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When were grounded three-slot receptacle outlets first required?
Wednesday, July 11, 2018
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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