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I have a Square D main panel with two separate load bus lines.

When I turn off main breaker I have 120 volts on each incoming supply wires to main breaker.

When I turn on any circuit breaker connected to the one bus line the voltage to that bus and to the incoming supply line drops down to anywhere from 20 to 2 volts.

But the voltage is great on breakers connected to the other bus line

What is going on.

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  • I am no expert on these things (not even close). But the experts will want to see pictures of the panel. Commented Dec 17, 2020 at 18:09
  • I see it. You measured 120V L1 to neutral, OK. You also measured L2 to neutral, also 120V, but you did not measure if they are opposite or same phase! You need to measure L1 to L2! The correct answer is 240V. I bet you get near 0V. Commented Dec 17, 2020 at 18:20
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    FYI, "Buss" is a brand of fuse cartridges (or a kiss, if you're a poet). A bus is a set of like things.
    – isherwood
    Commented Dec 17, 2020 at 18:53
  • And if you don't know the difference between a Buss and a bus when you are working on a panel, then the bus factor may come into play. Commented Dec 17, 2020 at 19:16

1 Answer 1

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You should check for 240 between lines, but I'm guessing you have it.

When I turn on any circuit breaker connected to the one bus line the voltage to that bus and to the incoming supply line drops down to anywhere from 20 to 2 volts.

That smells exactly like a very bad connection upstream on that leg. No current, full voltage. Turn anything on, current flows, high resistance in the bad connection, voltage you see at the panel input drops.

Given it's on the input side beyond your panel input, I think it's call the power company time in most jurisdictions.

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    Yep, you've got a bad hot leg connection. Time to call the POCO as well as check your panel's connections for severe deterioration. Commented Dec 17, 2020 at 23:08

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