This is a multi-wire branch circuit (MWBC) that has been wrecked by idiots, and made rather dangerous by their bungling.
North American houses use split-phase power. A normal MWBC takes advantage of this to bring two full circuits' worth of power on only 3 wires. Two hots share a neutral: the hots must be on opposite poles (240V between them) so the neutral only carries differential current, and can't be overloaded.
The neutral being pigtailed is a normal safety requirement of MWBCs. That's so you are able to remove a device (receptacle) without interrupting the neutral for the other half-circuit. That is still required, but it's a bit obsolete because now, both breakers must have a common shut-off.
Yes, this circuit is powered by two breakers. They should have been next to each other. Modern code requires they have common maintenance shutoff to avoid the problem you had finding the right breakers, so either a handle-tie or a 2-pole breaker. It's possible someone changed the panel and put both of them on a double-stuff breaker - that's a common blunder, and very dangerous on a MWBC.
The black supply wire is one half-circuit, the red supply wire is the other. The receptacle must have its tab broken off, because red and black are supposed to be 240V apart! Why isn't it? I don't know what happened. I suspect either
- somebody moved the breakers so both sides are on the same pole (0V apart) and then someone changed the receptacle, and forgot to break off the tab. (Probably didn't even realize tabs exist).
- somebody replaced the receptacle, didn't break off the tab, and got a big BLAM as they shorted the entire service. Chasing that problem, they moved the breakers around.
Either way, all the current from both breakers is being returned on the same neutral wire. The currents are not canceling each other out - they are stacking, with the 20A rated wire carrying as much as 40A. Neutrals do not have fuses. Nothing protects them except care not to do this!
What was originally intended
The original idea was to provide dual 20A circuits in the bathroom so you could run a heater and a hair dryer at the same time. They served them on the split receptacle. One pole, the black, also went off to serve other bathroom loads e.g. the light or fan.
This is a very deluxe setup, because it means not worrying about overloads. I highly recommend it for every bathroom or kitchen receptacle if you can afford the 2-pole GFCI breakers.
15A receptacles are allowed on 20A circuits **if there are two or more sockets*. If no tabs are broken, a duplex receptacle has 2 sockets. On the original setup here, a 20A receptacle would be mandatory because each half-circuit had only 1 socket.
How to fix it: Quick and dirty.
Cap off the red wire, and never use it again.
Somewhere in your panel will now be a breaker that does nothing. Next time someone has the panel open, eliminate that.
Get a GFCI+receptacle combo device, or "GFCI". Attach its
LINE terminals to the black and white pigtail. As always, leave the warning tape on the
LOAD terminals. They are not extra general purpose terminals, they are for wizards only.
How to fix it: As the original architect intended
Swap the current 15A receptacle for a 20A receptacle (required to meet Code as discussed). Break off the hot-side tab.
Plug a radio and a vacuum cleaner into them. Flip breakers until the radio OR vacuum cleaner are silenced, then keep flipping to find the other. Mark those breakers, on the breaker not the cover. You'll be taking the cover off and playing with live busbars.
Pull the red and black off those breakers and sanity-check to make sure they go to the same cable. Also find the accompanying neutral and pull it off the neutral bar.
Now, figure out how to rearrange space in the panel to give you two full spaces. Be careful: this house may be full of MWBCs, others may be wrong too, fix them and don't break any. This is the problem with this approach: you may keep stumbling across more problems.
The two spaces need to be adjacent. They need to be full spaces, you cannot use double-stuffs here because GFCIs are not made in double-stuff.
Now buy a 2-pole GFCI+breaker combo device, or even better a dual-mode GFCI, combo AFCI and 2-pole breaker. Expect to pay $90. On this super-breaker, land the two hots and neutral from this circuit. Take the breaker's pigtail and put it on the neutral bar.
Now you have full dual 20A capacity in the bathroom, and will never trip a breaker (except for GFCI/AFCI of course).