In am working on a 1920s era home. The electrical system in the house has been updated several times in a very hodge-podge way. Originally the house had conduit in the attic going from light receptacle to light receptacle. At a later date, romex was run from those light receptacles to plugs in the walls. Now I am replacing all the old rubber and cloth wire because it is disintegrated.

I recently checked the conductor limits for these boxes in the NEC and found that nearly every fixture is way over the limit. For example, the bathroom fixture has 13 "wires" (a mix of 12 and 14 gauge!) in it and each box is only a 4.5"x1.5" round box. The wire limit for this size box is 5.

The lights in use before I removed them were the type that are flush with the ceiling and thus didn't allow for much extra room. I am wondering if the light fixture size changes the number of allowed wires. For example, if the light fixture has a big ceiling mounted box like this one:

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Does that change the allowable wire limit?

And, if not, is the only answer to get rid of the current boxes and put in much bigger ones? Is there a better way of addressing this issue?

  • 1
    Have you done the box-fill calculations according to NEC (2014): 314.16? It allows you to include the "domed cover" if its volume is marked, as well as any listed/labeled "extension ring" you may add to the existing outlet box (assuming there is one). Also, you need only count a single "grounding" or "bonding" conductor -- the largest one -- no matter how many enter the box. There is no shame in replacing or adding new boxes to better conform to later codes. You need to impress your inspector.
    – Upnorth
    Aug 11, 2017 at 4:33
  • I used the NEC tables to do the calcs, but missed the part about domed luminaires. I did ground the grounds the way you say. Looks like I got to get some bigger boxes!
    – Murenrb
    Aug 11, 2017 at 12:54

1 Answer 1


Here is a Code language excerpt for your situation:

314.16(B) Box Fill Calculations. The volumes in paragraphs 314.16(B)(1) through (B)(5), as applicable, shall be added together. No allowance shall be required for small fittings such as locknuts and bushings.

(1) Conductor Fill. Each conductor that originates outside the box and terminates or is spliced within the box shall be counted once, and each conductor that passes through the box without splice or termination shall be counted once. Each loop or coil of unbroken conductor not less than twice the minimum length required for free conductors in 300.14 shall be counted twice. The conductor fill shall be calculated using Table 314.16(B). A conductor, no part of which leaves the box, shall not be counted.

Exception: An equipment grounding conductor or conductors or not over four fixture wires smaller than 14 AWG, or both, shall be permitted to be omitted from the calculations where they enter a box from a domed luminaire or similar canopy and terminate within that box.

So, the light fixture you have in your post would qualify as a domed luminaire and the wires from the fixture itself can be omitted from the calculation if there are only 4 or less. However, this does not add volume to the box calculation itself.

Unfortunately, junction box overfill is probably the number one violation of the Code.

Good luck and stay safe!

  • I was pretty sure this was the case. I guess with these old houses the only way to continue using the existing conduit is to upgrade every box that they run too. Right now given their size you cannot even run a light with a switch let alone handle all the different cables folks have added through the years.
    – Murenrb
    Aug 11, 2017 at 12:45
  • Alternatively, if you have easy attic (or other hidden) access, you can re-run everything to one or two large boxes and distribute a single pair to each fixture.
    – Upnorth
    Aug 11, 2017 at 15:16
  • You could also add more junction boxes in the attic, with fewer wires in each box. Buy the large, deep boxes that have the most room which will increase the allowed box fill, addressing the box fill concern in two ways.
    – user4302
    Aug 11, 2017 at 15:22
  • @ archonOSX, follow up question that I acknowledge is just looking for your opinion. I see several ways to fix this: 1) ditch the conduit and run romex from new larger boxes that are located right next to the ceiling fixtures that would now just have 1 14awg romex running to each. 2) replace all the ceiling fixtures with large 4-11/16 x 2-1/8 squares and reuse the conduit. Just not sure what is going to easier to do. Im fine with your off-the-cuff suggestion.
    – Murenrb
    Aug 11, 2017 at 16:00
  • Sounds like the romex idea would be easiest. If you change the boxes to 4 11/16 then the conduit will all have to be cut and re-fit. But that may not be too bad if it's just a few.
    – ArchonOSX
    Aug 11, 2017 at 16:16

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