Does it cause problems to overtighten connections on receptacles/switches?

Question

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?)

Answer 1

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.

Answer 2

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.

Answer 3

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.