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

Question

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?

Answer 1

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.)

Answer 2

First off the EVSE is usually hard wired if you are using a 240 volt station so you wouldn't need the plug at all. If you want to plug it in though refer to the Code.

The 2011 NEC does not allow you to do this at all for anything but a 120 volt system.

625.13 Electric Vehicle Supply Equipment. Electric vehicle supply equipment rated at 125 volts, single phase, 15 or 20 amperes or part of a system identified and listed as suitable for the purpose and meeting the requirements of 625.18, 625.19, and 625.29 shall be permitted to be cord-and-plug- connected. All other electric vehicle supply equipment shall be permanently connected and fastened in place. This equipment shall have no exposed live parts.

The 2014 NEC allows you to do it with conditions:

625.44 Electric Vehicle Supply Equipment Connection. Electric vehicle supply equipment shall be permitted to be cord- and plug-connected to the premises wiring system in accordance with one of the following:

(A) Connections to 125-Volt, Single-Phase, 15- and 20-Ampere Receptacle Outlets. Electric vehicle supply equipment intended for connection to nonlocking, 2-pole, 3-wire grounding-type receptacle outlets rated at 125 V, single phase, 15 and 20 amperes or from a supply of less than 50 volts dc.

(B) Connections to Other Receptacle Outlets. Electric vehicle supply equipment that is rated 250 V maximum and complying with all of the following:

(1) It is intended for connection to nonlocking, 2-pole, 3-wire and 3-pole, 4-wire, grounding-type receptacle outlets rated not more than 50 amperes.

(2) EVSE is fastened in place to facilitate any of the following: a. Ready removal for interchange b. Facilitation of maintenance and repair c. Repositioning of portable, movable, or EVSE fastened in place

(3) Power-supply cord length for electric vehicle supply equipment fastened in place is limited to 1.8 m (6 ft).

(4) Receptacles are located to avoid physical damage to the flexible cord.

All other electric vehicle supply equipment shall be permanently wired and fastened in place to the supporting surface, a wall, a pole, or other structure. The electric vehicle supply equipment shall have no exposed live parts.

Second, Follow ThreePhaseEel's advice on the ground wire for the circuit. Grounding for a circuit in a garage is especially important.

Third, an EVSE is considered a continuous load so the circuit breaker and wire for an EVSE have to be sized 125% of the ampacity of the load.

Here is a quote from the 2011 code (this is 625.40 in the 2014 code):

625.21 Overcurrent Protection. Overcurrent protection for feeders and branch circuits supplying electric vehicle supply equipment shall be sized for continuous duty and shall have a rating of not less than 125 percent of the maximum load of the electric vehicle supply equipment. Where noncontinuous loads are supplied from the same feeder or branch circuit, the overcurrent device shall have a rating of not less than the sum of the noncontinuous loads plus 125 percent of the continuous loads.

Good luck and thanks for helping to save the planet!

Answer 3

1. If there is a ground wire available in the box, it will probably be bare uninsulated copper, or have green insulation, solid green or green with a yellow stripe. If the wiring is run in metallic conduit to a metal box, the box itself may give you the ground you need.

2. You can't might be able to (see edit) connect to the ground for another branch circuit, but for a retrofit like this you may be able to run a ground wire to the ground rod or other grounding electrode, or the wire attached to the ground rod (grounding electrode conductor).

There is usually a grounding jumper in the dryer that you can install with a three wire cord. This provides protection in the event of a wiring fault without changing your wiring. Unless you're 100% sure you've got a good ground, it might be better to just set up the dryer for a three wire power connection.

edit: In locales where the 2014 NEC is adopted it is allowable to connect to the ground on another circuit provided the wiring goes back to the same enclosure, right size, etc. - that may or may not apply to the hot water heater or furnace equipment circuits.

Answer 4

1. A proper grounding wire would be a bare wire coming out of the same cable that has the 2 hot wires and the neutral. For example 10/3 w/g (with ground), e.g. 3 insulated wires and one bare copper wire, all coming from the same cable.

2. Define "safe". If "safe" means is there some way to jury-rig a ground wire to some nearby device that is itself properly grounded such that a short-circuit will definitely trip your breaker...maybe. If "safe" means that if something goes wrong while you're not around and your house burns down and your home insurance inspector sees that wiring and will still cover the damages...no. Do it the right way...if your existing cable doesn't have a ground, replace it with one that does.