Twist-on wire connector
Twist-on wire connectors are a type of electrical connector used to fasten together two or more low-voltage (or extra-low-voltage) electrical conductors. They are widely used in North America, but are not approved for use on low-voltage wiring in countries in the European Union and in many other countries.
Twist-on connectors are also known as wire nuts, cone connectors, or thimble connectors. One trade name for such connectors, Marrette, is derived from the name of their inventor (see #History).
In the UK, similar items to those now used in North America, but made from ceramic materials, were sold under the brand "Scruit". Under the name of "Porcelain Scruits", such ceramic connectors are currently available in South Africa.
Twist-on wire connectors are available in a variety of sizes and shapes. While their exterior covering is typically made from insulating plastic, a tapered conducting coiled metal insert, which threads onto the wires to hold them securely, is the means of ensuring connection. When such a connector is twisted onto the stripped and twisted together ends of the wires concerned, these wires are drawn into the metal insert of the connector and are squeezed together inside the tapered insert. Electrical continuity is maintained by both the direct twisted wire-to-wire contact and by contact via the metal insert.
Twist-on wire connectors are typically installed by hand, and molded grooves or blades may be included in their external design to assist in their manual installation. Wing-like extensions are commonly molded into higher quality connectors, in order to reduce operator muscle fatigue when installing a large number of twist-on connectors. Such extensions also allow these connectors to be installed with a common nut driver or a specialized tool,
Twist-on wire connectors are commonly color-coded to indicate the connector size and, hence, their capacity. They are commonly used as an alternative to terminal blocks or the soldering of conductors together, since they are quicker to install and, unlike soldered connections, allow easy subsequent removal for future rework.
Twist-on connectors are not often used on wire gauges thicker than AWG #10 (5.26 mm²), because the solid wires involved are too stiff to be reliably connected using this method. Instead, set screw connectors, clamps, or crimp connectors are used for heavier wire gauges.
Ceramic twist-on connectors are made for high-temperature applications, such as heating appliances.
Ordinary twist-on connectors are not rated for wet use (such as exposed outdoors or buried underground); a special gel-filled connector must be used in this circumstance.
Twist-on wire connectors are not generally recommended for use with aluminum wire. The US Consumer Product Safety Commission does not approve of their use with aluminum wire, and approves of only two (alternative) methods of connection. In spite of this, several companies manufacture twist-on connectors which they claim to be designed specifically for use with aluminum conductors, and which are rated for such use.
Special feedthrough twist-on wire connectors are available, which differ from standard wire connectors in that they have an additional opening at the top of the insulated cap. This allows a single-conductor bare wire to be pushed through the hole, forming a "pigtail" section which can be attached to a grounding screw. These feedthrough wire connectors are typically colored green, and are also called "screw-on grounding connectors".
Another specialized version consists of a twist-on wire connector which has a permanently attached wire pigtail protruding from the top of the insulated cap. The wire pigtail may be unterminated, or it may end in a pre-installed spade lug. If colored white, it typically may be used for splicing neutral wires in a device box, while leaving a pigtail free for connection to a device (such as a receptacle). If colored green, the assembly is intended to be used for a grounding pigtail, similar to the feedthrough twist-on wire connectors described in the previous paragraph.
William P. Marr emigrated to Ontario, Canada, from Scotland, early in the twentieth century. After settling in the Toronto area, Marr was employed as a contractor for Ontario Hydro, where he worked as an electrician, converting gas lit homes to electrical incandescent lighting.
At this time, the accepted practice for joining conductors was a process called “solder and tape”. Typically, a mechanic would first install the insulated wires, an electrician would then clean the exposed conductors, twist them together and the ends of the conductors then would be firmly joined by dipping them into a pot of molten solder. After cooling, the jointed exposed conductors would be wrapped with an insulating tape.
This process was time consuming and could be dangerous. Marr was injured when he inadvertently spilled molten solder on himself. To find a safer and more efficient way of joining electrical conductors, Marr worked in his home workshop to develop the first pressure type wire connector and, in 1914, produced a set screw version which was the forerunner to the present day twist-on wire connector now used in North America.
- "Electric Wire Connecter" U.S. Patent 1,583,479, Filed March 3 1923, Patented May 4,1926
- Canadian Patent CA 275586, Issued 22 November 1927
However, a wire connector closely resembling the present day twist-on wire connector was not patented (in Canada) by Marr until 1931.
- "Wire Connector", Canadian Patent CA 311638 Issued 26 May 1931. (US Patent not located.)
The table below shows the de facto standard color coding used by various manufacturers, to indicate the range of sizes of the conductors which may be joined with twist-on wire connectors.
|(0.326 mm²)||(0.581 mm²)||(0.823 mm²)||(1.31 mm²)||(2.08 mm²)||(3.31 mm²)||(5.26 mm²)|
|Gray [Note 1]|
|Blue [Note 2]|
|Orange [Note 3]|
|Yellow [Note 4]|
|Red [Note 5]|
- Mínimum: 1 conductor AWG 22 + 1 conductor AWG 20; Maximum: 2 conductors AWG 22 + 3 conductors AWG 20
- Mínimum: 1 conductor AWG 22 + 1 conductor AWG 20; Maximum: 3 conductors AWG 16
- Mínimum: 2 conductors AWG 18; Maximum: 3 conductors AWG 14
- Mínimum: 2 conductors AWG 18 + 1 conductor AWG 14; Maximum: 2 conductors AWG 14 + 1 conductor AWG 10
- Mínimum: 1 conductor AWG 22 + 2 conductors AWG 18; Máximum: 1 conductors AWG 14 + 4 conductors AWG 12