Electrical – Torque screwdriver hex bit for main panel breaker, etc

electrical-panel

I finally used the 10-50 in-lbs Torque screwdriver I received yesterday to all the breakers in the main panel and subpanel. The Siemens load center MBK100A breaker is the hardest to torque because of the 55 in-lb torque rating. I only have 50 in-lbs maximum.

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So far all breakers were under tightened, except for some 20 in-lb breakers that were tight already. Some were under tightened by 50% like the Siemens main breaker and the din rail MCB with recommended torque of 55 in-lbs and 30 in-lbs respectively.

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In the Siemens main panel, I was able to torque the right lugs well to reach the 50 in-lbs limit (when the torque setting was reached, the screwdriver slips instead of click sound, it is great design), but when tightening the left lugs. I couldn't reach 50 in-lb before the screws got so deformed the tool can no longer fit tight. So my three questions:

  1. How can you know whether the hex bit is the right size for the lugs or any screws?

  2. What is the biggest flat screw driver that can fit the Siemens MBK100A load center main panel perfectly?

Best Answer

It looks like your screwdriver bit was too small. The bit should have a snug fit and be wider, from what I can see. If the bit is not wide enough then the torque is applied to a smaller area and the screw can be damaged as yours was.

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