Wire Bundle Derating

The reduction in allowable per-conductor current when conductors are grouped into a bundle, because heat from each conductor reduces the bundle's ability to dissipate heat from any single one. AS50881 charts 1–5 provide the standard derating curves.

Technical detail

Derating is a function of the number of wires in the bundle and the percentage of those wires carrying current. Ambient temperature and radiant heat from nearby ducts must be combined with bundle derating to determine the effective ampacity of each conductor.

Example from §25.1707(j) practice: a harness rated to 150°C in a 50°C zone has approximately 40 A ampacity by AS50881 calculation; adding 20°C of radiant heat from a hot-air duct drops ampacity to approximately 36 A.

When it matters

Any new harness design or load increase on an existing harness. The most common failure mode is checking wire temperature class against ambient and forgetting the bundle and radiant contributions.

Related terms

FAA 25.1707 — System Separation Requirements
The Federal Aviation Regulation that requires EWIS to be physically separated and electrically isolated from other EWIS, aircraft systems, fluid lines, control cables, and heated equipment such that no credible failure can produce a hazardous condition.
SAE AS50881 — Wiring Aerospace Vehicle
The SAE aerospace standard that establishes recommended practice for the design, installation, and verification of aerospace wiring systems. Widely referenced as supporting guidance for FAA §25.1707 compliance.