out of space. Teeing off a 2-wire #14 circuit takes 12 cubic inches, already nearly the size of a Handi-Box, and you'll only find that many cube's in vastly oversized conduit. Does running a single #14 circuit in 1" conduit make sense? Absolutely not. Later, somebody will want to add a second circuit to that conduit, and boom, violation.

Grounds take space.

A nice feature of metal conduit is grounding is intrinsic to the conduit. You can run a green or bare ground wire if you really want to -- but it is redundant, and that wire counts as a wire for conduit-fill purposes (but not derate). And box fill.

As for box fill (314.16B) -- you're familiar with allocating one "wire unit" for all grounds inclusively? You don't have to do that if there aren't any ground wires.

  • Well, I wanted to run a green wire because the conduit could become separated, etc. and I wanted to guarantee a ground. I'm not doing any splices so that's cool. So it sounds like I can run the full 20A on both circuits. – Kerry Thomas Mar 15 '18 at 15:27
  • Also ... for the handy boxes I calculated 2 for the receptacle ... 2 for the hot going in and out ... 2 for the neutral going in & out ... 1 for the ground going in & out, total of 7, with 12 AWG that 2.25 * 7 = 15.75, and realized I would have to use 4" boxes for those in the middle. At the end of the of the chain I would 1 H/N/G in and the receptacle counts as two, at 5 * 2.25 = 11.25 so I would need at least a 3x2x2 1/2 handy box for that last one – Kerry Thomas Mar 15 '18 at 15:39
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    See NEC 314.16(C)(2), which talks about splices in conduit bodies. – Tester101 Mar 15 '18 at 19:59
  • @Tester101 Thanks for the ref. It seems to say what I'm saying. Splicing into a conduit body applies box c.i. rules, and you just won't have the cubic inches unless the conduit is insanely oversized for the wires. – Harper Mar 15 '18 at 22:58
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It's my understanding (and I could be wrong) that box fill only applies to conduit bodies, if you're making splices. Otherwise, you use conduit fill rules.

So if you're just passing through a conduit body, it should have the same fill limitations as the conduit coming into the CB.

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