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One of the things that's always been drilled into me is "loose wires cause fires," and that's something I always remain conscious of when changing out devices. But is there such a thing as "too tight" when it comes to tightening the screws on standard 15/20A receptacles and the like? What problems can it cause if one tightens things a little too far?

(There have been a handful of times in the past where I cammed out the screw head, stripped the threads, or bent some part of the structure by going a little too far. In all those cases, I discarded the device and tried again with a fresh one. But is there a torque danger zone before any of these things happen?)

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3 Answers 3

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Recent (last 10 or 20 years, maybe a bit more) code says to use torque wrenches/screwdrivers for all connections now.

You found some problems with going too tight. In addition, you also squeeze the wire too much. Most electrical wire is either copper or aluminum. Most screws and screw holes are made of harder materials. If soft wire is tighten/squeezed too much, it can be reduced in size, causing an increase of resistance (heating). Weakened/pinched wire may cause the wire to break off, including after you've closed the box.

All devices should have their torque values listed now, so no reason to be too loose or too tight.

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    I fail to see what is wrong with this answer, other than being an inconvenient truth. Aug 14, 2022 at 2:56
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    Strange downvotes. Maybe the individuals involved can explain themselves. Aug 14, 2022 at 3:33
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    If I make a mistake, I do like to know how or why, so I can learn. Do know that the exact code should have been mention, but I don't know the code that well.
    – crip659
    Aug 14, 2022 at 17:31
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    @FreeMan I argue that it addresses nothing. It merely re-stated OP's action rather than explaining a hazardous result of the action. "Too much" has no qualifier. If there was an explanation of what bad result does "too much" achieve then I'd be inclined to upvote.
    – MonkeyZeus
    Aug 15, 2022 at 14:52
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    @MonkeyZeus Think you might be right, the problems not fully explained. Will try an edit to try to make it better.
    – crip659
    Aug 15, 2022 at 14:56
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This was investigated more formally by researchers, and it was determined that your folk "knowledge" is only half the story. Too tight is also a problem.

Also, they discovered via testing that pro electricians could not "guess" screw torques any more accurately than their spouses or business managers.

The result of this was NEC 110.14, which requires torque tools be used to set screw torques to the specification - anywhere a torque is specified.

This is actually nothing new: NEC 110.3(B) already required installers to follow instructions and labeling, including torque specifications. It simply deletes the (proven wrong) excuse of "I know what 18 inch-pounds feels like!"

UL approves the instructions and labeling as part of listing equipment. So this allows UL to require that manufacturers either specify a torque, or show in testing why it doesn't matter.

times in the past where I cammed out the screw head

If you look closely at receptacles you'll see the combo standard/Philips head is "funny". It's a little extra hollow in the middle. This is actually a Robertson socket. You won't have cam-out problems if you use Robertson.

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    Yeah -- the kicker here is the OP is asking about wire binding screws, whereas the tests you mention were on mechanical set-screw type lugs (breaker/panel lugs IOW)...so it might be the case that overtightening is less harmful here, but I can't tell for sure Aug 14, 2022 at 5:39
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    Robertson is Canadian for square not Torx.
    – crip659
    Aug 14, 2022 at 10:31
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    Yeah, to be clear, Robertson and Torx are very different-looking screw drives (Robertson is a square, Torx looks like a six-petaled flower). But maybe Robertson is kind of the Canadian equivalent of Torx. Aug 14, 2022 at 10:45
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    @crip659 thanks for that info, edited. Aug 14, 2022 at 17:18
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    The screw heads for electrical receptacles are ECX drives. These are essentially a combination of flat head, Philips, and square drive. They can be tightened with any of these drives individually, but will work best with a specialty ECX bit.
    – Glen Yates
    Aug 15, 2022 at 14:28
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Conceivably, if you grossly over-torqued a connection, you could deform the wire and squeeze much of the conductor metal out of the connection. That could lead to a bad connection (excessive resistance) and/or mechanical failure (if there's less metal hanging on, it could break off more easily than it should).

I think it would probably be hard to achieve this by hand on typical receptacle/switch screws, but if a device specifies a torque range, you should just use a torque screwdriver and follow the specified range.

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  • It's not that hard to cut a #14 wire by accident by over torquing. It's happened to me a couple of times. Not enough to detect a "why" pattern, but possibly a) wires had hardened and/or b) device design, particularly the ones with a screw-down wire clamp accessed through a small hole in the back ... I think. It's definitely more to do with the design than the actual torque.
    – jay613
    Aug 16, 2022 at 0:38

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