The National Electric Code (NEC), which specifies the standards for electrical installations, is dedicated to safety and fire prevention. Every three years the code is revised and updated, and the 1962 edition of the code was the first to require that all 120-volt electric receptacles in a home be three-slot. That third, rounded slot provides grounding, which decreases the risk of electrical shock—or even electrocution—when using an appliance that is also properly wired.
The NEC gradually ramped up to this standard, beginning in 1947 with the requirement of three-slot receptacles in the laundry only. Then the 1956 edition of the code expanded the required three-slot outlet locations to include outdoors, garages, and basements. They were going for the wet locations first, where a person was most likely to be shocked, before raising the standard all the way six years later to include the whole house.
Two-slot receptacles immediately became a thing of the past, except that the code still allows replacement of an existing two-slot receptacle with another one. A similar sequence of gradually increasing the requirement happened in later years with both GFCI and AFCI protection for receptacles, which provide further shock and fire safeguards for home occupants.
The changes that come with each new edition of the NEC are not automatically adopted by local building departments. See our blog post Is the latest edition of the National Electrical Code (NEC) the standard used for the electrical system of new homes? for more on this.
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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 are electrical outlets and plugs polarized?
• How many electrical receptacles (outlets) are required in a hallway?
• What problems does having too many electric receptacle outlets on a single circuit cause?
• Is a house required to have outdoor electric receptacle outlets?
• How I can tell if a receptacle outlet is tamper resistant?
• Why is there a GFCI breaker in the electric panel for the bathroom shower light and exhaust fan?
• What is a false ground, bootleg ground, or cheated ground receptacle?
• How can adding wood paneling or a wainscot create an electrical safety hazard?
• How far apart should kitchen counter receptacles be spaced?
• How far above a kitchen countertop do electrical outlets have to be?
• What is reversed polarity at an outlet/receptacle? Why is it dangerous?
• How high above the floor do electric outlets/receptacles in a garage have to be?
• How far apart should electric receptacles be spaced in a bathroom?
• Is an ungrounded electric receptacle outlet dangerous?
• My bathroom electric receptacle/outlet is dead and there are no tripped breakers in the electric panel. What's wrong?
• Is there an adapter that can be placed on a two-slot receptacle to make it safe?
• How do the new tamper-resistant electric outlets work?
• Why is there no bathroom electric receptacle in this old house?
• How can I tell if the electric receptacle outlets are grounded?
• 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.