Replacing breaker with GFCI/AFCI causes trip

Question

As part of my basement finish, I need to replace a breaker with a AFCI.

This circuit currently services the new bedroom (outlets already there) and an outlet outside. One of the outlets is already GFCI but I decided to use GFCI and AFCI at the breaker.

I bought a GFCI/AFCI combo and replaced the old one. When I was replacing it, I discovered the cable coming in was 14/3 style, with the red and black wires each on their own breaker (call them breaker 1 and 2). I replaced breaker 1 with the new unit. So now the red is on breaker 1 (the AF/GFCI) the black is still on breaker 2, the white wire is connected to the AF/GFCI , and the curly pigtail from the new unit is connected to the neutral bar, as the instructions state.

With both breakers 1 and 2 off, if I turn on breaker 1 (the new one), everything seems to work. When I then turn on breaker 2, breaker 1 trips. The LED for it indicates the following from the instructions: "Ground fault/grounded neutral. Current has found an alternate path to ground, or the neutral and ground are in contact downstream of the breaker."

I replaced 2 others in the box with no issues, but these don't have GFCI outlets downstream, and they don't have a 14/3 cable. I plan to replace the GFCI outlet to a normal one next, but my hunch is that these 2 circuits sharing a neutral is the cause.

Does anything jump out to anybody about this setup? How would I resolve it? Do both of those circuits need to have AFCI/GFCI breakers?

Thank you!

Answer 1

What you have there is a Multi-Wire Branch Circuit

This is where 2 hot wires in a single cable share a netural wire. The neutral handles return current for both hot wires, and this is safe if certain rules are followed.

Despite being functionally two circuits (e.g. for "kitchen and bathroom dedicated circuits" purposes)... it is actually one circuit.

The rules:

• The two breakers must be phased correctly i.e. there must be 240V between the hots. (this a good thing. 0V between hots means overloading the neutral.)
• In locations where both hot wires are present, the neutral must be pigtailed to devices - you can't use a device as a splice block, as is often done with receptacles.
• The two breaker handles must be handle-tied, or alternately, use a 2-pole breaker.

GFCIs look for "current going missing"

The GFCI operates by comparing the current on the hot wire, and the current on the neutral wire. Under normal operation they should be the same. In fact, wires throw considerable magnetic fields. When current is equal and opposite on 2 wires, the magnetic fields cancel each other out.

If current is different on the 2 wires, it means current is going out on one wire and returning on some unexpected path, e.g. through a human being, shocking them, and then to ground and back around to neutral eventually. This "not all current accounted for" state is what makes a GFCI trip. It's easy to detect: the magnetic fields don't cancel each other out!

How does GFCI work with a 3- or 4-wire circuit such as a MWBC or 3-phase? Easy - we're already accounting for current direction, just do that with more wires. And again if everything is balanced, the magnetic fields cancel each other out. I.E. if the 3 current flows are +10, -9 and -1, that's OK.

So now you see your problem.

You have a 3-wire (+ground) MWBC, but you only have the GFCI operating on 2 of the wires. Any activity on the 3rd wire will trip it.

Most AFCIs also have a GFCI functionality. Those that don't have no neutral terminal.

The solutions are:

• use a 2-pole GFCI or AFCI (though a 2-pole GFCI+AFCI may be a tough find)...

• Use a 2-pole AFCI and then use GFCI receptacles. If you have a GE panel, their 2-pole AFCI offering is two single-pole AFCIs that do not supply neutral, and which can be handle-tied. These are designed for MWBCs. Siemens has a similar offering for Siemens/Murray/ITE panels. Do not use them in other panels.

• If this is a retrofit to an existing circuit, use a plain breaker and 120V AFCI/GFCI receptacles as needed. Of course this does nothing to detect wiring faults.