TIL about torque screwdrivers on breaker panel lugs

Used to just crank down on panel lugs by feel for years. Finally borrowed a torque screwdriver from a buddy last month on a Square D QO panel. An 8/0 lug needed 250 in-lbs and I was way over that by maybe 40 in-lbs. Anyone else check torque specs on your bigger connections or am I overthinking this?

3 comments

3 Comments

fox.jesse4mo ago

That "way over by maybe 40 in-lbs" part is exactly what scares me. Lugs get soft when you overtighten them, then they loosen up later from heat cycles. Ive seen a few service entrance cables get warm because someone gorilla'd the lugs. Its worth checking if you have the tool handy. Better safe than melting a neutral.

danielh814mo ago

The real issue nobody talks about is the torque spec itself. Those lugs are rated for a specific number of tightens before the threads start galling. If somebody overtightened it even once, the threads are already damaged. Then the next guy comes along with a torque wrench set to the right number, but the lug is already compromised. It might read 275 in-lbs on the wrench but the actual clamping force is way lower because the threads are binding. I have seen lugs that felt tight but the screw was just dragging against damaged threads. The lug was barely touching the wire.

skyler_white953mo ago

Oh word, so the threads can get wrecked from one overtorque and then the torque wrench reading is basically lying to you? That's a nasty failure mode right there. I've seen guys just zert them down with an impact and call it good, never thinking about how that damages the lug for the next install. And yeah, seen plenty of neutrals get cooked because someone cranked it too hard and the connection went bad over time. Honestly makes me wonder if we need some kind of one-time-use indicator or something on these lugs.