Museum of Plugs and Sockets logo, small US plugs and sockets
origin and early models
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US start
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Edison developed the incandescent lamp in the late 1870s, but initially they were rarely used in homes. Early electrical systems were very expensive and their reliability was unsettled. Moreover, Edison's preference for direct current was a limiting factor because it was only locally available.
The situation improved when Westinghouse and Stanley started to install AC lighting systems in the second half of the 1880s (see War of currents for details). Altogether it was not before the end of the 19th century that the use of electrical power in homes started to grow.

The need for devices to connect electrical appliances to the home network was recognized soon.
The main steps in development of plugs and receptacles in the United States are summarized below.


In homes electricity was initially used only for illumination. Lamps were restricted to a single light bulb socket, often in the center of the ceiling of a room.
Development of floor and table lamps, and other electrical appliances devices emerged at the beginning of the 20st century.
They had to be connected to the lamp socket, the only convenient access to power in a room.
Initially, to connect an appliance, a lamp had be replaced by a screw attachment plug - see image no. 1 -  or a technician had to install an additional lamp socket.

A drawback of screw plugs is that the appliance cord inevitably curls when the plug is attached to a lamp socket.

Edison screw plug
Screw attachment plug, developed in the mid 1880s by Edison. It fits in an Edison 26 mm base (outside diameter of thread) which became the most common lamp socket in the US.

Until the end of the 19th century the screw plug was mainly an industry standard. In the early 20st century they were commonly used in homes.

Harvey Hubbell (1857-1927) was a talented inventor and entrepreneur. Several of his inventions played a decisive role in the development of plugs in the US. Five patents related to plugs have to mentioned here.

Hubbell realized the need for a method that allows easy and safe (dis)connecting electrical appliances to a lamp or separate wall socket. His designs of separable attachment plugs were patented in 1904, see drawings A and B.

Patent 774251 ('B') shows a plug with two flat blades. Note the tandem position of the blades.

Images in 1906 Hubbell Electrical Catalogue.
(a) attachment plug 5406, similar to drawing B;
(b) multiple attachment plug 5327; (c) pull socket 35000*,
(d) side wall attachment plug 5426, (e) flush attachment plug 5418, (f) floor receptacle 5506; (g) theater plug and connector for an extension cord 5518.
* the light socket with pull-chain on/off switch was another very successful invention by Hubbell; patented in 1896.

Hubbell patent 774250
Separable attachment plug
invented by Harvey Hubbell.
Image redrawn from figure 1 of US patent 774250.
Application filed Feb. 26, 1903.
Patented Nov. 8, 1904.

Illustrations in patents were drawn in black and white.
Parts were numbered (see for example patent E, below).

For easy reference, most patent images on this page are redrawn. Numbers are deleted to enhance the visibility of structural elements.

Colors have been added to highlight parts. Screw bases are yellow, plugs (in patents indicated as caps) are green.

Images from 1906 Hubbell catalog
Images from 1906 Hubbell catalog

Hubbel patent 77451

In the early 1910s Hubbell worked on a simplified and improved construction of the separable attachment plug 'B'.
A remarkable novelty is the 90 rotation of the flat blades. Now they are positioned parallel next to each other. Both tandem and parallel blade plugs have been produced for quite a number of years, but soon the parallel type (image no. 3) became the standard 15 Amp - 110 Volt plug.

The coexistence of parallel and tandem blade plugs necessitated the construction of wall sockets that could accept both types. Image no. 4 shows a detail of an old parallel / tandem socket.

US pallel blade plug and tandem - parallel socket

Hubbell also developed the ubiquitous duplex receptacle, which is the mainstream in the US.

Hubbel patent 1064833

Three years after the parallel blade plug design, Hubbell received a patent for a polarized version of the plug; see drawing D.
One blade is both longer and wider than the other. A polarized plug (fig. 5) can be inserted in a matching polarized socket (fig. 4) in only one orientation.

In the modern polarized version of non-grounding 15A-125V plugs (NEMA 1-15P) both blades have the same length; only the width varies. Note that the plug shown in image 3 is polarized.

Hubbel patent 1180648
Attachment plug
invented by Harvey Hubbell. Image redrawn from figures 4 and 5 of US patent 1180648. Application filed
March 15, 1915.
Patented April. 25,

Hubbell and Arrows attachment plugs, small

The museum has examples of a Hubbell separable attachment plug and Edison screw lamps sockets with integrated outlets for straight blade plugs. They are shown on a separate page.

<<  Click image left to go to the page.

George P. Knapp, working at Hubbell Inc. in Bridgeport (CT), designed a separable attachment plug with an additional flat blade pin for equipment ground connection. A safety novelty in 1915.
Knapp described the aim of his invention in one sentence:

Knapp pUD patent lines 8-25
notes: cap = fig. 4 with supplemental blade 15;   base = fig. 2 with supplemental pair of contact plates 21 and binding screw 24. The attached ground wire 25 is shown in fig. 1.

