I have 6 power outlets in my kitchen. Two 20amp breakers in my panel supply power to them.
I wanted to update my GFCI outlets, with new ones (old ones were installed when the house was built, 1994). Cut off both breakers marked "kitchen outlets" in my panel. I pulled all six outlets, and thought I'd measure voltages at each junction box feeding the outlets.
I turned on the FIRST breaker, thinking the circuits were isolated. Measured the voltages with my multi-meter. Three registered 120v, and 3 registered 24 volts.
I turned on the SECOND breaker, and all 6 outlets now register 129v.
I turned OFF the FIRST breaker, and the the three that were 24v the first time, now had 120v reading, while the ones that had 120v, now had only 24v (the readings flipped).
Does this make sense? What's going on here?
Best Answer
This is a multi-wire branch circuit
aka MWBC. This is a special arrangement for delivering two circuits' worth of power onto a single cable with only 1 more copper wire, because it is putting two opposite-pole subcircuits arranged so the neutral only carries differential current. It's quite wire-efficient, but yours was installed quite improperly. They pretty much did everything wrong.
Also the six GFCI receptacles were not original; they were added probably by a previous seller because the home inspector red-flagged it. If you know anything about GFCIs, you only need 1 per circuit, but an MWBC is one circuit -- and the way they share neutral requires either one GFCI at the breaker, or a GFCI at each receptacle (expensive).
The first problem is that MWBC requires a handle-tied or 2-pole breaker. You don't have one. In fact, you haven't said, but you might even have the worst nightmare of MWBCs, which is a duplex breaker.
Fix the breaker... and revisit GFCI
OK, so the breaker needs to be replaced with a 2-pole breaker. (you can do handle-ties, but those are a PITA to find, and a 2-pole breaker is available pretty much anywhere.)
However, remember where I said an MWBC can have either
When you have more than 4 receps, it's actually cheaper to fit a single GFCI+breaker. Around $20 each for six GFCIs vs about $80 for a 2-pole GFCI breaker ($90 if you want AFCI also).
The only downside of this is that a GFCI trip with receptacles will only trip the single recep (and you can reset it right there), but a GFCI trip with a breaker will knock out everything, which may be confusing and will send you on a trip to the basement to reset. Your call.
But you need a 2-pole breaker. Remember, breaker types must match the panel. If the panel is Murray QP but the breaker is Eaton BR, the breaker is wrong and replace with a Siemens/Murray QP not a BR. Alien breakers are dangerous and burn up panel buses. This is especially important when spending $80 on a breaker!
If you currently have a duplex breaker (two handles in 1 space) then ask a new question about how to replace that with a 2-pole.
Neutrals need to be pigtailed
You didn't mention this, but the neutral wires in the boxes need to be spliced with a wire nut etc. and a single neutral wire brought out to the receptacle.
Removing a device must not sever power to the other half of the circuit.
However, it's perfectly OK to use the GFCI receptacle as a splice point for the hot wires. Note that ALL these wires will go on the LINE screws (most accept 2 wires in back-wiring), the LOAD terminals will never be used here.