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I have a plastic switch box with two circuits going to it.

One circuit is 15 amp which is a 3 way switch and another 2 way switch to control a different light. The other circuit is a 20 amp circuit and currently it has a 3 way switch... but the wires actually were not hooked up to it, but instead just the white and black wires tied together in a single wire nut.

There is no electric running through these wires but I know where the wire for this circuit runs to a outlet a few feet over and it’s on a 20amp breaker. I’m assuming this wire and switch was once used with some other switch to control a light but were eventually removed. I assume this only because I could not find any reason they would just be tied together and did not have power then running through either the hot or neutral.

My only concern is they still used the ground and tied it in with the other 15 amp switches/circuit. Is this allowable considering it’s a 20amp circuit and a 15 amp circuit considering the 15 amp has 14awg wire?

Seems if there was a ground fault it is a fire hazard for the high gauge wire. Also the ground from the 15 amp circuit seems to have 50 volts running through it so potentially a leak somewhere in that circuit which is carrying over to the 20 amp circuit.

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    Any current through the ground should be momentary until the circuit breaker trips, so the ground wire doesn't have the same sizing guidelines as hot and neutral. However, Code should have some guidelines, and the experts here should let us know what they are.
    – Armand
    Jun 13, 2022 at 21:45
  • Here's a related question: diy.stackexchange.com/questions/69645/…
    – Armand
    Jun 13, 2022 at 21:45

1 Answer 1

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The question is a little unclear. Assuming that:

  • The 15A circuit is in a cable with a ground wire
  • The 20A circuit is in a cable with a ground wire

Then connecting the two grounds together is exactly what you are supposed to do.

I am assuming that these circuits are in cables because if you had conduit then you would usually have metal boxes. You can have metal boxes with cables, but not (at least generally) plastic boxes with metal conduit.

The primary rule is "all grounds together". The secondary rule is "all grounds to the metal box" - but this is a plastic box so that doesn't apply.

As far as 15A vs. 20A, in a minor fault situation it wouldn't matter. In a major fault (hot shorted to ground), the breaker should trip long before wire heating is an issue.

There is a similar sort of question regarding using a ground wire from one circuit to add ground to other circuits that did not originally have a ground wire. In that case, the ground for a 15A circuit would (assuming it is the minimum size) not be valid for a 20A circuit, but the other way around would be perfectly fine.

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    It sounds like the unused wire is for a switch on one of the duplex outlets. Separate them and see what goes out. It is possible it is only effective on 1/2 of a duplex.
    – Gil
    Jun 14, 2022 at 0:38

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