John's got it exactly right here. The frequency on that circuit is being
distorted by a chopped current load on something else on that circuit. By the
time it goes all the way to the load center and back inductance has filtered it
enough that it's not seen on the other circuits. My bet would be a ceiling
fan or dimmer. I'd discount loose or poor connections as their frequency
wouldn't be regular.
This was a pretty common complaint here in the plant when we had a lot of the
early variable speed drives for motors. Nowadays they are much better so we
don't see it as often. Steve
it's not a matter of watts or for that matter volts... it's frequency.
Electric clocks get their time keeping ability from the 60 cycle frequency
of the A.C. supplied to the house.
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