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I have an AC unit on a dedicated 20A, 240V circuit running with 12ga THHN wires in 1/2 PVC conduit.

In the middle of the conduit I have a metal waterproof convenience receptacle wired on a separate 15A circuit with 14ga wire.

I have the receptacle and box bonded using the green ground wire of the receptacle. The 12ga wires pass through the box with no connection.

The question is whether it's acceptable to have the box grounded with 14 ga wire when technically the 12ga wires could short to the box and then the fault current would be running through the 14ga wire, which is too thin for the 20A breaker. In almost any real scenario the 20A breaker would trip before the ground wire was damaged, but I would like to know the code requirements.

3 Answers 3

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The grounding conductor needs to be suitable for the largest circuit in the box.

A circuit "just passing through" is still in the box.

You only need one ground, but it must be suitable for the largest circuit in that box.

Note that once the grounding wire gets to 10 AWG copper or 8 AWG aluminum, it's good for circuits up to 60A. The rules are more conservative for 15, 20 & 30A circuits.

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    I imagine both separate circuits have their own ground wire. Can the 20 amp ground be bonded to the box(combined with the 14 amp ground)?
    – crip659
    May 12, 2023 at 12:31
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    There's no reason (other than installer inexperience or the uncommon "isolated ground" outlet setup) for separate circuits sharing one conduit to have their own grounding conductor. If you have a grounding conductor for the 20A circuit, it should be joined to the metal box, and that's sufficient for both circuits sharing the conduit. But if you also have a 14AWG grounding wire, that can also be jointed to the box - or removed from the conduit where it shares with the 20A circuit as it's superfluous.
    – Ecnerwal
    May 12, 2023 at 12:39
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    This is my first time running PVC...with EMT I didn't have a ground wire at all. Is there no limit to "ground sharing"...if I had 10 different 20A circuits and 10 more 15 A circuits running through a conduit, I could just pull a single 12ga ground wire? May 12, 2023 at 13:02
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    Correct. Except that due to other rules you'll rarely have more than 3 circuits in a conduit. May 12, 2023 at 13:12
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Sharing the ground is fine as long as the ground is big enough for all circuits.

If this is a plastic box, then you don't need to worry about the 12 AWG wires shorting against the box.

If this is a metal box, then code requires the grounds be attached to the metal box first using the ground lug(s) provided on that box. Grounding to the receptacle is done as a secondary matter if that's even necessary - since many receptacles are able to pick up ground off the metal box.

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On this subject the NEC language keeps changing, it appears for clarity, not intent. 2023 NEC reads

250.148 Continuity of Equipment grounding conductors and connection in boxes

(C) Metal Boxes. A Connection used for no other purpose shall be made between the metal box and the equipment grounding conductor(s). The equipment bonding jumper or equipment grounding conductor shall be sized from table 250.122 based on the largest overcurrent device protecting circuit conductors in the box.

Above is a preview of the NEC. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2023 NEC® edition of NFPA 70.

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