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?


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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  • what size is the largest bit that can fit the lugs flat screw of main breaker? – Jtl Feb 22 '19 at 15:12
  • and how can I find a replacement screw for the lugs that is already damaged.. is it standard size? – Jtl Feb 22 '19 at 15:15
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    I have many different sized lugs that I stock the screw sizes vary depending on brand so they are not standard your best bet would be to take it to an electrical specialty store because the bottom of the screw is not a standard set screw. – Ed Beal Feb 22 '19 at 18:07
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    One of the biggest problems is just torqued screws is not enough with smaller feeders I torque back off and retorque 3 times with larger wire sizes I do the same 3 times then wiggle or force the wires on the feeder back and forth then left to right and retorque, with this sequence I have not had problems but if only torqued 1x have seen many problems. – Ed Beal Feb 23 '19 at 21:57
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    Pozzi , or pz2 in your case ? Drive is a slightly different than phillips, #2 in both sizes will be close , but if you want the correct bit ask for a pozzi drive not sure of the spelling but it has a slightly different profile than Phillips and can provide more torque with out stripping out. – Ed Beal Feb 28 '19 at 2:46

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