Questioning Torque Values

[don_resqcapt19]

Ill be hoonest I dont have a lot of faith in torque values. I read a study on it, I dont remember where maybe it was machinery's handbook, and due to differences in thread tolerance, cleanliness, lubrication, material, surface finish, etc, torque values really arent that accurate outside of the lab. Maybe I am wrong, but I am paranoid a lot of these torque values are one of these things that have been around forever (like lock washers - useless) and there isnt a lot of rigorous study behind them from an electrical standpoint. I think there are some good engineered joints, like often you will find in switchgear that employ belleville washers, but as far as a mechanical lug with a range of sizes and materials, hmmmm.....

They are not very accurate as they are trying to use torque to measure bolt tension in a typical bolted joint, or contact pressure on the conductor with a mechanical wire termination. As you stated there are so many things that effect the torque.

 

[GoldDigger]

thats what I have found as well. It is scary sometimes how little they want it tightened but, it works. Just have not found a way to use british electricians bits with a torque wrench.. Wish there was an adapter for a torque wrench to go onto a screw driver...

You could use the correct size hex socket to hold a hex shaft screwdriver bit. It should work, although not necessarily well. Often need to use an extension shaft.

 

[kwired]

Forgive me for asking this, I'm just want to be certain. Are you sure you're looking at inch pounds and not foot pounds on your torque wrench?

foot pounds is quite a bit more and would be noticeably more than most would tighten something without a torque wrench.

I think u should lube the terminal with penatrox, so as not to bind. Whats next, tourge switches and plate screws?

Lubrication is not called for in instructions, values given are intended for tightening as is. Lubrication would effect how much compression results on the conductor before the target torque value is reached.

Ill be hoonest I dont have a lot of faith in torque values. I read a study on it, I dont remember where maybe it was machinery's handbook, and due to differences in thread tolerance, cleanliness, lubrication, material, surface finish, etc, torque values really arent that accurate outside of the lab. Maybe I am wrong, but I am paranoid a lot of these torque values are one of these things that have been around forever (like lock washers - useless) and there isnt a lot of rigorous study behind them from an electrical standpoint. I think there are some good engineered joints, like often you will find in switchgear that employ belleville washers, but as far as a mechanical lug with a range of sizes and materials, hmmmm.....

I agree there is some importance in the torque values, but it is not all that precise in most conductor binding screw type terminations with a wide range of conductors that are acceptable to use in the termination either. Certainly isn't going to be the ultimate value on a terminal that has aged either or even for something being installed on a very hot or very cold day for that matter.

Every time I have followed a torque spec it has been lighter than I would have tightened freehand, and not just a little bit lighter.

Same here.


To OP: I have used torque wrench on Square D 225 amp frame size main breaker lugs for about as long as I have been in the trade. Won't say I torqued every single terminal but have used torque wrenches on them from about day 1. They have always had a tightening torque of 250 inch pounds, never have damaged the lug or the component it is connected to at that value. Never broke any free hand (Square D main breakers) for that matter either and it isn't too difficult to get maybe 300 -350 inch pounds when guessing on free hand.

Did one time have a POCO guy stand there and watch me torque load side of a meter socket and then told me he would connect line side - he didn't use a torque wrench and broke one of the line side terminals:blink: He wasn't a newbie at this either, was within 10 years of retiring and that was only thing he had done for adult career in his life was work at a POCO.

 

[mlnk]

I use a torque wrench or screwdriver for every connection including receptacles and switches. For Leviton that is 14 to 16 lb-in for residential and 20 lb-in for commercial grade. It is much faster to use a torque screwdriver, especially on circuit breakers. Torque is required by the NEC and by Mike Holt. I am an average electrician...so what does that make you who refuse to torque...below average electricians? Put that on your truck!

 

[jaggedben]

I use a torque wrench or screwdriver for every connection including receptacles and switches. For Leviton that is 14 to 16 lb-in for residential and 20 lb-in for commercial grade. It is much faster to use a torque screwdriver, especially on circuit breakers. Torque is required by the NEC and by Mike Holt. I am an average electrician...so what does that make you who refuse to torque...below average electricians? Put that on your truck!

Lol. I think you need to look up the definition of average. You might be average in general, but with respect to torque, you are well above it.

 

[Fulthrotl]

For those that use torque wrenches on their terminations, do you ever question them?

no, i question the quality of the chinese ungraded nuts and bolts.

now, when i am bolting up stuff where i can thru bolt it, and don't have to
depend on OEM stuff, i substitute 3/8-24 grade 8 allen head cap screws,
grade 8 flat washers on the cap screw side, grade 8 belville washers on the
nut side, and grade 8 nylock nuts. all american made. i am not a china hater,
but i've seen they seem to have a constitutional inability to produce a SAE
compliant thread.

