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Short introduction to flat blade plugs and sockets.

In homes electricity was initially only used for illumination. Houses were usually hard wired. Lamps did not need to be plugged in. The need for a way to temporarily connect a device to the house current was generated after 1900 when portable lamps and various other electrical home appliances came into use. At that time electric sockets in fixtures and sconces offered the only convenient access to power. As electrical conveniences proliferated in the 1910s, multi-outlet "plug clusters" and dangling electric cords became an increasingly familiar sight in American homes.

The most common systems were at that time either Edison's screw-in plugs (see image 1a) and Hubbell's flat pronged plugs. The latter was introduced in 1904 and has finally become the standard plug in North America and many other countries.

Source: Socket tutorial. by Paul Christ.

110 Volt versus 220 Volt.

Edison preferred 110V DC, but also the AC networks build by Westinghouse offered 110 volt to end-users. This low voltage was regarded as acceptable safe, an issue that made sense because in the early days of electrification bare or poorly insulated wires were not uncommon.

110 Volt is fine for home illumination, but it could deliver insufficient power when the number of more demanding appliances increased. Network stability was in danger.

The US and Canada were convinced of the necessity to raise voltage, but facing the costs of replacing all appliances it was decided to introduce nationwide a technique that offers both 110 and 220 Volt to each home. This split-phase or household wiring technique is explained in below (see image no. 1).


In the U.S. a variety of types of plugs and sockets were replaced when NEMA specifications became widely accepted. The National Electrical Manufacturers Association was created in 1926 by the merger of the Electric Power Club and the Associated Manufacturers of Electrical Supplies. One of its goals was providing a forum for the standardization of electrical equipment.

Initially, plugs and sockets have only two poles, because a protective (equipment) earth connection was uncommon. Earthed plugs and sockets were used only for (semi-) industrial purposes. The NEMA standard type for domestic use was designed in 1928 by Philip Labre.


US split-phase or household wiring scheme
Classic US parallel-tandem socket
Obsolete US tandem duplex socket

1 A standard phase-neutral voltage of 120V is commonly used in North American homes, but 240V is available for cooking equipment, air conditioners and other large appliances. There is a choice between the two voltages because standard U.S. household wiring has two line wires ('hot' in American English), whereas the neutral wire is a grounded center-tap of the secondary coil of the transformer; see scheme. A combination of line-1 (x) or line-2 (y) and neutral (w) offers 120V. Because of alternating current, line-1 and -2 are out of phase and can be combined to offer 240V. The system is also known as split-phase.

The scheme does not show protective earth ('ground') wires. Non-earthed 15A-120V and 15A-240V sockets (types shown in the scheme) are not permitted in new buildings in the U.S and Canada.
The scheme is based on information given on the HyperPhysics site (Georgia State University).
2, 3
Two examples of classic non-earthed socket with two parallel slots for 15A-120V plugs, and two slots in tandem position for 10A-240V plugs. Both sockets are not dual voltage types, because they have one line and one neutral connector and can only be wired to either a 120V or 240V circuit.

Even polarized, 120V NEMA 1-15P plugs can be used with socket nos. 14 and 15. Non-earthed 240V tandem blade plugs are no longer on the market. NEMA 2-20P is the only non-earthed 240V plug that is still available, but this type has two perpendicularly positioned blades and do not fit in socket no. 3.

Manufacturers: General Electric (no. 2), and Hubbell (no. 3). Sockets
became available during renovation of a 1937 home in California.   {BN}

Monolite quintet receptacle General Electric 4-plug outlet
Classic NEMA 1-15R socket with switch
Earth key for NEMA 1-15 socket


Monolite Quintet socket, rated at 15A-125V / 10A-250V (see no. 10 for explanation of different amperage and voltage ratings). The socket is polarized. Slots at right are 1.6 mm wider than corresponding slots at left (8.72 versus 7.12 mm), see inset.
Brand name: Monowatt Electric Corporation, New York, Providence, Chicago. Socket dates back to late 1930s, when Monowatt was a division of General Electric.   {RP}

Socket nos. 9 and 10 came in their original carton. They are shown on a separate page.
5 4-plug socket, rated at 15A-125V. The socket is polarized. Right and left slots are respectively 8.44 and 7.24 mm wide.
Manufacturer: General Electric (wiring device department, Providence). Socket design probably dates back to mid 1950s.
6 Classic model of a two-blade (non-grounded) 120V, 15A socket, with switch. The switch at left is an additional light switch.
Nowadays only grounded sockets are allowed, but non-grounded plugs (NEMA 1-15P) are still available for appliances that don't require a ground.
Manufacturer: Leviton. US patent no. 2704832 was granted on March 11, 1955.
7 US ground adapter for 15A - 125V two pole socket. The grounding clip of the adapter (green arrow) is connected to earth via the brass screw that is used to attach the wall plate to the body of the socket (which have to be grounded). These adapters are also known as cheater plugs. In 2003 they were still available in some hardware stores.

