Power over Ethernet has quietly changed what a cabling installation has to cope with. A decade ago a data cable carried data. Today the same cable is powering cameras, access points, door controllers, digital signage and increasingly the lighting itself, at up to 90 watts a port under IEEE 802.3bt Type 4.
That power turns into heat. And when cables are bundled together, the heat inside the cable bundle has nowhere to go. For CCTV that usually shows up as cameras dropping out with no obvious fault.
What actually happens inside a cable bundle
Every cable carrying PoE warms up. A single cable in free air sheds that heat easily. Put fifty of them in a bundle running through a containment tray and the ones in the middle cannot. Temperature builds toward the core of the bundle, and the cables in the centre run hottest.
Heat matters because copper resistance rises with temperature, and insertion loss rises with it. The effect is well understood and written into the standards: above 20°C, the usable channel length starts to fall. Unscreened cable degrades faster than screened, because screened constructions dissipate heat better.
The result on site is a link that passed testing on a cold Tuesday in February and starts throwing errors in August, or an installation that works fine until the client adds another twenty cameras to the same route.
What the standards say
ISO/IEC TR 29125 and TIA TSB-184-A both address heat rise in bundled cabling carrying remote power. BS EN 50174-2 covers installation practice in the UK. The common guidance across them is straightforward:
- Limit bundle sizes. Twenty four cables is a widely used working limit for bundles carrying higher power classes, and smaller is better.
- Derate channel length where ambient temperature exceeds 20°C.
- Prefer larger conductor gauges. Cat 6A at 23 AWG runs cooler than Cat 5e or Cat 6 at 24 AWG, because there is less resistance to generate heat in the first place.
- Prefer screened constructions where power density is high, for the heat dissipation as much as the noise performance.
- Keep bundles out of enclosed, unventilated containment where you can, and account for ambient temperature in plant rooms and roof voids.
Where installations go wrong
The failures we see are rarely exotic. They are almost always one of the following.
Cable specified for data, then used for power. A Cat 5e system installed in 2014 for desktop connectivity is now feeding 60 watt cameras. Nobody re-evaluated the bundle.
Bundles grown by accretion. The original install was sensible. Three subsequent contractors added to the same tray because it was there.
Ambient temperature ignored. A bundle rated comfortably at 20°C sitting in a roof void that reaches 40°C in summer is a different calculation entirely.
Testing at the wrong time of year. A channel that passes at 15°C ambient can fail at 35°C. If the test results do not record ambient temperature, they tell you less than you think.
Why cameras drop out at night
CCTV makes the problem worse at exactly the wrong time. When cameras switch to night mode their infrared illuminators come on, and outdoor cameras may run heaters in cold weather. Both increase the power drawn through the cable bundle, so a system that is marginal during the day can start dropping cameras after dark.
If your cameras keep dropping out overnight rather than at random, heat in the bundle and the PoE budget of the switch feeding them are the first two things to check.
PoE is now in the Wiring Regulations
From 15 October 2026 this stops being purely a data cabling question. BS 7671 Amendment 4, the current edition of the IET Wiring Regulations, introduces a dedicated section on Power over Ethernet and brings PoE within the scope of the UK standard for electrical installations for the first time. NICEIC sets out the changes here.
In practice the cable bundle is now an electrical design consideration as well as a data one, which is one more reason to have both designed together. We deliver electrical and data cabling through one accountable team for exactly that reason.
What to ask for
If you are specifying a system that will carry PoE, three questions are worth putting to whoever is installing it.
What bundle sizes are you working to, and what power class have you designed for? A specific number is a good sign. A vague answer is not.
What ambient temperature have you derated for, and what does that do to maximum channel length? This should be a calculation, not a guess.
Do the test results record ambient temperature? They should, and you should keep them, because they are the baseline against which any future fault is measured. We cover what else a proper set of results should include in what you should receive when a cabling installation is handed over.
The practical point
PoE is not going away, and power per port is going up rather than down. The cabling installed today will be carrying more power in five years than it does now, almost regardless of what it was specified for.
Designing for that is cheap at installation and expensive afterwards. Pulling a bundle apart and rerouting it in an occupied building costs a great deal more than getting the bundle size right the first time.
We design, install and test structured cabling systems across the UK, and we test with ambient conditions recorded so the results still mean something in five years. If you want a system reviewed before you add load to it, get in touch.