I'm replacing an duplex outlet in our kitchen and when I removed the outlet, I uncovered some surprises that have me baffled and concerned. There are two cables entering the box, one with three conductors plus ground, the other other with two conductors. Here's a crude diagram, with measured voltages:

Wiring and Voltages in Box

All three wires were connected to the outlet via backstab terminals, but I wasn't smart enough to take note of which wire went where.

In a lot of ways, this looks to be wired like a split outlet, with one separate phase going to each plug. In that case, wire 2 (white) would be the common wire for both outlets, with Wire 1 and Wire 3 beings the two hot wires from different phases. BUT the original outlet is not split - both tabs are in place and a check with my DMM shows continuity between the respective plugs. So how this was wired is a total mystery.

One or both of these circuits also seem to connect downstream to a built in microwave which is no longer functional with the outlet removed, regardless of which breaker I throw. I really wish I'd taken a photo of the existing outlet with wiring intact before I removed it, but I didn't. Can anyone help me figure how this was wired?

  • Is breaker A a 2-pole breaker or two adjacent breakers joined by a handle-tie? Are breakers A and B right next to each other in the panel? What makes you say the lower cable is controlled by breaker B? Is there a light switch anywhere around whose use is not accounted for? What is your ultimate endgame here? (Please tell me you didn't buy a 10-pack of GFCIs and are going to town). How old is this wiring? Commented Aug 19, 2017 at 22:28
  • Also don't install any more GFCIs until we have this conversation. There's a pretty good chance we can protect your whole kitchen and also correct the codevio, all by buying one single thing. Commented Aug 19, 2017 at 22:30
  • @Harper The breakers are not two pole and are not tied in or even adjacent on the panel. I don't have any light switches that aren't accounted for (both working and with an identified breaker). All I am trying to do is to replace the outlet. The old one has a broken TR bit that was preventing use of the outlet without some real brute force. The wiring was done by a licensed electrician (possibly my next stop) about two years ago as part of kitchen reno. This outlet has a twin on the other side of our cooktop, which is wired as conventional split outlet.
    – PhilW
    Commented Aug 19, 2017 at 22:38
  • Oh yeah, I know which breaker controls which hot wire because I initially tripped Breaker A which killed the outlet. But when I took the plate off and pulled the outlet, my non-contact tester told me I still had a hot wire. I identified Breaker B after some trial and error.
    – PhilW
    Commented Aug 19, 2017 at 22:42
  • Ok, easypeasy then, but you still have two serious (but easy to correct) safety codevio's that should be fixed ASAP - and separate from that, you might not have GFCI in your kitchen. My battery is at 9% so I will write something up when able Commented Aug 19, 2017 at 22:49

2 Answers 2


You have a multi-wire branch circuit (MWBC)

In this arrangement, the two hot wires are on opposite poles (VERY important) and they share a neutral. Two code requirements come out of that.

Neutrals must be pigtailed: removing a device (like that receptacle) must not sever the neutral for the other half of the MWBC. The way the last guy used the two (non-broken) neutral screws to splice the neutral violated that rule.

Both breakers in an MWBC must be handle-tied, so they have common maintenance shut-off. You must make the breakers adjacent, then either join them with a handle tie, or replace them with a 2-pole breaker.

It may help to disconnect and abandon any wiring you are not using. It is abandoned if you disconnect wires at both ends and cap them off.

While you're at it...

New work would require GFCI protection in the kitchen. To protect a MWBC, use a 2-pole GFCI breaker. Expensive, but possbly cheaper and definitely simpler than fitting GFCI receptacles on a MWBC.

  • So, in my diagram, Wire 1 was one hot, Wire 3 was the other hot and Wire 2 was the neutral? I kinda get this, but am curious as to how the outlet was wired in the first place. I still can't figure how the three wires were hooked up to the same outlet and why removal of the outlet caused the microwave to go offline. Replacing the two breakers with a 2 pole looks entirely possible in my box - there's an unused 2 pole in there right now. But what happens on the outlet end?
    – PhilW
    Commented Aug 20, 2017 at 0:03
  • The receptacle in question does not require GFCI protection as it doesn't serve a kitchen countertop surface (210.8(A) point 6 reads "Kitchen -- where the receptacles are installed to serve the countertop surfaces") Commented Aug 20, 2017 at 0:03
  • @PhilW i suspect your diagram is quite wrong, but I did't want to bother reviewing it. Unsplice the two blacks and measure again, I suspect you will get quite a different result. Commented Aug 20, 2017 at 0:08
  • @Harper The diagram is literally correct - all three wires shown come from the only two cables in the box and the hots respond to their respective breakers. All three wires were attached to the outlet and none of the tabs were broken. I can unsplice the two blacks, but what am looking for?
    – PhilW
    Commented Aug 20, 2017 at 0:11
  • When you unhook the blacks from each other, one of them will be 240V from the red, the other will be open-circuit. Which one matters a lot. Also when you unhook them, the white should cease to be 240V from the red. Also, something has lost power. Something powered 24x7. Commented Aug 20, 2017 at 1:13

Depending on the year built this could have been a totally coda compliant multi wire branch circuit. The red being 1 leg and the black being a second leg with the white neutral common to both circuits. In the past the breakers did not require handle ties for the last few code cycles this has been a requirement. Some electricians will split the hot tab on an outlet and wire the top outlet in kitchens with 1 leg and the bottom on the second . others would wire every other outlet with 1. Then the other on the next. All of this was up to code and before the late 70's gfci's were not required. Installing GFCI'S on multi wire branch circuit can be problematic.

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