Can two circuits with dual function breakers share a box?

Question

Corded Dishwasher and Disposal each have own 20a circuit with dual-function breakers; however, they each go to a single receptacle that share the same 2-gang box.

Am I correct that splicing the grounds together - as one would normally do when sharing a box - would render the GFI of the dual-function breaker useless? Would the only way to appropriately resolve this be to utilize two separate single-gang boxes? Are there any exceptions to having two different circuits sharing a box yet not sharing ground?

Answer 1

It's the neutrals you need to firewall

GFCI and DFCI devices simply do not care about ground. (Note how a GFCI breaker has no ground connection -- it has line-hot, line-neutral, load-hot, and load-neutral, and that's all!) In fact, one could pull two hots and two neutrals through an EMT conduit to a metal box, thus having a single ground path all the way from the panel to the box, and have this wiring setup work, provided the receptacle box is wired correctly.

And in that, I mean that each DFCI needs to supply both its own hot and its own neutral to its corresponding receptacle, with no cross-connections between hots or neutrals; if you get this wrong, you'll get unwanted tripping due to wayward neutral current bypassing the DFCI that it originally ran through to get to its corresponding appliance. This is because GFCI protection relies on the First Rule of Electricity; namely, that current flows in closed loops. As a result, if the current going out on the hot wire doesn't match the current coming back on the neutral wire, that current has to be going somewhere else, and the GFCI assumes that "somewhere else" is you and kills the power to save your bacon.

So, once you have two independent hot/neutral pairs as well as a grounding path to the box (this can be done as 12AWG wires in a conduit, as a single 12/2/2 cable, or as two independent 12/2 cables), you can wire this all up in a single 1-gang box of sufficient size (18 in³ minimum). You simply need to use a regular NEMA 5-20T duplex receptacle, preferably specification grade, and break both tabs off (i.e. not just the tab/fin on the hot side, but the one on the neutral side as well). As a result, you'll have two independent 20A receptacles on the same mounting yoke -- each receptacle is connected to its corresponding pair of brass and silver screws, but not to the other receptacle or the other receptacle's terminal screws.

From there, it's just a matter making sure that your first hot/neutral pair goes to the terminal screws for the top receptacle and your second hot/neutral pair goes to the terminal screws for the bottom receptacle. If you mix the wires up at this point, once again, you'll get reminded when your DFCIs trip on you as soon as you put a load on the circuit, for the same reason as before.

Answer 2

No, combining the ground will not render the GFCI useless. Just imagine for a minute any metal j-box, or any metal stud construction building, all those grounds become interconnected through the yokes of the receptacles.

A GFCI receptacle doesn't use the ground to function, it is required to be grounded if a ground is present, but notice if you look at the GFCI installation instructions usually around step 7 says to Connect the grounding wire (only if there is a grounding wire).

A GFCI receptacle trips when the magnetic field on the hot and neutral don't cancel each other. The test button on the receptacle just allows about 5mA of current to bypass the sensor on one of the wires allowing a magnetic field to be present in the current sensor. Sometimes two-wire circuits cause some confusion when using a receptacle tester, the tool does need a ground to function, a Caution in user instructions of some of the testers points out that a false fail will result.