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Variations in RJ Connectors
Last Updated: 01/31/2017
  • 4P4C


  • 6P4C


  • 6P6C


  • 8P8C



Jack – female port which accepts cable terminated with associated modular plug

Modular plug – male connector used to terminate twisted pair cable

Application (Most Common):

RJ-11: used primarily to connect telephones to faceplates/boxes for up to 2 telephone lines

RJ-12: used primarily for voice/data applications: telephone systems (i.e. with PBX), networking, extended-distance peripherals

RJ-22: used primarily to connect telephone handsets to telephones

RJ-45: used primarily for data networks such as Ethernet (i.e. connect from a patch panel to a network switch; connect a computer's NIC to a data port). Sometimes used in audio/video applications such as security cameras or converting HDMI to Ethernet for long distance installs.

Installation Considerations:

The RJ- designation refers to the wiring scheme employed, not the physical construction of the plug. For example, a 6 Position, 6 Contact plug can be referred to as an RJ-12, RJ-18 or RJ-25 depending on how it is terminated.

To avoid the confusion of RJ connector names it is often easier to refer to a modular connector by the number of contact positions (positions or slots whether occupied by a metal contact or not) and the number of contacts (positions occupied by a metal contact). For example, an RJ-12 has 6 positions and 6 contacts (6P6C). If you refer to this modular plug as a “6 position, 6 contact plug” it will be understood even if somebody else refers to it by another RJ designation.

Other Options/Variations:

Stranded and Solid conductor types: the shape and number of prongs of the contacts can vary depending on whether the wires are constructed from a single strand of solid copper or several strands twisted together to form one conductor.

  • RJ-11C and 12C – designation sometimes used to refer to an RJ-11 or RJ-12 port in a surface mount box or a faceplate; when the port is in a wall mount telephone faceplate it may be referred to as an RJ-11W or RJ-12W

  • RJ-14 - wired for 2-line telephone service

  • RJ-21 - 25-line telephone service using a 25-pair Telco (or Amp) connector

  • RJ-31X - used where alarm systems need to temporarily disable station equipment so that the system may be used to report an alarm condition.(i.e. When an alarm is triggered an in-use phone line may be “hi-jacked” so that the alarm condition can be sent to the fire department)

  • RJ-25 - wired for 3-line telephone service

  • RJ-48 - 8 position, 8 contact (8P8C) jack/plug with several wiring variations of 4-wire data service; applications include T1 and DDS lines

  • RJ-50 - 10 position, 10 contact (10P10C) jack/plug used selectively by various manufacturers for data transfer on particular products like UPS’s, servers, industrial equipment

Popular Manufacturers:

Related Industry Terms/Acronyms:

  • RJ – Registered Jack: modular plug design originally developed by Bell Labs

  • Pinout (or pin to pair assignment) - refers to the wiring configuration between the conductors in a cable and the pins in the connector or between the connectors at each end of the cable

  • UTP - copper Ethernet cable consisting of Unshielded Twisted Pairs surrounded by an outer jacket

  • STP (ScTP) - Shielded Twisted Pair cable encases the conductors in a shield (usually foil) as a means of reducing the potential for electromagnetic interference (EMI). The shield may also be composed of a braided screen (ScTP)

  • Straight through - refers to a pinout wherein pins (or contacts) 1-8 at one end of the cable are connected to corresponding pins 1-8 on the other end of the cable (as opposed to pin 1 at one end being connected to pin 4 at the other end for example)

  • Cable OD - outside or overall diameter of a cable

  • Insertion/Extraction Life or Cycles - the number of times a port or plug can be mated (inserted or extracted) before it begins to degrade, resulting in possible loss of performance

Comparison Chart:

Positions Contacts Width
RJ-11 6 2 or 4 0.3799”
RJ-12 6 6 0.3799”
RJ-14 6 4 or 6 0.3799”
RJ-22 4 4 0.3031”
RJ-25 6 6 0.3799”
RJ-45 8 8 0.4598”
RJ-48 8 8 0.4598”
RJ-50 10 10 0.4598”

See Also:

Further References:

  • BICSI(Building Industry Consulting Services International)
  • IEEE(Institute of Electrical and Electronics Engineers, Inc.)
  • ISO(International Organization for Standardization)
  • TIA(Telecommunications Industry Association)
  • ANSI(American National Standards Institute)

Frequently Asked Questions:

Can a 6 position plug be used in an 8 position (RJ45) port?

You will find that the 6P plug will mate and lock in an RJ45 port; however, they are not designed for use together. The smaller width of the 6P plug leaves room on either side that can result in unintended shifting if the cable or device is moved or jostled. When this occurs contact is lost between the conductors and performance is reduced or lost entirely.

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