Obsolete type-A plug and connector Obsolete type-A plug and connector Obsolete 2-pin plug in T-configuration Comarison of T-slot pluf and NENA 2-20P

8, 9 Standard flat blade plug and connector made of ebonite, a vulcanized natural rubber. A relative high sulfur content and prolonged times of vulcanizing results in a solid product. Manufacturer: Arrow Electrical Wiring Devices. Dating: 1920s.  {ChR}

As many older North American devices the connector shows two different amperage and voltage ratings:
10A - 250V
15A - 125V

This has to do with a peculiarity of the National Electrical Code from 1923 to the 1950s. Originally, sockets were rated at 10A 250V, because the NEC limited lighting circuits to 10 amperes. In 1923, the code changed to allow lighting circuits to be fused at 15 amperes; however, the old rule still applied to circuits over 125 volts.
10, 11
Obsolete plug with two flat blades in T-configuration, rated at 10A - 250V. The U.S. company Bryant* has made T-slot sockets for this type of plugs. They were rated at 15A - 125V and 10A - 250V. T-slot plugs do not fit in socket no. 3 and are incompatible with NEMA 2-20 sockets (see image no. 11). Manufacturer of the shown plug is not indicated; it could have been made in Australia, since the type has been used also in Australia and New-Zealand in the 1930s - 1950s.   {ChR}
* Bryant Electric Company in Bridgeport, CT (1888-1988, thereafter part of Hubbell).


pre-NEMA 15A-125V / 10A-250V receptacle Drawings 1-4 from US patent 1179728

   Hubbell pre-NEMA 15A-125V / 10A-250V plug

One of the oldest earthed flat blade plugs for domestic use is the 1915 Hubbell design shown here.
However, the 3-prong plug and corresponding grounding socket that became the standard NEMA 5-15 plug for domestic use was invented by Philip Labre in 1928. Labre's earthed sockets accept standard not earthed NEMA 1-15 plugs with parallel flat blade pins. They do not fit in earthed sockets designed by Hubbell.

12 Non-NEMA duplex socket with angled slots for line and neutral. It is rated at 15A-125V and 10A-250V (see caption to image no. 13 for explanation of dual amperage/voltage).
The two sockets are each other's mirror image, an unusual orientation, but the top and bottom angled slots are correctly
(crisscrossed) wired and therefore identical, as indicated in image no. 19. Manufacturer: Hubbell (model 7051).

The outdated model is based on a design which was first disclosed in a 1915 patent application, see image no. 13 for details. The socket is used in the U.S. for some decades, but was likely never manufactured by other electrical manufacturers in the US, due to the patent. The domestic earthed plug designed by Philip Labre in 1928 - based on the commonly used plug with two parallel flat blades - became the standard earthed plug (NEMA 5-15).

Early 1930s the - at that time - 17 years U.S. patent protection on Hubbell's socket and plug with angled pins ended. The design has served as model for Australasian 10A-250V devices. Standard Australasian 10A plugs fit perfectly in socket no. 12. See AS/NZS 3112 page for details about the history of the standard. Identical plugs are used in Argentina (IRAM 2073 standard), Uruguay and China (GB 2099-1 standard).

The model predates NEMA, and evidently NEMA has never approved the design. However, there are two NEMA standard types that have identical angled hot and neutral (W) slots, but different earth pin slots: [1] NEMA 10-20R (larger offset of earth slot), and [2] NEMA 7-15R (U-shaped earth slot). Unearthed Australasian plugs fit in those NEMA sockets.
13 Drawings 1-4 of George P. Knapp et al. US patent application no. 1179728, filed Jan. 11, 1915 and patented Apr. 18, 1916.
The invention is about a portable separable attachment-plug especially adapted for use in connection with electric implements. George P. Knapp was working for the Harvey Hubbell company in Bridgeport (CT). It is the first US patent for a polarized plug.

