How to wire GCFI's with 2 Hots and a single Neutral

Question

I'm getting my house that was built in 1978 ready to sell. I'd tried adding GFCI's in the kitchen. I have an L shaped counter. I have 3 outlets on one wall and 2 outlets on the other. The long wall used to have a frig to the right of the sink. This outlet box has 1 Bk, 1 Red and 1 White coming from the basement. Bk is connected the Circuit Breaker in slot 1 and Red is connected to Circuit Breaker in slot 2. Black and White connect to the duplex outlet. The Bk and White then go to another box towards the left. The Red is wire nutted and goes up the same pipe as the Bk and White.

This 2nd box; with the same 3 wires has another duplex outlet and a switch for the sink light. This 2nd duplex is also connected to the Bk wire. These same 3 wires also go to the left and land in another box on the other side of the sink. This 3rd box also contains another duplex outlet connected to the Bk wire. The Red is again nutted in box 2 and box 3.

So, I replaced the duplex outlet in box 1 with a GFCI. I used the Bk and White wire from breaker 1 and connected to the Line side. I used the load to feed box 2. When I turned the light on it tripped the GFCI. Putting a circuit tester in the outlet in box 3 also tripped the GFCI.

Looking closer, I noticed the light switch was connected to the Red wire from breaker 2. So I moved the switch to the hot on the duplex outlet. The light now works. Using a circuit tester, the GFCI and duplexes on both sides of the sink check good.

Going around the corner to the left are 2 outlets on ether sides of a stove. This next outlet in line (box 4) has a Red wire for hot. So, I replaced it with a 2nd GFCI. The green light came on and all was fine; until I plugged my circuit tester in. That caused GFCI 1 to trip. Now this 4th box has a Red coming in and a Red going out. It also has the Bk wire nutted. Looking at box 5, I see the outlet has Bk wires and the Red is nutted and going somewhere. It may be going to an outlet near the door where there used to be another counter/island. (New Frig location)

So now I'm lost. I fixed the light problem by putting it on the same circuit as the other outlets. (Using the Bk circuit) I'm thinking this outlet is causing the same issue as the switch when it used the Red. I think GFCI 1 sees a current draw on its neutral and trips whenever I try to use the Red circuit 2. Any suggestions?

This is an old house with #12 solid wire in conduit running into the attic for all of the hops. Please tell me I can fix this without pulling another wire. That looks painful. Will switching from GFCI outlets to breakers buy me anything?

Thanks in advance to any and all who can help.

Bob

Answer 1

In your situation where you have one neutral shared between two circuits, you can't use the "load" side of the GFCI receptacle.

That's because stuff downstream on the other circuit will feed return current on the neutral, so the GFCI will see an imbalance and trip. To put it another way, if you have 5A of current on the red wire and 2A on the black wire, you'll have 3A on the neutral wire (the red & black cancel, with the remainder on the neutral). Since the current on the black & neutral aren't in balance, the GFCI trips.

There are two options in this situation:

1. Install a 240V double-pole GFCI in the breaker panel. This type accounts for varying amperages in the two "hot" wires. This is also the expensive solution, and it's not ideal because if you trip one circuit the other one trips, too.
2. Replace all of the receptacles on the circuit with GFCI, using only the "line" side of the GFCI. Use wire nuts & pigtail jumpers to join the wires coming in & going out of the box. This option is more work, but may be cheaper. It's also more convenient because if you trip the GFCI on one receptacle, only that receptacle should be affected.

The refrigerator ideally should NOT be on a GFCI, because you don't want it to trip and ruin all your food. However, if the fridge is plugged into a GFCI that's within 6 feet of the sink, it still must be GFCI protected because of the proximity to the sink (though it may not be a problem if it's behind the fridge or is a single rather than duplex receptacle).

Here's a good primer on how GFCI works.