3/8-24" grade 8 has the same torque value and compressive strength that 1/2" hardware
possesses, assuming the 1/2" hardware is grade 2 compliant. a lot of it isn't.

torque them to 35 ft. lbs. lubed, and be done with it.

for setscrew lugs, i don't use them, i hipress. if i have to use them, as on a breaker,
i use antisieze, and torque to specified torque, unless i see metal deformity, then
i stop.

 

[Mil113]

Terminal Torque requirements

Terminal Torque requirements

Section 110.14 (D) 2017 NEC requires use of calibrated torque tool, my recent inspections have only seen 1 company with calibrated torque screw drivers, they do have Torque wrenches for main lugs, I do believe the intention is if a manufactures torque value is on the device then a calibrated Torque tool is required under this code.
Questions have risen from companies stating that only if the Client requests the requirement. My understanding is that if they are requiring the manufacturer to be in accordance with 2017 NEC, then calibrated torque devices are required (this includes wrenches and screwdrivers)

 

[Jraef]

Forgive me for asking this, I'm just want to be certain. Are you sure you're looking at inch pounds and not foot pounds on your torque wrench?

I think it's worth asking. 250 in.-lbs. is only about 21 ft.-lbs., it's not that much. I'm wondering if that was the problem here.

 

[sameguy]

On resi. I never used a torq. just my left hand, I'm right handed. On industrial we use torque, power plant work you are watched by QC as you torque. I too feel the values seam lower than what I get when I use my left hand.

 

[oldsparky52]

I think it's worth asking. 250 in.-lbs. is only about 21 ft.-lbs., it's not that much. I'm wondering if that was the problem here.

Add me to that list.

 

[GerryB]

An interesting post. Caused me to read an old (2007) article from AEIA about inspecting a 200 amp service and asking the electrician to see his torgue tools. So I'm curious if any one ever failed a residential service or was even asked about torque tools. I never have in my area. (never really torqued anything but switch gear).

 

[mopowr steve]

What bothers me, is how do you know the proper torque is being supplied to the compression of a conductor in a lug when the lug itself is already factory made so tight that it’s already close to torque value to even make it budge. Example: Siemens panel neutral bar. Man those screws can be difficult.

 

[Tony S]

Some of the trade association in the UK actually require an electrician to have torque drivers calibrated.

After 40 years in the trade I think my wrist is still within the calibration parameters.

 

[sameguy]

I think I told this before,
At a power plant QC argued with one of my men over the use of a crowsfoot, I had chosen a value that fell between the min/max of the spec. and with the +/- (4%) of the tool it wouldn't matter what the men did about the crowsfoot angle of attachment. QC and I got it fixed as no rework or bad paperwork was generated.

 

[steve66]

At that point, I felt like the aluminum lug wouldn't take much more, certainly not 100 in.lbs more, before stripping out.

Every time I have followed a torque spec it has been lighter than I would have tightened freehand, and not just a little bit lighter.

Sounds like something is amiss. Are you sure your wrench is calibrated correctly?

I don't ever use torque wrenches (engineer- not electrician) but I have noticed that every time I "bump" a tire pressure gauge it instantly becomes worthless. (And by "bump", i mean accidentally dropping it on the garage floor.) Never reads anything close again.

Also have a 2 thermometers that I use for grilling. One reads about 20 degrees too low, and the other about 20 degrees too high. They have both been that way since I bought them.

I guess my point is that as things get cheaper and cheaper, its getting harder and harder to find any kind of gauge or meter that has any accuracy even when new, much less after its been in use a while.

 

[dkidd]

Probably, as well as the need to follow manufacturers' instructions, but, since we're still on the '11 NEC in VA, I'm not concerned . . . yet.

Virginia adopted 2014 on Sept. 4.

 

[sameguy]

Steve66,
Take your grill stats out and on the back is a nut for calibration; use boiling water to set them then tighten the nut; with or without a torque wrench!

 

[ActionDave]

Sounds like something is amiss. Are you sure your wrench is calibrated correctly?

I don't ever use torque wrenches (engineer- not electrician) but I have noticed that every time I "bump" a tire pressure gauge it instantly becomes worthless. (And by "bump", i mean accidentally dropping it on the garage floor.) Never reads anything close again.

Also have a 2 thermometers that I use for grilling. One reads about 20 degrees too low, and the other about 20 degrees too high. They have both been that way since I bought them.

I guess my point is that as things get cheaper and cheaper, its getting harder and harder to find any kind of gauge or meter that has any accuracy even when new, much less after its been in use a while.

We've always used high quality torque wrenchs and screwdrivers. That is not an issue.

 

[steve66]

We've always used high quality torque wrenchs and screwdrivers. That is not an issue.

I was suggesting it might be Mark32's issue. Not your issue.

I'm glad you can still find quality wrenches.

More and more, anytime I want something that works right, and something that will last, I can't find it. Especially locally. The big box stores sure don't have it.

 

[readydave8]

So I'm curious if any one ever failed a residential service or was even asked about torque tools.

Never

For a while I would lay torque wrench on top of meter base if I was going to be there when inspector came, can't remember anyone noticing