Fig. 1 illustrates the use of the novel - grounded ! - attachment-plug. Electricity from a lampholder socket can safely be used, via an adapter (10) and plug (13), to heat water in a chafing dish, iron or any other electric implement.
Fig 2 shows details of the adapter, with at left a screw shell (12) and center contact (19) which fit in commonly used screw base lampholder sockets.
Fig. 3 shows the layout of contact slots in the outlet part of the adaper. One of the angled slots 17 is connected to the screw shell (12), the other to center contact (19). These two slots correspond to hot and neutral (W) in image no. 19. The third slot (GROUND) serves for an equipment ground connection, through a separate ground wire that has to be attached to screw 24 of the adapter.
Fig. 4 shows the plug and part of the cable that is connected to the implement. Flat blades 14 are hot and neutral slots. The longer ground pin 15 is connected by wire 26 to the metal support of the chafing dish.
Image source: 
I am grateful to Frits Riep who has drawn my attention to the origin of the angled Hubbell plugs and sockets.
14 Hubbell 10A-250V, 15A-125V plug (order of ratings as indicated on plug). The plug is essentially identical to the model in the patent application (Fig. 4), but it is a more recent specimen (1950s?). The plug has an Underwriters Laboratories* certification mark.  * U.S. testing facility and developer of safety standards.


Classic Acamemy three-way connector
NEMA 1-15R made by Busch-Jaeger, Gemany; accepts also Europlugs Terko - NEMA 5-15R adapter, Terko side Terko - NEMA 5-15R adapter, US side

15 Obsolete 15A, non-earthed 3-way multi-plug (outlets at top, bottom and one side). The 'automatic' wiring of this plug is unique; details are given on a separate page. Manufacturer: Academy Electrical Products Corp., New York. Dating: 1950s.

↓ note

Nos. 16 and 17-18 have been made for the U.S. Army in Germany. They were for sale in Germany in the 1950s - ca. '80s.
16 Not earthed socket with two slots in parallel position for 15A-120V plugs and two tandem slots for 10A-240V plugs (see also no. 3). Slots accept also not earthed continental European 2-pin plugs with 4.0 or 4.8 mm pins. Original wall plate is missing. Manufacturer: Busch-Jaeger in Lüdenscheid, Germany. The socket has a type of Busch-Jaeger logo that has been used from 1951 until 1979.   {FSE}
17, 18
Adapter plug that fits in Terko* sockets (image no. 17) and accepts NEMA 5-15 plugs (image no. 18). Note that NEMA 5-15 is a 120 Volt standard whereas the German Terko system is rated at 250 Volt. The adapter does not have a transformer.
The adapter has a MPAD molding mark. No. 76 refers to the company that has produced the cast: Bezet-Werk, Herman Buchholz GmbH in Berlin-Lichtenrade. No. 131 stand for an urea-formaldehyde resin with cellulose as filling agent. It is not clear whether Bezet-Work also has assembled and sold the adapters.
* See Terko page for details.


Knob and tube wiring (K&T)

Knob and tube wiring was an early standardized method of electrical wiring. It used individual wires supported by porcelain insulators (knobs) and run inside porcelain cylinders (tubes) when going through wood beams. Until the mid-1930s, knob-and-tube wiring was installed in many U.S. homes and it can still be found in some older houses. Whether or not they have to be replaced is discussed on several web sites.
See for example, and
In short: knob and tube wiring properly installed and in good condition is both reliable and safe, but it has no ground wire and is not appropriate for electrical equipment that requires grounding. At present K&T is not permitted in new constructions in the US, except in a few industrial and agricultural related situations specifically listed in the US National Electrical Code.

Knob and Tube wiring (1) Knob and Tube wiring (2) Knob and Tube wiring (3) Knob and Tube wiring (4)

19, 20 Porcelain knob insulator. This example consists of two pieces and has on each side of the nail a pass-through groove to guide the wire. Knobs are nailed into wall studs and floor joists. Usually hot and neutral wires are separated from one another, which allows the wires to readily dissipate heat into free air. Therefore wires are capable of carrying higher currents than the same conductors in close proximity. Another type of insulator (not shown) consists of a single piece of porcelain. The wire loops in a circular groove running around the circumference. Also block-shaped ceramic cleats are occasionally used. {BN}
21 Porcelain tube, used to keep wires from coming into contact with, or being compressed by wood framing parts of a building. {BN}
22 K&T wiring inside a 1930 Pittsburgh house. Part of a photo taken in 2006 by Laura Scudder and shown at the Wikipedia page about knob and tube wiring.


US radio outlet

In many homes, build in the 1920 and '30 radio antenna coils were installed at the attic. A wire connected the antenna to a grounded radio outlet downstairs. Plugs existed for the outlet to  radio connection. These plugs had a pin configuration that never has been used for 120 or 240 Volt power plugs.

It is confusing that quite some of these outlets were a part of a dual socket; the other half was a standard 15A-120V outlet. Read more about radio outlets on Antique Radios.

When home antenna coils fell into disuse, radio outlets were often put out of sight by wall paper or layers of paint, as the image shows. Photo has been sent by Joshua Hodges.


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