I have a 30 amp circiuit running to a high power outlet in my dining
room. I discovered if I flip one of the the 15 amp breakers on the
circuit in the breaker box, the power to the basement lights shuts
off.. How did they run at 15 amp line from the 30 amp circuit? there
is a junction box in the basement cieling with a line going to the
basement lights right were the 30 amp line runs up to the dining room.
Actually, when I read other response and re-read OP's question, I see it
does seemingly preclude my earlier response. Agree it sounds like
240V/15A service w/ one side to the lights.
Physically not any problem to do, not certain about whether code would
have allowed it at some time in the past or not but agree breakers
_should_ be ganged.
If as described; possible, yes. Likelihood would depend on the size of
the A/C unit.
You could verify the way it's connected measuring the voltage between
the two "hots" of the 240V A/C outlet to each other and to ground then
after tripping the breaker w/ the lights and repeating. If it's as you
think, on the second measurement instead of 120V from each to ground and
240V between you'll get 120V between and 120V from one side to ground
and 0V on the other. The other breaker test would show the same values
on the other side, obviously.
Two 15A breakers tied together does not make a 30A 110V branch circuit. In
most cases this is a 15A 220V branch. If that is the case, either hot black
wire plus a neutral white wire will make a 110V circuit.
The unanswered question is what is the true voltage and purpose of that
socket in the dining room. electric heat or dedicated window A/C perhaps.
Could be 110V x 2 plugs or a single 220V but 15A in every case. There is no
combination of 15A breakers that will get you a 30A circuit, they are never
run in parallel on the same phase to the same load. If it is 220V 15A then
you shold not be able to turn off the individual breakers, they should be
tied together with a bar and if that is broken you should replace it with a
new double slot breaker. If the basement light blows half the circuit, it
still may be energized in the dining room without you knowing it.
You should not have 110V lighting split off 220V service but it is
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