Ceiling electrical boxes come in different depths: 0.5", 1.5", 2.25", 2.75" seem to be fairly common. I'm going to be hanging some lights as well as ceiling fans. I want to have the boxes as shallow as possible in order to keep the service chase shallow.

How do I determine how deep of boxes I need? I'm looking at 1.5" deep boxes -- in what situations would I need to use boxes deeper than that?

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    I will say, the difference between a 1/2" deep pancake box and a 2"+ deep box will have no effect on insulation. Think about the tiny amount of space a box occupies. ... Now, cutting many 6" recessed lights, that's a different story. .... Personally I think you are looking for a solution to a problem that doesn't exist. Jan 2 '16 at 17:52
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    I edited my question to better reflect the situation. Insulation has part to do with it but the main constraint is because the boxes will be in a service chase which I want as shallow as possible so as to maximize headroom...the whole reason would require more explanation and isn't really relevant to the question...suffice it to say, shallower boxes would be more useful so I don't want to just use the deep ones automatically (but of course want to follow code)
    – Nick
    Jan 2 '16 at 18:06

The number of wires in your box determines the size of the electrical box you need. I personally find 1.5" boxes rather shallow for a fan install since you are often hiding some of the fan wires in the box - this is a bit brand dependent though.

Also the depth of your box shouldn't effect insulation. We are talking about the difference of an inch. The insulation will have a little hump and will perform the same.

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    Nice link, I saved it. I need to add that fan boxes by code have bolts included in the box for hanging the fan. There are stories where the fans have dropped using standard octagon boxes.
    – Jack
    Jan 2 '16 at 19:56
  • @Jack - you need a fan brace or toggle through a cross framing. They have kits in big box in a variety of sizes. I usually pick the biggest.
    – DMoore
    Jan 3 '16 at 4:28

When there's going to be more in the box, you'll want a bigger / deeper box. In most places in the US the National Electrical Code (NEC) requirements for box fill must be followed. The NEC details a system for determining how big of a box (volume) you need for your wiring, depending on how many wires and devices are going to be terminated in that box. The main reason for this is to limit heating inside the box - when you consider that, the insulation is probably less of a concern.

In most cases using boxes smaller than the code requires would make wiring and installing the devices much more difficult. I'd never trade a tiny bit of insulation for better, code compliant wiring.

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