Why a colour pair is not an address

On a 96-fibre cable, blue tube and green fibre is one specific fibre. On a 288, it is two.

The sequence

Twelve colours, in a fixed order: blue, orange, green, brown, slate, white, red, black, yellow, violet, rose, aqua.

The same twelve are used twice over. They identify the tubes within the cable, and they identify the fibres within each tube. Fibre 15 on a standard cable is the third fibre — green — in the second tube — orange.

Mnemonics exist and most are worse than simply learning the order. The one property worth internalising is that it repeats: after aqua, back to blue.

Where the numbering comes from

Fibres are numbered continuously through the cable, not restarted per tube. With twelve fibres to a tube:

• Fibre 1 is blue in the blue tube.
• Fibre 12 is aqua in the blue tube.
• Fibre 13 is blue in the orange tube.
• Fibre 144 is aqua in the aqua tube — the last fibre a single pass of the sequence covers.

Tube sizes vary. Six-fibre tubes are common on smaller cables, and the arithmetic shifts accordingly — which is why the tube size has to be established before any lookup means anything.

The ambiguity that matters

Twelve tube colours cover 144 fibres at twelve to a tube. Beyond that, tube colours repeat.

On a 288-fibre cable there are twenty-four tubes and only twelve colours, so the blue tube appears twice — tubes 1 and 13. “Blue tube, green fibre” is fibre 3 and fibre 147, and nothing about the colours distinguishes them.

This is not an edge case. Cables of 288 fibres and above are ordinary in backbone and metro builds, and the moment a cable crosses that line, colour stops being an address and becomes a hint.

Binders: the layer nobody documents

Large cables group tubes into binders — bundles held with coloured thread, following their own colour sequence. A 432-fibre cable might be six binders of six tubes of twelve fibres.

The binder is what resolves the repeated tube colours. It is also, reliably, the detail missing from the drawing you were handed.

If you are working on a cable large enough for tube colours to repeat, establish how binders are identified before the closure is open. Working it out while holding two identical blue tubes is a considerably worse time to start.

Ribbon cable counts differently

In ribbon cable, fibres are bonded side by side in flat arrays. The ribbon carries the identity — usually printed on it — and the fibres within follow the colour sequence in physical order from the marked edge.

So a ribbon lookup is ribbon number plus position, not tube colour plus fibre colour. Applying loose-tube reasoning to ribbon is a specific and avoidable mistake.

The convention is not universal

The twelve-colour sequence is the one most commonly encountered, and it is not the only one. Cable manufactured to other conventions exists, older cable follows older practice, and some markets differ.

No tool can know which convention a particular drum was built to. The cable's own datasheet is the only authority, and it takes a minute to check against a cost that does not.

Three things to confirm before cutting

The convention. From the datasheet for that cable, not from a table.

The count direction. Whether fibre 1 is at the marked end, and which end carries the mark. Getting this backwards mirrors the entire cable.

What is live. Colour tells you which fibre. It tells you nothing about whether it is carrying traffic — that is the records first, and a light source second, and the light source is the one that is actually true.

Frequently asked questions

What is the fibre colour order?

Blue, orange, green, brown, slate, white, red, black, yellow, violet, rose, aqua. The same twelve identify both the tubes and the fibres within them, and the sequence repeats after aqua.

Which fibre is number 87?

With twelve fibres to a tube, fibre 87 is the third fibre — green — in the eighth tube — black. The arithmetic changes with tube size, so establish that first.

Why is a colour pair ambiguous on a big cable?

Because twelve tube colours only cover 144 fibres at twelve per tube. On a 288-fibre cable the blue tube appears twice, so blue tube and green fibre is both fibre 3 and fibre 147. Above that size, colour is a hint rather than an address.

What resolves the ambiguity?

The binder — a coloured thread bundling groups of tubes, with its own colour sequence — or the tube's physical position. The binder detail is also the one most often missing from the drawing, so establish it before opening the closure.

How are 24-fibre tubes coloured?

Usually by repeating the twelve colours with a black tracer stripe or ring for fibres 13 to 24. As with everything here, confirm against the cable's datasheet.

Is the colour sequence the same everywhere?

No. The twelve-colour sequence is the most commonly encountered, not a universal law. Other conventions exist, older cable follows older practice, and no tool can know which one a drum was built to.

Does colour tell me whether a fibre is in use?

No. It identifies which fibre, not what it is doing. Check the records, then check with a light source — and the light source is the one that is actually true.