Twist-on wire connector
Twist-on wire connectors are a type of electrical connector used to fasten 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, items similar to those used in North America, but made from ceramic materials, were sold under the brand "Scruit". Such ceramic connectors are currently sold in South Africa under the name of "Porcelain Scruits", such .
Twist-on wire connectors are available in a variety of sizes and shapes. While their exterior covering is typically made from insulating plastic, their means of connection is a tapered coiled metal insert, which threads onto the wires and holds them securely. When such a connector is twisted onto the stripped and twisted-together ends of wires, the wires are drawn into the connector's metal insert and squeezed together inside it. Electrical continuity is maintained by both the direct twisted wire-to-wire contact and by contact with the metal insert.
Twist-on wire connectors are typically installed by hand. They may have external grooves to make them easier to handle and apply. Winglike extensions are commonly molded into higher quality connectors to reduce operator muscle fatigue when installing a large number of the 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 modifications.
Twist-on connectors are not often used on wire gauges thicker than AWG #10 (5.26 mm²), because such solid wires are too stiff to be reliably connected with this method. Instead, set screw connectors, clamps or crimp connectors are used.
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). Special gel-filled connectors 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 are designed specifically for, and rated for, use with aluminum conductors.
Special feedthrough twist-on wire connectors 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 connectors are typically green, and are also called "screw-on grounding connectors".
Another specialized connector version has a permanently attached wire pigtail protruding from the top of the insulated cap. The pigtail may be unterminated, or it may end in a preinstalled 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 as a grounding pigtail, similar to the feedthrough twist-on wire connectors described in the previous paragraph.
William P. Marr emigrated from Scotland to Ontario, Canada early in the twentieth century. After settling in the Toronto area, he was employed as a contractor for Ontario Hydro, where he worked as an electrician, converting gas-lit homes to electrical incandescent lighting.
At that time, the accepted practice for joining conductors was a process called “solder and tape”. Typically, a mechanic would first install the insulated wires; then an electrician cleaned the exposed conductors, twisted them together, then firmly joined the conductors's ends by dipping them into a pot of molten solder. After cooling, the jointed exposed conductors were wrapped with insulating tape.
The process was time-consuming and potentially dangerous. Marr was injured when he spilled molten solder on himself. Seeking a safer and more efficient way of joining electrical conductors, Marr, working in his home workshop, developed the first pressure-type wire connector and, in 1914, produced a set-screw version which was the forerunner to the present-day twist-on 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
A wire connector more 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.)
This table shows the de facto standard color coding various manufacturers use to indicate the range of sizes of conductors that may be joined with twist-on wire connectors.
|Color||AWG Wire Range|
|(0.326 mm²)||(0.581 mm²)||(0.823 mm²)||(1.31 mm²)||(2.08 mm²)||(3.31 mm²)||(5.26 mm²)|
- Minimum: 1 conductor AWG 22 + 1 conductor AWG 20; Maximum: 2 conductors AWG 22 + 3 conductors AWG 20
- Minimum: 1 conductor AWG 22 + 1 conductor AWG 20; Maximum: 3 conductors AWG 16
- Minimum: 2 conductors AWG 18; Maximum: 3 conductors AWG 14
- Minimum: 2 conductors AWG 18 + 1 conductor AWG 14; Maximum: 2 conductors AWG 14 + 1 conductor AWG 10
- Minimum: 1 conductor AWG 22 + 2 conductors AWG 18; Máximum: 1 conductors AWG 14 + 4 conductors AWG 12