This guy is talking about resilient channel while he is playing with another thing, a hollow box pointed out by red arrow. What is that?

enter image description here

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    These all seem like very basic questions. Typically questions on this site show some basic understanding and ask to solve a problem or how to complete a task. If you look at this help page, good questions should inspire detailed answers. If your questions are getting simple yes/no answers or a one line identification, its not a quality question.
    – JPhi1618
    Jan 30, 2020 at 16:12
  • @JPhi1618 Would please give a link to somewhere else for such very basic questions? or tutorials for fresh new guy?
    – zghqh
    Jan 31, 2020 at 0:04

2 Answers 2


That's an electrical junction box. Likely for a ceiling light, but could also be for a ceiling fan, smoke detector, or anything else mounted on the ceiling that needs power attached.

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    The box is surface-mounted on the joist because the channel and furring strips bring the finished ceiling down somewhat. Normally you'd see side-mounted boxes or boxes on integrated brackets.
    – isherwood
    Jan 30, 2020 at 15:59
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    @isherwood So, side-mounted boxes or boxes on integrated brackets are more common than the one in OP, right?
    – zghqh
    Jan 31, 2020 at 0:11
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    @zghqh a typical use case would only have 1/2" of drywall, maybe 5/8". Either way, surface mounting wouldn't be flush with that style box. They make thin boxes for special cases, but most of these are side mounted, which is preferable for adequate space for wiring in the box.
    – BMitch
    Jan 31, 2020 at 0:33

That is a 4" deep Octagon Box.

It is a standard electrical junction box for mounting lights and other ceilingey things. (not a fan; that takes a reinforced box). Note the single Romex (NM-B) cable entering it.

The person is applying a junction box extension ring. They're modestly expensive, especially since, as an obscure item, the big-box stores will be wildly overpriced (for some reason, they do not specialize in big boxes). Regardless, it isn't necessary. A junction box that easy to remove could simply be removed and remounted with a spacer behind it. Use a slice of the very same stuff being used to thicken the ceiling; solved. (as it happens, drywall is a good firestop, which just goes to prove, nothing is completely useless). The excellence of the firestop helps the steel junction box do its job, which is to distribute arc heating so nothing reaches combustion temp.

Also the person is using a power tool to run down screws on electrical boxes. That's done for "coolness" or for TV-show pacing; or because "if all you have is a hammer" (the drywall screws need it). You shouldn't use power drivers on electrical boxes in real life, because if you cam out the screw or God help you, strip the screw hole - you'll regret it.

  • huh... I use a drill on nearly all my electrical-work screws. Granted, I'm not an electrician doing it daily, but I've not had issues with either caming out the heads or stripping the screw holes. I'll trust that if I did more, I'd likely run into the problem, but I tend to be pretty careful with the screw gun, and pretty lazy about turning a screwdriver by hand... I also tend to use the drill to get it almost tight, then finish by hand so I don't over tighten...
    – FreeMan
    Jan 30, 2020 at 18:39
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    @FreeMan A pro might not. Amateurs have worse luck with power drivers; some think "rat-tat-tat-tat-tat-tat-tat-tat-tat-tat-tat-tat-tat-tat" is the happy sound of technology. Jan 30, 2020 at 18:50

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