electrical geek question---

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.
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Chris wrote:

One possibility not precluded by this description is there are two separate circuits run physically parallel to that point where the lighting circuit branches.
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dpb wrote:

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.
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so what that means is if I am running an airconditioner in the dining room, and turn on the lights in the basement, I could trip the circuit, then?
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Chris wrote:

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.
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Chris wrote:

As adjunct to other comment, what does the outlet for the A/C look like?
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it's a huge three pronged outlet that reads 250 V/ 30 amps on it..
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Chris wrote:

OK...guess that's what they did.
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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 technically possible.
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Probably a 15A "Edison" circuit.
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