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I have a tandem circuit breaker in my main breaker box with two 20 amp breakers. To turn off power my wife's office, I need to throw both breakers. I can throw either one and as long as the other is on, the circuit is still live.

My guess is the electrician who installed this tandem breaker wired it in parallel. In other words, current is flowing through both breakers when closed and both have to be open for it to break the circuit. Does that sound right?

If so, my understanding is that it would take 40 amps of current to flow before this breaker would trip. (20 amps X 2 = 40 amps because it's in parallel)

They used 12 AWG wire on the circuit. That means the wire would basically catch fire before the circuit would actually break. This seems incredibly dangerous.

Are my assumptions correct in this case? I'd like to know so I can get a electrician out to inspect the wiring, since I've spotted a few other oddities in this house.

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    How do you know the circuit is still live? A single outlet or light will remain powered, regardless of which breaker is turned on? Are you sure you didn't just confuse yourself while testing? It's pretty easy to get things mixed up as you run back and forth to the breaker box. – Jonathon Reinhart Mar 26 '18 at 1:33
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    Pretty sure I didn't confuse myself. Checking that the lights in a room are on when either breaker is on is not rocket science. – Mark Maunder Mar 26 '18 at 3:23
  • Sounds like a British ring circuit, except the British terminate both ends of the ring in the same breaker. Never been a fan of duplex/double-stuff breakers, monkeyshines like this seems to follow them around. – Harper Mar 26 '18 at 5:11
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    It's unclear whether this is a single-sided (120v in the US) circuit or a double-sided (240v in US) circuit. If the latter then the two breakers are presumably side-by-side and on opposite "legs" of the double-sided main, and the problem is that somewhere along the way the tie bar between the two breakers has gone walkabout. – Hot Licks Mar 26 '18 at 12:17
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    @MarkMaunder, not rocket science, but we don't know you, and it's amazing the convoluted ways people will devise to try and test circuits when they could just do something easy like look at a lamp plugged in. – JPhi1618 Mar 26 '18 at 14:02
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Intentionally wiring the two breakers together in the breaker box would be a really stupid (borderline malicious) thing to do.

My guess is that there are supposed to be two independent circuits, but someone has somehow unintentionally joined them together in a junction box or outlet box somewhere along the circuit.

This could be pretty complex. I would recommend having an electrician come check out the situation.

If I were solving this, here is what I would do:

  • Work carefully! It's a dangerous situation when you can't be sure the power is off. Assume every box you encounter is live. You'll want a contact-less voltage sensor.
  • Open the breaker box and verify that there are two 12/2 cables entering, the black wire of each connected directly to one lug on the tandem breaker.
  • Start drawing a diagram, including every switch and outlet involved.
  • Turn off both breakers.
  • Open a box that's affected, and disconnect the black wires from the device. Cap off the wires. I would start with the box that you think would be closest to the breaker box and first on the circuit.
  • Turn a breaker on, one at a time, and note which wires/devices still have power. This will help you figure out which wire heads upstream / downstream. Label the wires with numbers or descriptions (if you can write small enough) e.g. "From hall switch"
  • Repeat until you have the situation mapped out.
  • Report back here to tell the curious people what you found.
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    Thank you. The house was wired for a generator with a separate breaker box to switch between line/off/generator. This particular circuit is not wired into the generator. But perhaps when they did that work, they moved things around and this was the result. – Mark Maunder Mar 26 '18 at 2:37
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    @MarkMaunder Or perhaps something else altogether. This requires detective work - you'll have to open every device box on (both) circuits and trace. It would be dangerous to assume anything, I think. – J... Mar 26 '18 at 12:09
  • @MarkMaunder Make sure to let us know what you find, I'm very curious. If you are comfortable investigating this yourself, please be careful. I've added a rough flow chart of how I would go about tracking this down. – Jonathon Reinhart Mar 26 '18 at 12:50
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    If they were wired together as the OP suggests, would turning one off for the time being be a good idea? If everything still works on one, it would seem the second is not needed. I'm assuming these are not AFCI. – JimmyJames Mar 26 '18 at 13:58
  • To Me it sounds like some one accidently tied 2 hots together from the tandem breaker. I would start at the first junction box probably the switch. For those thinking this could be 240v (both l1 and l2) dpst breaker having those tied would result in an immediate trip, tandem or double stuff breakers are side by side but take only 1 space and are on the same leg. – Ed Beal Mar 26 '18 at 19:01
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The British have an idiomatic design called a ring circuit, where the circuit starts at the panel as normal, visits several outlets, then loops back to the panel. It lands on a breaker rated for 2x the safe wire capacity - which assumes no wire failures. Mind you, the UK also requires switches on every receptacle and fuses on every plug, which helps make this safer for them. Doesn't work in North American standards (or anywhere else in the world).

It looks like here, the homeowner non-electrician was too clever for himself. He decided to fashion a British ring circuit, except fuse each end of the wire for 1x wire capacity. True, that helps with a big defect in UK ring circuits, and a duplex breaker is a savvy choice. It's still wrong for many reasons: least of all, the breakers need a handle-tie for the same reason MWBCs do, and there's no such thing as a handle-tie between breakers on the same pole. You'd have to use a 3-phase breaker and disuse the middle space, which means 3 spaces instead of 1, not that you should even be trying to fix this.

Rewiring to be correct is rather straightforward: Sever the circuit at some opportune location, so instead of a ring circuit, it is 2 circuits. And you're done.

Simply find the right receptacle, remove the downline wires, cap them, tape them so the caps don't fall off (as they are prone to do with a wire-nut on 1 wire), and push them into the back of the box.

The art of it will be in choosing the opportune location. This is where you map the circuit to see where the outlets are, and also where the loads are. You want to split it so it places half the load on each side, or so the expected loads won't overload either circuit. For instance "between a gaming PC and a laser printer" is a great place to split it.

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