Tandem breaker needs both thrown to break circuit. Incorrectly wired?

Question

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.

Answer 1

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.

Answer 2

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.