The wiring for a bathroom light and a receptacle are in a 3.5 x 1-1/2 in. octagon electrical box. Is this the correct size for this number of wires? There are 3 blacks, 3 whites, 1 red, and 3 grounds. It's a 15 amp circuit. I'm not sure whether the gauge is 14 or 12. The wires in the bushing on the side of the box are Romex. The other wire is 50 yrs old. The house is in the USA. I've read some fill charts on-line, but I don't understand them as there are various increases/decreases in the conductor count for certain situations.

Correction: The box is 3.5 in., not 4 in.

enter image description here

  • What devices are actually mounted into that junction box? May 15, 2017 at 1:37
  • @Laxmidi I added a box fill calculator link and a Succinct PDF link to my answer but Harper has the more detailed info and in the comments there I posted a link to more clarified information. There is plenty depending on what is mounted on that box for example a dome cover provides more volume, I think you will be fine with what you have, the wires that are 50 yrs old are probably 12 AWG if you do not have a gauge - check to see if the bare wire will insert into a brand new outlet push fit wire hole you will know right away.
    – Ken
    May 15, 2017 at 18:13
  • @ThreePhaseEel There's one 2-bulb light fixture and a GFCI outlet.
    – Laxmidi
    May 16, 2017 at 2:06
  • @Laxmidi -- o.O how are the devices even mounted to the box? You can't get a standard duplex yoke into a 3.5" octagon to begin with (there's no way to mount it) May 16, 2017 at 2:07
  • 1
    @ThreePhaseEel I was wrong. OP might be wrong too. The box I fit a receptacle in, is called a 4” octagon box. Those are 3-1/2 in the square sides and scant of 4" corner to corner. There is such a thing as a 3-1/2 box, but it's freakysmall and again a scant 3-1/2 corner-corner and only 3" side-side. May 17, 2017 at 17:01

2 Answers 2


A random check of a manufacturer shows 3-1/2 x 1-1/2 octagon boxes are 11.8 cubic inches. It may also be stamped on your box. You need:

  • 1 wire unit for all the grounds
  • 4 wire units for your hots
  • 3 wire units for your neutrals
  • 1 wire unit for the internal clamp
  • 2 wire units if a receptacle is inset into that box. If the item is surface mount, 0 wire units.

Total 8 wire units. A wire unit takes 2.0 cubic inches for 14AWG and 2.25 cubic inches for 12 AWG. So that's 18 or 20.25 cubic inches. Your box has 11.8?

You have a few options.

  • Add a box extension, a 1-1/2" extension would solve your space problem. Nobody cares which holes the wires come in, I never use knockouts in extensions because then it's hard to remove.
  • Put a second nearby box to offload some of the wires.
  • Convert to a 4" square box (21 cu. in.) with a mud ring (? cu. in.)giving a face similar to a 3-1/2" octagon box. This may also help you address your depth problem mentioned in the other post. It's ridiculously easy to change boxes since all your wires are flexible and terminate in the box. (by "ridiculously" I'm contrasting it with thru wires which must be pulled, steel conduit, substituting a wider box so you have to cut the conduit, and doing it 21 feet in the air in a boom lift!)
  • Thanks for the excellent info, Harper. I gave incorrect box size.. The electrical octagonal box is actually 3.5 in., not 4 in. If I add a 1.5 in. extension ring to the 3.5 in. octagonal box, will that be sufficient? Do I need to plug the side knock-out in the original box and use the extension's knock-out. Or can I simply use the old holes with the extension bolted on the front?
    – Laxmidi
    May 15, 2017 at 15:09
  • @Harper from my understanding the code requirements for wire spacing is not specific to receptacle and luminaire boxes as they are not considered junction boxes.
    – Ken
    May 15, 2017 at 17:02
  • @Ken I have to admit, that would make a lot of sense, because it's real common to have 2 in, 2 out 2 yoke and a ground in a 3.5" octagon box, not so different a situation than OP. I just wish I could get a cite on that. Just that the appearance here is that he's using it as a junction box owing to the third cable coming in. May 15, 2017 at 17:08
  • 2
    Box sizes can also be found in the NEC table 314.16.a. My question to OP. Is that a hickey in the box (a device that attaches the fixture to the box) if it is 1 wire unit needs to be added to the calculation.
    – Ed Beal
    May 15, 2017 at 17:25
  • @Harper - I searched but could not find anything specific only mentions that implied the luminaire was not counted the same. Here I found a link and it seems to be accurate and detailed enough. ecmweb.com/code-basics/box-fill-calculations his 3.5 inch box - I have seen just like that. Hate to say it - I have seen 3 unit boxes with 3 way switches in them and power lines - tight fit but I guess inspectors are either not required in some places or do not pay it any attention.
    – Ken
    May 15, 2017 at 17:32

