Two-prong receptacle ungrounded
Photo: Keate
Two-prong receptacle (ungrounded)
Old-fashioned two-prong receptacles connected to two-wire cables don't have the ground wires that protect people and electrical devices in case of a fault. Yet it is possible to retrofit a new three-prong or GFCI receptacle into the same outlet box without any rewiring, as long as the box itself is grounded.

Luckily, metal boxes attached to armored, or BX, cable—a type of wiring commonly found in old houses—generally are grounded; the cable's flexible metal jacket serves the same purpose as a dedicated ground wire.

To switch out your receptacles, just follow the steps.

1. Check for ground. Insert one prong of a circuit tester into the receptacle's hot slot (the shorter one), and touch the other to a screw that secures the cover plate. The tester should light up. If it does not, the box is not ­grounded. You can install a GFCI (see tip at bottom), or call an electrician to fix the wiring.

2. Remove the old receptacle. Turn off the power at the ­breaker panel or fuse box. Unscrew the old receptacle from the box and detach the wires.

3. Connect the new receptacle. Attach the black (hot) wire to the brass terminal and the white (neutral) wire to the silver. On a GFCI, use the terminals in line with the "line" label on the back of the receptacle. (If your box is not grounded, skip to Step 6.)

4. Fasten the ground screw. This green screw, sold in hardware stores, fits in a threaded hole in the back of the box. Hook one end of an 8-inch green grounding wire or pigtail (also available at hardware stores) to the screw and tighten it.

5. Ground the receptacle. Secure the other end of the 8-inch grounding pigtail to the green grounding terminal on the three-prong or GFCI receptacle. Insert the new receptacle into the box.

6. Turn on the power. Use a circuit tester to make sure the circuit is working.

TOH Tip
Even if an outlet box isn't grounded, installing a GFCI in it will still protect you (and your tools and appliances) from ground faults. But an ungrounded GFCI can't safeguard sensitive electronics, such as a computer or phone, from the interference caused by stray currents. The National Electrical Code requires you to stick a label on the receptacle that reads, "No equipment ground." These labels come in the box with a new GFCI.
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