Knapp US patent 1179728
Separable attachment plug invented by George P. Knapp (Hubbell Inc.). Figs 1-4 of US patent 1179728. Application filed Jan. 11, 1915. Patented April 18, 1916.

In the US plug with angled power pins are now used for 15A - 277V (with a different type of ground pin), and a larger version for 120/240 Volt split-phase wiring (see scheme below).
The incompatibility of angled blades with recptacles for parallel blades could have been the reason why Knapp's design has not become the standard US grounding plug.
However, plugs - similar to image no. 5 - have become the standard 10A-250V plug in Australia, New Zealand, Argentina and Uruguay, and are also commonly used in China. See appendix.

Grounding plug with angled power blades made by Hubbell Inc. (model 1742).
Rating: 15A-110V / 10A-220V.

Nos 13, 14 and 15 refer to fig. 4 in drawing E.

Grounded 15A plug with angled pins

Philip Labre's 1927 patent for a grounding receptacle and plug (drawing F) is based on the non-grounding parallel flat blade plug (image no. 3) that was most commonly used from the 1920s.

Grounding was achieved by adding a bracket strap and spring clip (fig. 3) to a receptacle (fig. 1). The matching flat blade plug (fig. 4) has an additional, slightly longer, pin that makes contact first with the receptacle spring clip.
The bracket strip is secured to the edge of a metal wall box. A ground wire is necessary to connect the wall box to ground.

Fig. 2 shows that there is only one orientation in which a grounding plug can be inserted. Hot and neutral connection cannot be exchanged.

In 1948 Labre's design has been adopted by NEMA (see below) as the standard grounding 15A-125V configuration. Only the ground pin is now U-shaped, rather than flat.

Philip F. Labre US patent 1672067
Grounding receptacle and plug invented by Philip F. Labre. Image redrawn from US patent 1672067. Application filed May 12, 1927. Patented June 5, 1928.

Edison's 1880s light bulbs worked on 110 Volt. It was regarded as a reasonable safe voltage. In the 1960s - '70s the nominal voltage was raised to 120V. For powering air conditioners, electric heaters and dryers etc. raising voltage to 240V would be better. However it meant that all lamps and electrical equipment had to be replaced, and wiring had to be upgraded in all homes. Instead the US and Canada have chosen for an alternative approach: split-phase wiring. It was implemented in the 1940s - '50s.

240 Volt is offered at the distribution board. When a load is applied from either 120V conductor to the neutral center tap (see scheme right) 120V is available to lamps, wall receptacles etc.
The two 120V connectors (hot-X and hot-Y) are out of phase and can be combined - without using neutral - to offer 240V power.

Split-phase wiring
* upper pole of NEMA 10-20 receptacle remains unused for 240V.
  Both W+X and W+Y would offer a 120V connection.

The National Electrical Manufacturers Association (NEMA) has been founded in 1926 in New York. In the 1940s NEMA defined standards for, among others, plugs and receptacles. Although these standards are not compulsory, they promote interchangeability between products made by various manufacturers and are often regarded as "the US standards".

NEMA wiring devices are made in current ratings from 15 to 60 Amp with voltage ratings from 125 to 600 V (singe-, split- and/or three-phase). More than 70 different connector types have been defined. Some of them are phased out.

Each connector is unique for a combination of voltage, amperage and grounding system. For most connectors two variants exist: straight blade and twist-locking; see a examples right.

NEMA configuration codes exist of two numbers. The first refers to a series (e.g. 125V grounding), the second indicates the current rating. Twist lock series start with L or ML (Midget Locking).
Split-phase and three-phase devices are categorized in separate series.
Find more information on Wikipedia NEMA connectors page.

Examples of NEMA plug configurations.
Examples of NEMA configurations


Patent protection of the plug design with angled flat blades (drawing F) ended in 1933. Shortly thereafter three Australian entrepreneurs in electrical industry suggested that the Hubbell plug could become the standard domestic plug in Australia. The model was easy and cheap to make and free of rights. In 1938 it became the official standard plug in Australia and later also in New Zealand. See AS/NZ 3112 page for additional information.

Australasian 10A socket
AS/NZ 3112 standard rating: 10A - 250V
The standard domestic socket in Australia and New Zealand. Blade configuration of matching plugs is identical to Hubbell plug (image no. 5).
The plugs are also used in Argentina (IRAM 2073 standard), Uruguay and China (GB 2099-1 standard).


Socket Tutorial by Paul Crist
History of Hubbell Incorporated and Harvey Hubbell
Prof. Hannington's Speaking of Science: Hubbell Sockets  (unavailable in Europe because of EU privacy regulations)
Hubbell Electrical Catalogue (1906)
Hubbell and Labre's patents:  (xxx is number of patent, e.g. 774250).
Split-phase wiring drawing is based on the scheme on HyperPhysics site (Georgia State University).

I am grateful to Frits Riep for sharing his knowledge of the history of US plugs.


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