Somebody sends you a set of floor plans and asks what the cabling will cost. The plans are to scale, the outlet positions are marked, and the arithmetic looks straightforward: measure from the cabinet to each point, add it up, price it.
Do that and you will be short. Sometimes badly short — and on a job of any size, short cable is not a small error. It is a return visit, a joint where there should not be one, or a run that misses its length limit and has to be redone.
The gap between the drawing and the pull is predictable, though. Every part of it can be accounted for before anyone quotes.
What the drawing does not show
A floor plan is a flat projection. Cable is not flat.
The vertical component. The plan shows a cabinet in one corner and an outlet forty metres away. It does not show that the cable goes up into the ceiling void at one end and back down at the other. On a commercial floor with a three-metre ceiling height, that is around six metres of cable on every single run that appears nowhere on the plan. Across dozens of points per floor it becomes hundreds of metres.
Slack, and it is not symmetrical. Every run needs spare at both ends, but not the same amount. At the cabinet you need enough to dress the cable properly, route it through management, reach the highest patch position and still allow the panel to be pulled forward for work later. At the outlet you need much less — enough to terminate comfortably and make a service loop.
Allowing roughly two metres cabinet-side and one metre user-side reflects that asymmetry. It is not generosity. Cable that is exactly long enough to reach cannot be dressed, and cable that cannot be dressed makes a cabinet nobody can work in.
Route, not distance. Cable follows trays, trunking, structure and fire compartmentation. It goes around things. The straight line on the plan is the shortest possible path and almost never the actual one.
Buildings are not stacks of identical floors
This is the one that catches people on multi-storey work.
It is tempting to measure one floor, multiply by the number of floors, and move on. But many towers taper — the floorplate at the bottom is larger than the floorplate at the top. On the refit this example draws on, that taper was roughly forty percent across the building’s height.
Multiply the ground-floor measurement by the number of storeys and you over-order significantly. Multiply the top-floor measurement and you come up short on the lower half, which is worse. Either way you have quoted a building that does not exist.
Measure a representative floor, establish the taper, and scale accordingly. If the plans show materially different layouts on particular floors — plant rooms, double-height spaces, a sky bridge — treat those separately rather than averaging them away.
A worked example
From an estimate made on a central Durban high-rise refit:
| Input | Value |
|---|---|
| Drawing scale | 1:100 |
| Ceiling height allowance | ~3 m |
| Slack, cabinet side | 2 m |
| Slack, user side | 1 m |
| Points per floor | 38 |
| Building taper allowed | ~40% |
| Estimated cable, per floor | ~1,476 m |
That works out around 39 m average per point on a floor where no single run was anywhere near 39 m in a straight line. The difference is the vertical travel, the slack, and the actual route.
Add to that a contingency of a couple of hundred metres per floor, for two specific and very likely scenarios:
- The cabinet gets split. One comms room becomes two, and every run’s length changes.
- A tenant occupies parts of two floors and needs proper throughput between racks, which is a link nobody drew because nobody knew who the tenant would be.
On a commercial refit, neither is speculative. Tenants are frequently unknown at cabling stage, and the layout that gets signed off is rarely the layout that was drawn.
(These figures are estimates made from drawings, not as-built measurements. They are shown as a method, not as a benchmark for your building.)
Why this matters more for cameras than for desks
A data outlet that comes up slightly short is an inconvenience. A camera run that comes up short is a design failure, for one reason: the 100 m limit.
Structured cabling standards specify a maximum channel length of 100 m for Ethernet — conventionally 90 m of fixed horizontal cable plus 10 m of patch leads at both ends. That is not a guideline that degrades gracefully. Beyond it, behaviour becomes unreliable, and with PoE cameras you have power delivery to worry about as well as data.
Now apply the estimating gap to it. A camera position measuring 85 m on the plan looks comfortably inside the limit. Add six metres of vertical travel, three metres of slack, and a route that goes around a lift shaft rather than through it, and the real channel is over 100 m. It may even work on the day, and then fail intermittently in a way that costs days to diagnose.
Do the length calculation on the real route, including vertical and slack, before you commit to a camera position. If a position lands near the limit, you have three options and they are all cheaper than discovering the problem after installation: move the cabinet, add an intermediate switch position, or go to fibre for that leg.
Include the strip-out
One line that belongs in every refit estimate and is frequently forgotten: removing what is already there.
On a building being refitted, there is usually an existing system — old cabling, old cameras, redundant containment. Somebody has to strip and dismantle it, and that is labour, access and disposal. On the project here, the strip-out of the existing system was written into the proposal alongside the new cabling from the beginning.
Leave it out and you have either an unfunded variation or a new system installed alongside abandoned cable that nobody will ever be able to trace.
The method, condensed
- Measure from scaled drawings, not from impressions.
- Add vertical travel — ceiling height at both ends of every run.
- Add asymmetric slack: more at the cabinet, less at the outlet.
- Follow the actual route, around structure, not the straight line.
- Establish the building’s taper; do not multiply one floor by the storey count.
- Add per-floor contingency for cabinet splits and unknown tenants.
- Check every camera position against the 100 m channel limit on the real route.
- Price the strip-out of the existing system.
None of this is difficult. It is simply the part that gets skipped when a quote is wanted quickly — and it is the part that decides whether the number was ever real.
Based on estimating and programme correspondence from a high-rise commercial refit in central Durban, 2015–2016. The building, contractors and tenants are not identified. Quantities are drawing-stage estimates shown as a worked method; calculate any real project against its own drawings and standards.