Two-slot receptacles, which do not have the additional round slot for a ground connection, were standard until grounding for electric receptacles was mandated by the National Electric Code in the early 1960s. They are still considered acceptable for locations in older homes where appliances like a table lamp or alarm clock are used that do not require a 3-prong (grounded) cord. But a refrigerator, microwave, washing machine, or other new appliance that requires grounding cannot be plugged into an older receptacle.

   The adapter plugs sold at hardware stores for about a dollar—nicknamed “cheater plugs”—provide a way to connect a three-prong cord into a two-slot receptacle, but do not actually provide the necessary ground connection. A small clip that protrudes from one side of the adapter plug is designed to be screwed to the face of the cover plate with the set-screw at the center of a duplex receptacle, thereby providing an illusion of a ground connection through the screw to the metal components in the receptacle and backwards to the panel and ground. But that rarely happens, because there is often no continuous, electrically conductive route between most receptacle boxes and the main electric panel. In the photo above, the homeowner didn’t even bother to set the screw connection.

    It is possible to test an ungrounded 2-slot receptacle for grounding of the box to see if an adapter would actually function properly, by using a “wiggly” tester, like the one shown below. They are available at most hardware and home-improvement stores. After confirming that the receptacle is live by placing a lead in both the hot and neutral slots so that the wiggly lights up, leave one lead in the hot slot (the smaller slot) and put the other lead on the center screw. If there is a ground connection, the pen will light up. Some wiggly testers have only a red light, but the one shown has two lights that so that it can differentiate between voltages.

    But, again, you will very rarely get the wiggly to light up at a two-slot receptacle. So hardware store adapter plugs essentially make a safe two-slot receptacle into an unsafe three-slot receptacle. They are a safety hazard that we regularly write up in inspection reports on older homes.

    A modern receptacle has a third bare-copper wire for grounding embedded in the electric cable serving it. Running a new ground wire back to the panel when upgrading an old receptacle to three-slot is difficult-to-impossible. But there is a code-approved alternative: install a three-slot GFCI-receptacle. The shock protection provided by a Ground Fault Circuit Interrupter receptacle (the kind with a TEST and RESET button in the center) is considered an acceptable alternative to grounding. The only additional step required is to apply a small sticker that says “No Equipment Ground” to the cover plate, which comes in the package with all GFCI-receptacles.

   This solution is only acceptable when replacing a receptacle and leaving the existing, older wiring in place. Any new wiring must be three-wire (grounded).

    Also, see our blog post How can I tell if the electric receptacle outlets are grounded?

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

  To learn more about electrical wiring, devices, and receptacles, see these other blog posts:

 • What is the difference between what trips a GFCI (ground fault) receptacle and a circuit breaker?

• What is the code requirement for GFCI protection for receptacles near a wet bar sink? 

• What is the requirement for a service receptacle outlet for heating, air conditioning, and refrigeration (HACR) equipment? 

• Why is an opening in the wall around the side of an electrical receptacle outlet a safety defect?  

• When was GFCI-protection for kitchen dishwasher receptacle outlet first required? 

• What is allowable voltage range at a wall receptacle outlet in a house?

• When should I replace electric receptacle outlets? 

• Does an electric receptacle outlet in a storage shed require GFCI protection? 

• What are "self-contained" electrical receptacle outlets and switches?

• What is the difference between an electrical receptacle, an outlet, and a plug?

• Does a washing machine receptacle outlet require GFCI protection?

• What is the building code requirement for receptacle outlets at stairs and stair landings?

• Can I remove a 240-volt range receptacle and hard-wire the range? 

• What is a "backstab" receptacle outlet? 

• Why are some electric receptacle outlets upside down (ground slot up) in a house?   

• What is the height requirement for an electric receptacle outlet? 

• Where are GFCI receptacle outlets required?

• When were GFCI receptacle outlets first required?

• Does a home inspector remove receptacle outlet cover plates?

• What is the minimum height for an exterior receptacle outlet? 

• When was the current receptacle/outlet spacing of 12-feet first required? 

• When was the three-slot (grounding) outlet/receptacle first required? 

• Why does painting an electric receptacle (outlet) make it unsafe?

• Why is there no bathroom electric receptacle in this old house? 

• How far apart should the electrical receptacles be placed?

• What are the most common problems/defects found with electric receptacle outlets during a home inspection?

   Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.