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Torquing for Techs

June 22, 2019

Dear Swami,

 

According to the local electrical inspector, I need a certified torque wrench to torque the lugs in our portable power generator, distros, and other items, but I'm having trouble finding the right tightening torque range. Can you give me any insight?

 

Signed,
Torqued in Toledo

 

Dear Torqued,
 

Great question! Back in the day, I used to do my own mechanical work because I couldn’t afford to pay a real mechanic. I had a torque wrench, and it’s basically a socket wrench with a long handle and a gauge that tells you how much torque you’re applying. It’s the same for torquing lugs and terminals in electrical systems. The idea is to tighten it enough so that it won’t loosen, given the repeated cycles of heating and cooling, but not enough to damage the threads on the screws. Equipment manufacturers know how much torque should be applied for best results and they sometimes provide that information in the documentation for the devices.

 

Appendix L in the National Electrical Code (NEC) provides torquing values for tightening slotted head screws, Allen bolts, and square bolts. It says, “In the absence of connector or equipment manufacturer’s recommended torque values, Table L.1, Table L.2, and Table L.3 may be used to correctly tighten screw-type connections for power and lighting circuits…For proper termination of conductors, it is very important that field connections be properly tightened. In the absence of manufacturer’s instructions on the equipment, the torque values given in these tables are recommended. Because it is normal for some relaxation to occur in service, checking torque values sometime after installation is not a reliable means of determining the values of torque applied at installation.”

 

The recommended torque depends on the size of the screw or drive, and it’s given in inch-pounds (lbf-in), which is one way of measuring torque. (Torque is force multiplied by the length of the handle.) Besides socket wrenches, there are also screwdrivers that allow you to set the amount of applied torque.

 

Portable power generators typically have lugs with recessed ¼” Allen set screws, in which case, according to the NEC, you would need to torque it to 200 lbf-in. Most torque wrenches in North America are calibrated in foot-pounds (lbf-ft), not inch-pounds. Fortunately, it’s easy to convert between them. To convert inch-pounds  to foot-pounds, just divide by 12. For example, 200 lbf-in is the same as about 17 lbf-ft (200 divided by 12 is about 17). 

 

You said you need a certified torque wrench. You can buy a calibrated torque wrench and have it certified by an ISO 17025 accredited lab. Keep in mind that, even with a certified torque wrench, there are no guarantees that it’s being used properly. There is a great article about how to properly use a torque wrench at https://www.tekton.com/torque-wrench-accuracy-and-calibration. Look closely at the four tips for maximizing accuracy.

 

Happy torquing!

 


 

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