Installing a new light at base of stairs to lower level. Old box is a round fan type and new light is a square mounting plate. Can I make this work or is there a need to change the old junction box?
Electrical – Round junction box – new light has square mounting plate
electrical
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The base you show is ~1" and page 8 here: http://www.farnell.com/datasheets/722224.pdf Shows a standard relay at just over 2".
It should fit just fine in a 4" deep screw cover junction box if you mount the base to the bottom of the box. Don't mount it to the cover.
However, if you plan to use a time delay relay or some other type then check the dimensions before you buy your jbox.
Depending on your application, you might consider a solid state relay such as this: http://www.amazon.com/AC-AC-80-280VAC-24-380VAC-SSR-40AA-Solid/dp/B00976EJXC/ref=sr_1_1?ie=UTF8&qid=1447848272&sr=8-1&keywords=AC+relay
It is only 1.5" high so could be mounted in a 2 ⅛" deep 4" or 4-11/16 box. Happy Day!
Why not use a standard round junction box? They come in several widths and depths, for example
If the space is very shallow, there are round pancake boxes as well
Two safety issues:
- 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.
- 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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Best Answer
Generally it's either no problem at all - there are mounting holes that match, and the box will be covered, or you need some sort of adapter to go from the holes you have to the holes you need - which MOSTLY only works if the holes in the box are further apart than the holes in the device.