Often called “wire nuts,” and in Canada “Marettes®” (a trade name), these insulated connectors usually contain a small metal coil. Wires to be connected electrically are stripped of a specified length of insulation, held together, and the connector twisted on by hand. (Connectors that don't contain a metal coil may be used to connect fixtures, but not for branch wiring.)
As pressure-type wire connectors, wire nuts may be used in branch circuit electrical wiring–house wiring, for example. But each size is tested and approved by Underwriters Laboratories and the CSA for certain specific combinations of wire, which are listed on the packaging. Code prohibits a connector's use for any other combinations.
Below are the specifications for two sizes in a popular brand. The gauge numbers in the given combinations apply to both stranded and solid wire. This table is provided only to illustrate what lists of approved combinations look like, and to show how wire nut sizes differ in the combinations that may be wired with them. You MUST rely only on the directions issued by the manufacturer of the connectors you are using.
|#3 connector||#4 connector|
|Strip stranded wire||5/8 inch||3/8 inch|
|Strip solid wire||½ inch||3/8 inch|
|Approved combinations||two to five #18||four to six #18|
|two to four #16||three to five #16|
|two #14||two or three #14|
|one #14 + one to three #18||two #12|
|one #14 + one or two #16||one #12 + one #14|
|two #14 + one #18||one #12 + one to three #16 or #18|
|one #16 + four #18||one #14 + one to four #16 or #18|
|one or two #16 + one to three #18||one #14 + five #18|
|one to three #16 + one #18||two #14 + one to three #16 or #18|
|three #14 + one #18|
|three to five #16|
|four to six #18|
When a connector is properly installed, no bare metal shows. Insulating a badly-made connection by wrapping it with tape is not acceptable.
In the 1970's a number of new homes in the United States were wired with aluminum wire, which is cheaper than copper. Aluminum is an excellent conductor (utility transmission lines are aluminum). However, it is also very active chemically, and a freshly exposed aluminum surface will be covered with aluminum oxide, an insulator, in a matter of seconds. Aluminum also expands and contracts as it heats and cools more than copper does. These characteristics make it difficult to make good electrical connections. In some cases, as the connection ages its resistance increases, and so its temperature rises, which ultimately can lead to a fire. A number of home fires in the 1970s, some with fatalities, were traced to these connections.
In response manufacturers changed the nature of aluminum wire and improved devices connected to it, such as circuit breakers, switches, outlets and so on. These second-generation devices are marked “CO/ALR.” Note the “R”.
The owner of a home wired with aluminum in the 70's has a number of options. Obviously, one is to rewire the house with copper wire, an expensive step and one that can be destructive. (14 AWG aluminum wire, however, should always be replaced with copper.)
Alternatively, all the switches, outlets, etc. may be replaced with CO/ ALR types. Replacing every connection in a house is more difficult to do than it sounds, and some authorities believe that even then complete protection will not have been achieved (see below).
Another solution is to attach a short length of insulated copper wire between the aluminum wire and the existing switch, outlet, etc. This practice is called “pigtailing.” The aluminum-to-copper connection must be made with the special COPALUM system now made by TYCO, though retaining the former brand name, AMP. The COPALUM system may only be used by licensed electricans specially trained and certified by TYCO. Other crimping systems sold to the public and even electricians are not comparable and cannot make a safe connection.
In the mid-1990's, a new type of wire nut was introduced that was specifically designed for aluminum-to-aluminum and aluminum-to-copper connections. These nuts are purple. (If the packaging says the connectors it contains are approved for aluminum or copper-clad aluminum wires but they are not purple, the nuts were made before January 2, 1987 and should not be used.) The use of this connector for “pigtailing” is controversial.
The Consumer Product Safety Commission stated¹ in 2003 that “The COPALUM crimp connector, which has been available for more than 20 years, is the only system recognized by CPSC that provides a complete and permanent repair and reduces the fire hazard in aluminum wire circuits. ... CPSC believes that 'twist-on' connectors, receptacles and switches and other devices that connect directly to aluminum wires, are an inadequate solution.”
1. CPSC Release #03-120 (May 1, 2003) www.cpsc.gov/CCCPSCPUB/PREREL/prhtml03/03120.html
The Consumer Product Safety Commission booklet on this topic can be accessed at
UL 486B, Standard for Wire Connectors for Use with Aluminum Conductors.
UL 486C, Standard for Splicing Wire Connectors.
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Last revised: 15 June 2006.