BS 7671 Amendment 4: Cable Sizing with PoE Derating

Section 716 in Amendment 4 introduces specific derating factors for cables carrying Power over Ethernet. This is a new addition to the cable sizing methodology that sits alongside the established derating factors for ambient temperature, grouping, thermal insulation, and burial depth. Understanding how PoE derating interacts with these existing factors is essential for accurate cable sizing.

The physics behind PoE derating is straightforward. Every PoE cable carrying current generates I²R heat. A single Cat 6A cable with 23 AWG conductors (0.26 mm² per conductor) carrying 600 mA for PoE+ (30W) dissipates approximately 1.8W over a 90-metre run. For PoE++ at 960 mA (60W), heat dissipation reaches 3.5W per cable. In a bundle of 24 such cables, the cumulative heat generation is 42-84W — equivalent to a small panel heater operating inside the cable containment.

Section 716 provides derating factors based on three variables: the number of cables in the bundle, the PoE power level per cable, and the installation method (open tray, enclosed tray, conduit, or unventilated void). The factors range from 1.0 (no derating needed — single cable in open air) to as low as 0.65 (large bundle, PoE++, unventilated enclosed space).

The critical interaction is with standard grouping derating. PoE derating is applied in addition to grouping factors from Appendix 4 Table 4C1. If PoE cables share containment with power cables, both the PoE heating and the mutual heating from grouping must be accounted for. This compound derating can be severe — for example, a power cable grouped with 24 PoE++ cables might see a combined derating factor of 0.45 (PoE factor 0.70 multiplied by grouping factor 0.65).

Ambient temperature correction adds a third derating layer. In ceiling voids reaching 40°C ambient, the temperature correction from Table 4B1 further reduces the effective current-carrying capacity. The total derating is the product of all three factors: ambient × grouping × PoE.

Practical mitigation strategies include: separating PoE bundles into smaller groups (reducing the cable count per group below 12 significantly improves the PoE factor), using cables with higher temperature ratings (90°C instead of 70°C shifts the baseline), and providing ventilation in enclosed containment.

ECalPro's Cable Size Calculator applies Section 716 PoE derating factors alongside standard Appendix 4 corrections, providing accurate compound derating for any PoE installation scenario.