25-pair Color Code

The 25-pair color code is a color code used to identify individual conductors in a kind of electrical telecommunication wiring for indoor use, known as twisted pair cables (often seen with RJ21 cables). The colors are applied to the insulation that covers each conductor. The first color is chosen from one group of five colors and the other from a second group of five colors, giving 25 combinations of two colors.

• The first group of colors is, in order: white, red, black, yellow, violet.
• The second group of colors is, in order: blue, orange, green, brown, slate.

The 25 combinations are shown to the right in the image. The combinations are also shown in the table below showing the color for each wire ("1" and "2") and the pair number.

The first five combinations are very common in telecomms and data wiring worldwide but beyond that there is considerably more variation.

The color violet is sometimes called purple, but in the telecommunications and electronics industry it is always referred to as violet. 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 10 colors.

Sometimes each wire in a pair will have a colored stripe or rings (often called a "tracer") matching the color of its paired wire. This makes it easy to identify which pair a given wire belongs to. This means that the first pair is a mate of white with a little blue and a colour wire of blue with a little white. Pair 17 would be a mate of yellow with a little orange and colour wire of orange with a little yellow. Otherwise, to determine which pair a wire belongs to one has to note which color codes are physically twisted together in the lay of the cable as the sheath is stripped back.

When used for common analog telephone service, the first wire is known as "tip" and is connected to the positive side of the direct current (DC) circuit, while the second wire is known as "ring" and is connected to the negative side of the circuit, following the tip and ring convention. Neither of these two wires has any connection to the local ground. This creates a balanced audio circuit with common-mode rejection also known as a differential pair. This convention works in the UK with the first or mate wire as the +ve A leg and the colour wire as the -ve B leg.

These terms are based on the ¼″ (6.5mm) TRS phone connector where the "tip" of the connector is separated from the "ring" of the connector with a ring of insulation. The connection furthest from the wire is known as the tip, the middle connection is the ring and the (largest) connection closest to the wire is the sleeve (unused in this case). The female side or "socket" end is normally wired with the "tip" and "ring" configuration also, to accommodate the "plug" and maintain correct polarity when connections are established. For the female connector the connection order with respect to the wire is of course reversed.

For cables with over 25 pairs, the first 25 pairs (called a binder group) are marked with mylar ribbons using the colors of the color code starting with a white/blue ribbon, the second 25 pairs with a white/orange ribbon, and so on through the 24th binder group (600 pairs), which has a violet/brown ribbon, and forming a "Super" binder. In cables more than 600 pairs, each of the 100 pair binder groups within the 600 pair of 24 binder groups 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 will have 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.

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, you count the groups counter-clockwise. If the cable is the "field side", you count the groups 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.

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