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What type and size of screw should I use to mount a 4”x4” metal electrical box to the back of the under the sink cabinet?

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For box mounting I use 2 #8 pan/ washer head screws. All the ones I have seen are Phillips or square drive #2, but there may be straight. Inspectors like the square head #2 because usually only electricians use these so it can help you pass inspection. At least 3/4” but 1” is the max I normally buy because the next size is usually 1.5” and that’s too long, especially if no drywall (don’t want to drive a screw through a cable).

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  • My answer would've been "almost anything". What criteria are you trying to meet with these specific suggestions? What are inspectors looking for? – isherwood Dec 4 '20 at 15:21
  • Not sure of your issue I stated, inspectors like the washer head because they won’t damage a plastic box (box type not listed). Also when they see a square drive the think electrician and may not be as big of a jerk to a home owner. I was hired to fix an entire house that the owner wired he used bugle head screws and cracked some of the boxes the inspector flagged all of them. I pulled the screws and installed washer head square drives. Several of his wires coming out of the boxes were 5.99” not kidding I pulled the slack for 6”. Owner told me he inspected 1 box saw square drive and signed. – Ed Beal Dec 4 '20 at 15:40
  • To be fair, you didn't mention box damage in your answer. That makes sense. – isherwood Dec 4 '20 at 15:42
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Driving that screw is a pretty simple task and you could make due with anything close to suitable - something in the size 8-10 range, at least 1/2" long but not so long it goes through the cabinetry into who knows what, hopefully threads coarse enough to hold well in manufactured wood-like substrates. Ideally a washer head but a pan head will do, a tapered flat head won't sit right and will leave a little sharp bit exposed.

Working inside a box, inside a cabinet, half lying on the floor, is pretty awkward. A slotted screw will be a real pain, a phillips screw will be better but could still be difficult. A screw that stays on the driver well will help a lot. You could use a hex-washer head screw, and a nutdriver with a strong magnet. My preference is a square drive screw, they stay on very well for me. Of course, using a power tool rather than a hand driver will also make things easier.

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