That many conductors is fine - you have no issues with that. Your load ratings on that box are important. You are reworking it - so it will need to be in code.

EDIT 5-15-2016 Something else that might be helpful a box fill calculator

PDF Link for a nice short quick reference.

The following paragraphs of text Were taken from here >

Outlet Boxes

The 2008 revision of the NEC significantly changed 314.27. Significant changes often bring numerous errors. The 2011 revision corrects an error in the previous NEC by requiring that boxes installed in a wall for luminaire support must be marked on the interior of the box to indicate the weight ratings of the box. The 2011 revision also: breaks up the requirements into a list format, for easier reading; incorporates the provisions for lampholders into the title of the rule; and revises the luminaire box ratings for clarity and ease of reading. Ceiling Fan Boxes

Outlet boxes used as the sole support of a ceiling-suspended (paddle) fan must be listed and marked as suitable for this purpose. If the maximum weight isn’t marked on the box, it is allowed to support a fan up to 35 lb. If the box is marked with a weight, it can support a fan up to that weight, but not more than 70 lb. Ceiling paddle fans more than 70 lb must be supported independently of the outlet box [314.27(C)].

Occasionally, an electrician may install an extra unused switched conductor in a luminaire outlet box installed in the ceiling. While this may seem like a nice feature — because it allows for a future ceiling fan — it’s a Code violation when the ceiling fan is finally installed, unless the box is listed to support a ceiling fan. This change was accepted after initial reluctance, because the NEC isn’t in the habit of dealing with future violations, no matter how likely they might be.

The result of this change in 314.27(C) is that such an installation must incorporate a box that’s suitable for ceiling fan support, despite the fact that a ceiling fan is not installed. Considering the effort required to install the separate, independently switched conductor, this change does not add much of a burden to the electrical community. However, it does add an enforceable requirement for AHJs who did not have one before.

Where spare, separately switched, ungrounded conductors are provided to a ceiling-mounted outlet box in a location acceptable for a ceiling-suspended (paddle) fan in a dwelling unit, the outlet box must be listed for the support of a ceiling-suspended (paddle) fan [314.27(C)] (click here to see Fig. 1).

  • Downvoter, care to comment? I'd like to learn something here.
    – isherwood
    May 15, 2017 at 14:55
  • @isherwood - I certainly would like to know why the down vote - if they do not like my answer - fine. But worthy of a downvote I do not think so. Maybe it would be good to track downvoters in the meta and allow voting on them - if someone is just a drive by downvoter .. remove their ability. They are not adding value to the site.
    – Ken
    May 15, 2017 at 16:58
  • 2
    I did not down vote but as Harper shows above the size of the box for wire fill is not fine.
    – Ed Beal
    May 15, 2017 at 17:18
  • The down vote is because this answer starts out with incorrect information, then goes on to quote a bunch of irrelevant text. Here is the relevant text from the NEC "314.27(A)(1) Vertical Surface Outlets. Boxes used at luminaire or lampholder outlets in or on a vertical surface shall be identified and marked on the interior of the box to indicate the maximum weight of the luminaire that is permitted to be supported by the box if other than 23 kg (50 lb).". So as you can see, as long as the fixture is less than 50 lbs, there's no problem attaching a light fixture.
    – Tester101
    May 16, 2017 at 16:16
  • @Tester101 I included links in my edit of the answer the other day. Perhaps I am wrong on the interpretation.
    – Ken
    May 16, 2017 at 22:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.