Single-core 25-pair, 50-conductor cable

The 25-pair color code, originally known as even-count color code,[1] is a color code used to identify individual conductors in twisted-pair wiring for telecommunications.

Color coding

With the development of new generations of telecommunication cables with polyethylene-insulated conductors (PIC) by Bell Laboratories for the Bell System in the 1950s, new methods were developed to mark each individual conductor in cables.[2] Each wire is identified by the combination of two colors, one of which is the major color, and the second the minor color. Major and minor colors are chosen from two different groups of five, resulting in 25 color combinations. The color combinations are applied to the insulation that covers each conductor. Typically, one color is a prominent background color of the insulation, and the other is a tracer, consisting of stripes, rings, or dots, applied over the background. The background color always matches the tracer color of its paired conductor, and vice versa.

The major, or primary group of colors consists of the sequence of white, red, black, yellow, and violet (mnemonic: Why run backwards? You'll vomit!).[3] The minor, or secondary color is chosen from the sequence blue, orange, green, brown, and slate (mnemonic: Bell operators give better service.[4]).[5][6][7]

25-pair color coding using twisted pairs with solid color coding only, without tracers. This method was rarely used.
25-pair color code
Major color Minor color
1 White Blue
2 Orange
3 Green
4 Brown
5 Slate
6 Red Blue
7 Orange
8 Green
9 Brown
10 Slate
11 Black Blue
12 Orange
13 Green
14 Brown
15 Slate
16 Yellow Blue
17 Orange
18 Green
19 Brown
20 Slate
21 Violet Blue
22 Orange
23 Green
24 Brown
25 Slate

The wire pairs are referenced directly by their color combination, or by the pair number. For example, pair 9 is also called the red-brown pair. In technical tabulations, the colors are often suitably abbreviated.

Violet is the standard name in the telecommunications and electronics industry, but it is sometimes referred to as purple. Similarly, slate is a particular shade of gray. The names of most of the colors were taken from the conventional colors of the rainbow or optical spectrum, and in the electronic color code, which uses the same ten colors, albeit in a different order.[citation needed]

When used for plain old telephone service (POTS), the first wire is known as the tip or A-leg (U.K.) conductor, and is usually connected to the positive side of a direct current (DC) circuit, while the second wire is known as the ring lead or B-leg (U.K.), and is connected to the negative side of the circuit. Neither of these two sides of the line has a connection to the local ground. This creates a balanced audio circuit with common-mode rejection, also known as a differential pair. The tip and ring convention is based on the 14 inch (6.35 mm) TRS phone connectors, which were employed in telephone switchboards in the 19th and 20th centuries, where the tip contact of the connector is separated from the ring contact by a spacer of insulation. The connection furthest from the cable is known as the tip, the middle connection is the ring, and the (largest) connection closest to the wire is the sleeve.

Older Bell System wiring inside customer premises used 4-conductor untwisted wire cable. The 4 conductors were solid red, green, yellow & black wires. They match to the current 25-color code as follows:

green ("Line 1" tip) white/blue
red ("Line 1" ring) blue/white
black ("Line 2" tip) white/orange
yellow ("Line 2" ring) orange/white

25-pair telco cable pinout

A common application of the 25-pair color code is the cabling for the Registered Jack interface RJ21, which uses a female 50-pin miniature ribbon connector, as shown in the following table. The geometry of the pins of the receptacle (right hand image) corresponds to the pin numbers of the table. The left column of pins are the ring (R) conductors, while all tip (T) conductors are on the right.

