If this is newer wiring, the white NM 14/2 cable is rated for 15 amps and the yellow cable NM 12/2 is rated for 20 amps, so there is a mismatch. If its on a 15 amp breaker there is no safety issue, but it can be confusing for an electrician if devices are added later on and the only visible cable is the yellow 12/2.

Bruce

Yep, but it's a big reg flag. I'd check the breaker. Frankly when I see two yellow wires and one white one, I'd lay odds on the fact that the circuit has a 20A breaker on it.

Surprise, surprise the breaker is 20 amps. The white cable the comes out of the ceiling box is a switch loop, not that I think that makes a difference.

The space is an attached storage area (shed) but what's behind that wall is even more interesting...a bathroom. Conveniently the contractor thought to protect that light and a receptacle (even labeled) with the GFCI in the bathroom. Me thinks that's a problem.

Looking a the panel (it's actually a small, 4 space sub-panel) there's what appears to be MWBC in there and I suspect it's part of the same work. Actually I know it's a MWBC, the only cable isn't big enough to be anything else. It uses one of the single pole, tandem breakers. I think it's Cutler-Hammer BR or Siemens.

You're right, the switch loop doesn't make a difference. Unless someone is living in the shed, you can probably get a pass on sharing the bathroom receptacles with other outlets. It is illegal in dwelling units.

Hold the phone. Are you saying you have a MWBC on a tandem breaker. EIther BR or SIemens both poles of the tandem are on the same "phase." You're potentially overloading the shared neutral.

Tying the two handles together in that breaker isn't proper. Now if you had two side by side tandems there are (For some manufacturers) handle ties that will bridge the two adjacent poles or wrap around to get the two far poles. The current code requires handle ties (or other means to assure both sides trip together) on all MWBCs. Older versions of the code only required it when the two sides terminated on the same yoke (such as the practice often seen in Canada of feeding the two different receptacles on a duplex with opposite sides of the MWBC). [By the way there appears to be an error in the non-normative figure in the 2014 handbook on this].

Oh h-e-double toothpick, I just picked up on the single pole and a MWBC. Thanks for the reminder! That I'm going to have to check but I'm pretty darn sure it can't be anything else. There's only three cables (beside the feeder) going to the sub-panel. One for 240v A/C, the yellow one we see in the picture, and what I suppose is 12/3. Unless someone makes one with two neutrals but the cable just didn't look large enough for 4 wires.

This is U.S. construction, circa 2009-2010. No idea what revision the AHJ was on at the time.

I've seen tandems tied as you described.

Good to know about the living space and restrictions on a bathroom circuit. There's a receptacle in the shed as well but I don't think it's ever been used.

Well, good news and bad news. I was wrong about the MWBC, it's just a 12/2 cable that was twisted enough that I thought it might have been a 3 wire cable. I also missed a cable coming in from the other side which would add a wire for the tandem breaker.

For the bad news - the neutral and ground are bonded. I also don't like what appears to be feeding the subpanel. Looks to be the same size wire as the A/C circuit and I suspect the panel is only fed by a 30 amp breaker.

Sub panel in question:

Main service panel. Double pole breaker sizes, clockwise from top left - 15, 40, 30, 30.

The breakers in the sub-panel are (from left to right) ITE/Siemens, Westinghouse, and GE (I think).

And just to make things interesting the entire mess sits on the wall over an open sump pit. ;-)

Gak, FPE panel, too. Rip it all out and do it right. By the way the goofy subpanel that isn't an FPE STABLOK does indeed appear to be an ITE Siemens but it has Westinghouse (Type BR) breaker. While it fits and works, it's not legal either.

And a couple failure-prone Federal Pacific panels to throw into the mix. I'd recommend having one or more new panels installed.

The main panel is circa 1958, the second FPE panel is circa 1970, and the Siemens panel is circa 1986.

As I said the whole mess sits almost above an open sump pit, certainly within 3 feet open space required in front of a panel. I'm guessing it would have to be moved??? Because of the layout of the basement, mechanicals, etc. I'm guessing the nearest spot would be roughly 25 feet away.

The fun never ends!

Any chance of covering the pit? Since two of the panels are problematic and the third is defective in installation (improper grounding and wrong breakers), I'd put one panel in with a suitable number of poles.