One big disadvantage of screw connectors was that the cord inevitably got twisted after being connected to the receptacle. In the early 1900s, table and floor lamps became more popular, and sockets were mounted on walls for secondary connections. The Edison screw was very successful, and quickly became the first de facto standard for electric connection. Along with his lightbulb, Thomas Edison developed the Edison screw in the early 1880s, for which he received a patent in 1881. In the early days of electrification, residential use was almost exclusively for illumination, with rooms normally having just a single spot in the center. Main article: History of AC power plugs and sockets The dimensional standard for electrical connectors is ANSI/NEMA WD-6 and is available from the NEMA website. Other plug and receptacle types are for special purposes or for heavy-duty applications. NEMA 1-15P (two-pole, no ground) and NEMA 5-15P (two-pole with ground pin) plugs are used on common domestic electrical equipment, and NEMA 5-15R is the standard 15-ampere electric receptacle (outlet) found in the United States, and under relevant national standards, in Canada ( CSA C22.2 No.

Different combinations of contact blade widths, shapes, orientations, and dimensions create non-interchangeable connectors that are unique for each combination of voltage, electric current carrying capacity, and grounding system. NEMA wiring devices are made in current ratings from 15 to 60 amperes (A), with voltage ratings from 125 to 600 volts (V). NEMA connectors are power plugs and receptacles used for AC mains electricity in North America and other countries that use the standards set by the US National Electrical Manufacturers Association. The NEMA 5-15R device on the far left is most common the two rightmost designs are typically seen in older buildings.

All accept a 1-15P plug the two on the left also accept grounded 5-15P plugs the second from the left also accepts 5-20P plugs. Main article: AC power plugs and sockets Ungrounded (L) and grounded (R) power plugs Common North American 125-volt receptacles.

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