Electrical – 3 240v circuits connected to 2 double pole 30a square d breakers

The last guy got the 2-pole breaker thing right, the amperage wrong, and the GFCI wrong. Here's what you need.

The breaker needs to be correct for the smallest wire in the circuit. If it's all 12 AWG (no 14 AWG extensions) then the breaker must be 20 amps or smaller. A 30A breaker is wrong and should be removed from service immediately.

Since it's a multi-wire branch circuit (MWBC), the circuit needs common shut-off, and with circuit breakers, that means either a 240V/2-pole breaker, or single breakers with handle-ties. A handle-tie is a bit of plastic that sits in between the handles so they jointly shut-off. It's sold by proper electrical supply houses; Big Box won't stock it.

Since it's a bathroom, at least part of the circuit needs to be GFCI protected. Single-pole GFCI breakers with handle ties won't work. The only options are:

• A 2-pole, 120/240V GFCI breaker (expensive)
• GFCI receptacles, making sure if you connect any additional loads on the "LOAD" side of the GFCI receptacle, that they use a dedicated hot-neutral pair separate from the MWBC shared neutral.

By "multi branch circuits" you mean multi-wire branch circuits or MWBCs.

How do MWBCs and tandems go together? THEY DON'T.

Either

• the electrician ran new wire, eliminating the MWBCs, or
• he carefully placed them on opposite poles (you have to know how to do that) and totally ignored the Code requiring handle ties, and is just hoping the next guy doesn't carelessly rearrange them onto the same pole, or
• he slapped them on a tandem and the neutral wire is glowing cherry red right now.

If you want to do that same thing, your best bet is to put both pairs of your MWBCs onto a 2-pole/2-pole quadplex. That's what he should have done.

Making your own handle tie is out of the question. Listed handle ties arent available for duplex breakers, but they don't need to be, since quadplex breakers are readily available.