(R) (T) Color
The corresponding
pin order in the
female RJ21 connector
Pin no.
blue/white 1 26 white/blue
orange/white 2 27 white/orange
green/white 3 28 white/green
brown/white 4 29 white/brown
slate/white 5 30 white/slate
blue/red 6 31 red/blue
orange/red 7 32 red/orange
green/red 8 33 red/green
brown/red 9 34 red/brown
slate/red 10 35 red/slate
blue/black 11 36 black/blue
orange/black 12 37 black/orange
green/black 13 38 black/green
brown/black 14 39 black/brown
slate/black 15 40 black/slate
blue/yellow 16 41 yellow/blue
orange/yellow 17 42 yellow/orange
green/yellow 18 43 yellow/green
brown/yellow 19 44 yellow/brown
slate/yellow 20 45 yellow/slate
blue/violet 21 46 violet/blue
orange/violet 22 47 violet/orange
green/violet 23 48 violet/green
brown/violet 24 49 violet/brown
slate/violet 25 50 violet/slate

Larger cables

For cables with more than 25 pairs, each group of 25 is called a binder group. The binder groups are marked with mylar ribbons using the same color coding system, starting with a white/blue ribbon, then a white/orange ribbon, and so on. The 24th binder group has a violet/brown ribbon, completing a super binder of 600 pairs.[5][6]

In cables of more than 600 pairs, each of the 600-pair super binder group bundles is wrapped with a mylar binder ribbon, or string, matching the "tip" colors of the color code, starting with white. The pattern then starts over with the first 25-pair group as white/blue, and continues indefinitely, in multiples of 600 pairs or parts thereof. For example, a 900-pair cable has the first 600 pairs in 24 groups of 25 pairs in a white binder, and the remaining 300 pairs in 12 groups of 25 pairs wrapped in a red binder.[5][6]

Some cables are "mirrored" or "clocked" with a pattern that is known throughout the telephone industry. Starting with the first binder group in the center, the technician counts the cable's groups in a spiral direction depending on the location of the central office or switch. If looking at the cable's core and the switch is in that direction, the groups are counted counter-clockwise. If the cable is the field side, the count is clockwise. There are indicators on the mylar ribbons to know where to begin for each layer and a diagram for the different cable sizes should be readily available for reference.[5][6]

Other color schemes are sometimes used for outdoor cables, particularly outside the U.S., but this color code is common for aerial and underground cables up to several thousand pairs in North America. In the UK, the British Post Office (later BT) used this color code for what is now known loosely as CW1308 specification cables, referring to the Post Office's "Cable and Wire" specification No. 1308.

Extra pairs and colors

Colors of additional pairs in multi-pair cable as per Bell Standards

When working on aerial cable splicing and installation, it is common to use a telephone lineman's set or "butt set" to communicate over long distances. To facilitate this, extra pairs of wires are embedded in cables using "major" colors for both wires (instead of the major/minor color combinations used for the rest). One extra pair (red–white) may be embedded into cables that are 6 to 75 pairs; two pairs (red–white and black–white) may be encapsulated in cables of 100 to 300 pairs; and three pairs (red–white, black–white, and yellow–white) may be included in cables of 400 to 900 pairs.[6] These extra pairs are often referred to as "talk pairs", and are never used for subscriber service.

Optical cable

A color coding method used for fiber-optic cables, TIA-598-C, starts with the same first ten colors, adding the color rose for 11, and aqua for 12.[8]

See also


  1. ^ AT&T, Bell System Practices, Section 461-200-101 Issue 7, Connector Cables—Identification (May 1979)
  2. ^ F.W. Horn, Even-Count Cable, Bell Laboratories Record 37(6), 208 (June 1959)
  3. ^ Abruzzino, James: Communications Cabling (2E), page 187. CNC Press, 2000
  4. ^ "Modern Telephone Cable Color Code".
  5. ^ a b c d Highhouse, John (1997). A Guide for Telecommunications Cable Splicing. Cengage Learning. ISBN 9780827380660.
  6. ^ a b c d e American Telephone And Telegraph Company (February 1959). "Even PIC Cables" (PDF). Bell System Practices – Outside Plant Construction and Maintenance, Section G50.607.3 Issue 2.
  7. ^ A compilation of mnemonics for Electronics Engineering (ECE) by Jemery Gentiles lists both the standard mnemonic pair and a not-safe-for-work alternate.
  8. ^ Leroy Davis (2007-02-21). "Fiber wire color coding". Archived from the original on 2007-12-12. Retrieved 2007-12-01.