The copper cabling standards are built around a hundred-metre channel, conventionally ninety metres of fixed cable plus ten metres of patch leads. Beyond that the answer is a switch in between, or fibre.
One number governs almost every copper installation, and it is a channel figure rather than a drum figure.
The hundred-metre channel
The structured cabling standards define a channel: everything between the equipment at one end and the equipment at the other. It is a hundred metres, and it is conventionally made up of ninety metres of fixed horizontal cable plus ten metres of patch leads at the two ends.
That division is the useful part. If the run inside the walls is ninety metres, there is only ten metres of patch lead left to play with — five at each end if you are being even-handed — and a long lead in a cabinet is spending part of the budget.
Why it is a limit at all
Signal weakens as it travels and the pairs interfere with one another more as frequency rises. The grades are specified to hold their transmission limits across that distance, and beyond it nobody has committed to anything.
That is why exceeding it produces such varied results. A long run may come up at the expected rate, or negotiate down, or run for months and then start reporting errors when a warm summer changes the cable’s resistance. There is no clean failure to diagnose, which is what makes over-long runs so unpleasant to inherit.
Measuring the length you will actually have
The route, not the plan. Cable follows joists, goes up and over, comes back down, loops into a cabinet and leaves a service loop at each end. A run that measures forty metres on a floor plan is regularly sixty in reality.
Add the patch leads at both ends to the total before comparing it with the limit, because the channel includes them.
What to do when the distance is genuinely too far
Put a switch in the middle. Each run then starts its own channel, and the hundred metres begins again. The switch needs power where it is, which is the practical difficulty, and PoE can sometimes supply it from the far end.
Use fibre. The distances are in a different class entirely and the grade of fibre and the module decide the reach — multi-mode for the shorter kind, single-mode for the long. This adds cages, modules and a fragile-until-terminated cable, and it is the right answer for anything between buildings.
Reconsider the route. The shortest path is frequently not the one first imagined, and a different riser or a straighter run under a floor can bring a marginal distance back inside the limit.
Two things that are not the length limit
Power over Ethernet travels along the same cable and is specified over the same channel grades, so it does not shorten the run — but the cable takes its share of the power, which is why the standards publish one figure for what leaves the source and a lower one for what the device is entitled to.
A long patch lead between two units in one room is not a problem in itself. It becomes one when it is quietly consuming the channel budget of a run that was already close to the limit.
The questions that come up before an order
What happens if a run is slightly too long?
There is no line at which it stops. A channel beyond its specified length is outside what anybody guaranteed, and it may work, or come up at a lower rate, or report errors under load, or work in winter and not in summer. The failure is unpredictable, which is the actual reason the limit exists.
Does the limit change with the category?
The hundred-metre channel is the model the standards are built around, and the grades are specified to reach it at their own rates — Category 5e at gigabit, Category 6A at ten gigabit. Category 8 is the exception, specified over a much shorter channel, which is why it belongs between equipment in a rack.
Last reviewed 10 September 2026