25-pair color code
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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. 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.
| Pair # | First wire | Second wire |
|---|---|---|
| 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 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, red through violet are also in spectral order, 2-7.)
Sometimes each wire in a pair will have a colored stripe or rings matching the color of its paired wire. This makes it easy to identify which pair a given wire belongs to. Otherwise, to determine which pair a wire belongs to one has to note which color codes are physically twisted together.
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. 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.
These terms are based on the ¼" (6.5mm) TRS 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 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. The pattern then starts over with 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/blue binder, and the remaining 300 pairs in 12 groups of 25 pairs wrapped in a white/orange binder.
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.
[edit] Mnemonics
The first group colors can be remembered with the mnemonic: Why Run Backwards, You'll Vomit (White, Red, Black, Yellow, Violet)
The sequence of second group colors can be remembered with the mnemonic: BOGBRuSh (Blue, Orange, Green, BRown, Slate). (This mnemonic is attributed[citation needed] to a British radio engineer, "bog brush" being a somewhat derogatory British term for a toilet pan cleaning brush.)
Another mnemonic common for the second group colors is: Bell Operators Give Better Service.
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