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I am installing a new light fixture into ceiling. There is a round hole there right now (from a recessed light that I removed).

The problem is this round hole is just short in diameter of standard junctions boxes (octagonal ones). A rectangular junction box will fit through the hole however.

The question is: are there any code regulations that I cannot have a rectangular junction box in a round hole?

I am in Ontario, Canada.

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  • Simply cutting a larger hole is easier said than done: there is a metal sheet on the top of ceiling board.
    – Slav
    Commented Mar 5, 2016 at 15:27
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    If you install a typical rectangular junction box (as used for e.g. wall sockets) you may find the mounting holes are not in the right spots for your fixtures. Ceiling boxes have different screw-hole locations than wall boxes.
    – Shimon Rura
    Commented Mar 7, 2016 at 21:20

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Why not use a standard round junction box? They come in several widths and depths, for example

round ceiling box

If the space is very shallow, there are round pancake boxes as well

pancake box

Two safety issues:

  1. It is imperative that a fixture be solidly mounted, so, if it attaches to the box itself, the box must be attached to framing members or an adequate brace.
  2. Small round boxes have limited space, so you must be careful about overfilling them with wires and attachment devices. If there is a single cable with two or three wires, you will usually be okay, but pancakes generally cannot handle several cables.

SUPPLEMENT: As to the question about a square box in a round hole, NEC Section 314.21 says

Repairing Noncombustible Surfaces. Noncombustible surfaces that are broken or incomplete around boxes employing a flush type cover or faceplate shall be repaired so that there will be no gaps or open spaces greater than 3mm (1/8 in.) at the edge of the box.

Section 314.19 also says that flush fixtures have to be have to be completely enclosed at back and sides, so gaps around a box behind the fixture would probably be considered to violate that as well.

That said, it is routine to apply spackle and other noncombustible fillers around junction boxes that have gaps where they meet the wall. But it is critical to ensure that whatever box is used, it is securely mounted to underlying structure.

Finally, cites are to NEC which may or may not be applicable in your jurisdiction, but is generally viewed in North America as standard practice.

Images and links are for illustration only, not an endorsement of goods or sources.

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  • All I have here is Home Depot and Lowes (Canada). None of them had those.
    – Slav
    Commented Mar 5, 2016 at 20:21
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    @Slav Both of the illustrated items are listed at Home Depot. Any electrical supply store would have something similar.
    – bib
    Commented Mar 5, 2016 at 20:27
  • Not in Canada: homedepot.ca/en/home/…
    – Slav
    Commented Mar 6, 2016 at 14:43
  • My browser says the fan pancake type are available in Ontario, Upland and Rancho Cucamonga (zip 91758).
    – bib
    Commented Mar 6, 2016 at 15:31
  • I am in Ontario Canada, not Ontario USA, but thanks for checking. Even then, it still doesn't answer my original question: are there regulations that say the hole in wall must match the shape of junction box?
    – Slav
    Commented Mar 7, 2016 at 15:40

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