Wiring – Looking for or adding a ground to an existing NEMA 10-30 socket

240vgroundingwiring

I have an unused older 3-wire dryer socket (NEMA 10-30R) in my garage, I would like to upgrade this to a NEMA 14-30 socket with an separate ground. I'll be using this for an EVSE unit for electrical vehicle charging.

The previous owner did some newer wiring around the box containing this socket and I suspect he might have either run a ground to the location or installed a ground wire to a water pipe or something.

I have two related questions:

  1. If I shut off the breaker and take off the socket, what am I looking
    for that will indicate a ground wire? Would it just be an unused
    fourth wire in the box?

  2. If I find only a 3-wire run to this box, is there a safe DIY way to
    add a ground myself without running back to the breaker panel?
    There's a very new furnace/air-handler and water heater very close,
    would either of those have a ground wire I could tie into or should
    it be something else?

Best Answer

A ground wire in a NM run for a dryer is indeed an unused fourth wire in the box, and it will be bare. If your garage dryer socket was instead run using AC (BX, in other words) or metal conduit, then the box itself is grounded, and should be pigtailed to to ground the receptacle. (In the rare case the circuit was run with MC, there will be a green ground wire in the box.)

If you're comfortable using a multimeter, you can turn the circuit off at the panel and measure the resistance to the neutral wire -- the ground will only have a few ohms to the neutral as they are connected at the service entrance.

Adding a ground to a three-wire, 120/240VAC circuit is best handled by running a separate ground wire back to an allowable grounding point as per NEC 250.130(C), using single conductor bare armored ground cable. In the 2014 NEC, point 4 does allow you to tap the nearest EGC of a suitable size originating from the same enclosure as the recipient branch circuit; but, since a 30A circuit needs a 10AWG copper EGC, you can't use a 15 or 20A circuit's EGC as the donor.

Also, if your appliance can do it, you can install a neutral bonding jumper and change it over to a NEMA 10-30 plug on a 3 wire cord. (Just about all dryers can do this.)

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