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I'm ordering some quick-disconnect boxes.

How come almost every AC quick disconnect boxes I find seem to have raised bumps on the back of them that would prevent me from mounting it flat against the wall?

For example, this box: https://a.co/d/7LZip6L

enter image description here

This box has the two screwhole wings on the side, and a big bump on the bottom of the back, which seems like they would add a 1/8" gap or more between the box and the wall?

I was hoping to drill through the wall, and enter directly into the back of the disconnect box via one of the back knockouts, anf caulk all around the border of the box where it meets the wall.

These bumps mostly prevent that. How should I then use the knockout? Run pipe through the wall, romex to the knockout?

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3 Answers 3

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Another reason is that there are threaded holes pre-drilled in the box for mounting auxiliary equipment such as a ground lug, and that bump allows you to run a threaded bolt a few turns through the mounting hole so you are able to properly tighten the machine screw. Notice that there are two screws already coming through on you picture.

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  • That makes alot of sense.
    – Jamin Grey
    Dec 21, 2022 at 5:04
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To avoid interfering with the drainage plane of the wall or trapping water behind the box, for that matter

The reason for the stamped-in "standoffs" on the back of NEMA 3R enclosures is so that they neither trap water against the mounting surface nor interfere with the ability of their mounting surface to serve as a drainage plane. Without them, you'd get perched water at the top and/or trapped water behind the box, both of which would cause serious damage if the water-resistive barrier that forms the drainage plane has any imperfections in it.

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  • Should I caulk on both sides and the top, but leave the bottom open to drain, or should I not caulk at all? I will still caulk the pipe coming into the back and perhaps where the screws connect to the house wall.
    – Jamin Grey
    Dec 21, 2022 at 5:03
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One reason: Many walls or wall finishes are not perfectly flat. If they have small bumps in the back it makes the box easier to mount on some types of surfaces without it being a significant downside on a flat surface.

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    It also allows for moisture to escape and not rot the wall. Dec 20, 2022 at 9:43
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    One tutorial pointed out I should only caulk the top and both sides, allowing any trapped water to drain out the bottom. Good idea.
    – Jamin Grey
    Dec 20, 2022 at 15:01

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