Guide
How to estimate low-voltage cable
Short answer: for every run add the horizontal path, the drop from the ceiling to the device, the termination allowance at the closet and a service loop, then 10–15 % slack – and order full boxes per cable type.
The parts of a run
- Horizontal path – along the route the cable really takes (corridors, cable trays), not the straight line.
- Drop – from the ceiling down to the device: ceiling height minus mounting height.
- Closet allowance – up and down the rack or into the panel, often 10–15 ft.
- Service loop – extra at the device for moves and re-terminations.
- Slack – a percentage for everything you cannot see on the plan.
Example
A camera 120 ft from the IDF along the corridor, mounted at 9 ft under a 12 ft ceiling, 10 ft closet allowance, 5 ft service loop, 10 % slack: (120 + 3 + 10 + 5) × 1.1 = 151.8 ft.
Ethernet length limit
Twisted-pair Ethernet (Cat5e, Cat6, Cat6A) is limited to 100 m (328 ft) per channel: 90 m of permanent link plus 10 m of patch cords. A run longer than that needs an IDF, a switch closer to the device, fibre or an extender.
From feet to boxes
Cable comes in 1,000 ft (305 m) boxes. Divide the total per cable type by 1,000 and round up – but check the longest runs too: a box with 180 ft left cannot make a 200 ft run. On larger jobs, count how many runs fit in each box.
Typical cable types
| Use | Typical cable |
|---|---|
| IP cameras, access points, IP door controllers | Cat6 / Cat6A |
| Maglocks and strikes | 18/2 (16/2 on long runs) |
| Card readers (Wiegand / OSDP) | 22/6 shielded, or as the reader requires |
| Request-to-exit, door contacts | 22/4, 22/2 |
| Alarm sensors | 22/4 |
Questions
How much slack is normal?
Usually 10–15 %. Older buildings, hard ceilings and unclear routes need more.
Should labour follow the footage?
Partly – most estimators price labour per run (terminations, testing) plus per foot for long or difficult paths.
Related
Let GITECHY Design do the math
Cable lengths, PoE and power budgets, batteries, voltage drop and camera coverage – on every design. One flat price per company.