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I’m looking to add a new fixture to a masonry ceiling of the living room an open floor plan home.

Current Setup

The existing electrical raceways to all ceiling fixtures are done with exposed 1/2” conduit mounted directly to the masonry ceiling. The dining area, which is open to the living area, already has a track lighting fixture with a raceway running to it. This raceway originates in a standard 4” square junction box in a hallway wall and emerges from the wall at ceiling level where it then extends 10-15’ to the fixture. The junction box contains a switch for the track lighting fixture, a switch for a wall outlet and some other spliced connections for non-switched outlets. All of these circuits (switched light, switched outlet, non-stitched outlets) are on the same 15 amp breaker.

Planned Addition

The new fixture would be added to this same 15 amp branch circuit and would be wired to the existing switch in the junction box that’s now used for an outlet (the outlet would just be moved to the common hot and become non-switched). I would prefer not to run new conduit through any finished walls and am instead hoping to leverage the existing raceway of the dining area’s track lighting to get the new fixture’s wiring from the junction box up to the ceiling. I would then use a simple T-style conduit body to split-off a new section of raceway toward the new fixture. This T-style conduit body would be necessary as the existing track lighting does not have its own junction box to branch from (the conduit just connects directly to the track).


This seems perfectly logical, but I don’t think I’ve seen it done like this anywhere else. All the other raceways in the home appear to be dedicated to a single switched circuit, even if it means running lots of conduit from the same branch circuit physically in parallel over large distances. Because of this I’d like to know if there is a NEC-based reason not “share” and “branch” a raceway like this, specifically with a conduit body as opposed to a new junction box. Maybe I don’t see it anywhere else because it just doesn’t “look” as nice when the raceways are exposed?

I’ve confirmed that:

  • Adding the new fixture will not mean exceeding the recommended 1440W limit for the 15 amp branch circuit.
  • Sharing part of the raceway (4x14 AWG conductors — 2 hot and 2 neutrals) would come nowhere near the 1/2” conduit fill capacity. Derating is also not a factor AFAIK.
  • There would be no splices in the T-style conduit body (I would pull separate neutrals for each fixture), so its fill capacity would be the same as the raceway, and therefore well within limits.
  • The bend radius allowance for a standard off-the-shelf 1/2” T-style conduit body with 14 AWG wire appears to be well within limits.
  • The box fill for the existing junction box would still be within allowed limits, even with these minor changes.

Are there any factors that I’m missing, or is this actually a very common setup?

  • You are not overloading the circuit or exceeding the fill these are the most common problem's I find. Sounds like you have done your homework. Sometimes I run conduit into fixtures sometimes boxes it is legal both ways. – Ed Beal Sep 8 '16 at 16:16
  • Thanks Ed. So it sounds like conductors to different switched fixtures can share a raceway without issue. I'm still at a bit of loss as to why the original contractor seemed to go to great lengths to avoid doing this (they placed lots of parallel conduit where it would seemingly not be needed). Do you think it could have simply been to avoid the "negative aesthetics" of having exposed/visible junction boxes or conduit bodies on a ceiling? – rjacobs Sep 8 '16 at 22:42
  • That may be the wire size derate can play a part when more than 3 current carying conductors are in a pipe for more than 24" then the wire size needs to be increased. – Ed Beal Sep 8 '16 at 22:46
  • My understanding was that in standard temp environments #14 AWG is actually rated higher than 15 amps, so its already derated for >3 conductors on a 15 amp branch circuit. It actually looks like I could go to 9 conductors in a raceway before derating necessitates <15 amps. I suppose I could pull #12 AWG for this "shared" section of raceway instead to be extra safe though. I hope I'm getting that right. – rjacobs Sep 9 '16 at 4:18
  • Code rates 14 awg wire for 15 amps and the normal load is supposed to be 80% of that or 12 amps. 12 awg is rated for 20 amps and the load at 16 amps the ampacity ratings can be found in tables 310.15.B.16 not more than 3 current carrying conductors. Adjustments for more than 3 are in table 310.15.B.3.A. . 4-6 current carrying conductors require 80% derate. – Ed Beal Sep 9 '16 at 13:00
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The discussion in the comments revealed that the "planned addition" as described is perfectly fine but that the only detail that really needed additional confirmation was the conduit fill calculations, specifically regarding derating.

Since I'm using a T-style conduit body to branch a raceway for two fixtures there is not room to have a code-compliant splice between the common junction box and the fixtures themselves. This then means that separate neutrals need to be pulled and that there will therefore be more than 3 conductors in the shared part of the raceway. I had initially read that derating didn't come into effect for small wires (#14 and #12) until a much higher number of conductors are involved, but it was a good exercise to truly confirm this.

The NEC requires derating for 3-or-more conductors in one raceway. My initial impression for #14 THHN was that it's rated to only 15 amps, which would then mean that any derating requirements could be an issue for this 15 amp branch circuit. Looking deeper tough it turns out that 15 amps is only the required overcurrnet protection for circuits with #14 wire (from NEC from 240.4(D)(4)) and that #14 THHN is actually rated slightly above 15 amps at low temperatures. In my case I needed to derate 80% (for 4-6 conductors), and for #14 THHN at low temps this worked out to 80% of 25 amps = 20 amps, which still appears quite fine for the existing 15 amp branch circuit that I'm working with. This handy online tool, and of course the appropriate tables from the NEC (from 310.15(B)(16)), confirmed this.

So it looks like this planned addition is fine as stated assuming a wire gauge <= #14.

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