BS 7671 Amendment 4: PoE in Office Buildings

Power over Ethernet has become the default power delivery method for office devices — IP phones, wireless access points, LED luminaires, and security cameras. Amendment 4 introduces Section 716, which addresses the thermal effects of carrying power and data on the same cables in bundled installations. This is a significant addition because standard structured cabling designs did not previously account for PoE heating under BS 7671.

The core issue is that PoE cables carrying current generate heat. A single Cat 6A cable carrying 60W (PoE++ / 802.3bt Type 4) dissipates approximately 3.5W per cable over a 90m run. In a typical office floor box or containment run where 24 or more cables are bundled together, the cumulative heating can raise cable temperatures by 10-15°C above ambient, reducing the current-carrying capacity of adjacent power cables and potentially exceeding the cable's own temperature rating.

Section 716 introduces derating factors for PoE cable bundles based on the number of cables, the PoE power level, and the installation method. For a bundle of 24 cables carrying PoE++ (60W per port), the derating factor can reduce the effective current capacity by up to 30% compared to an unbundled installation. This factor must be applied when sizing cables and selecting cable types for PoE-heavy installations.

The practical impact for office fit-outs is significant. Where a traditional structured cabling design might use standard LSZH Cat 6A in 100mm cable trays, Section 716 may require either larger containment (to reduce bundling density), cables with higher temperature ratings (e.g., 75°C rated instead of 60°C), or a combination of both. For new builds, this should be addressed at the containment design stage. For refurbishments, existing containment may become a constraint.

Cable route separation between PoE bundles and power cables also receives attention. Where PoE cables share containment with power cables, the heating from the PoE bundle must be factored into the power cable derating calculation per Appendix 4. This is a double derating scenario that can catch designers off guard.

ECalPro's Cable Size Calculator applies Section 716 PoE derating factors alongside standard grouping and thermal corrections, giving designers accurate cable sizing for modern PoE-heavy office environments.