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. NEMA wiring devices are made in current ratings from 15 to 60 amperes (A), with voltage ratings from 125 to 600 volts (V). 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 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. 42), Mexico (NMX-J-163-ANCE) and Japan (JIS C 8303).
Other plug and receptacle types are for special purposes or for heavy-duty applications.
The dimensional standard for electrical connectors is ANSI/NEMA WD-6 and is available from the NEMA website.
Precedents
In the early days of electrification, residential use was almost exclusively for illumination, with rooms normally having just a single spot in the center. Along with his lightbulb, Thomas Edison developed the Edison screw in the early 1880s, for which he received a patent in 1881. The Edison screw was very successful, and quickly became the first de facto standard for electric connection. In the early 1900s, table and floor lamps became more popular, and sockets were mounted on walls for secondary connections. One big disadvantage of screw connectors was that the cord inevitably got twisted after being connected to the receptacle.
Harvey Hubbell's inventions
In 1903, Harvey Hubbell filled the , for a lightbulb socket adaptor and plug, also a standalone receptacle. The adaptor was screwed into the lightbulb socket, leaving a flat face with two holes to conveniently attach the plug. The same patent had a second design, with a wall attachable receptacle, capable of receiving the same plug, thus being the first socket and plug design patented in the US. Later in 1904, he changed the design to flat blades (a design later incorporated in the NEMA 2 series), filled under the Both these patents were granted in .
In 1910, Hubbel worked on improving his popular flat blade design, filled in 1912. The new design had parallel blades, a more compact design and was easier to manufacture. This design was improved once again in 1915, introducing a polarized plug under the . The flat blade plug which in subsequent years evolved to become NEMA 1-15 was born.
Other American manufacturers
It is worth noting that in addition to Hubbell's system, circulated a large variety of different plugs and receptacles, some of them compatible with Hubbell's, some not. In 1919, Hubbell unsuccessfully tried to prevent other manufacturers from making receptacles and plugs to the dimensions used by Hubbell. The report of the court proceedings includes a comprehensive review of the development of the art in the US prior to 1919, based on evidence presented to the Court. Separable plugs had been available for more than a decade prior to Hubbell's 1904 design.
NEMA
In 1926, the National Electrical Manufacturers Association (NEMA) was founded by the merger of the Electric Power Club and the Associated Manufacturers of Electrical Supplies and represented manufacturing companies of the electrical segment in a national level. NEMA provided a forum between companies for reaching standardization, but it was not until the 1940s that NEMA started to publish standards on receptacles and plugs, much later than UK, France and Germany. Since NEMA was a forum between manufacturers and not an governmental body, any decision had to be thoroughly discussed and agreed upon by its members, which substantially slowed the standardization process.
Nomenclature
NEMA connectors are named following an alphanumeric code consisting of: prefix "L" (locking types), numerals, a hyphen, numerals, suffix "R" or "P" for "receptacle" or "plug".
There are two basic classifications of NEMA connectors: straight-blade and locking. The metal conductive blades are often informally called "prongs" (as in "3-prong plug"). Numbers prefixed by 'L' are curved-blade, twist-locking connectors. Twist-locking types are used for heavy industrial and commercial equipment, where increased protection against accidental disconnection is required.