Heated Equipment Separation under FAA 25.1707(j)
FAA 25.1707(j) requires EWIS to be separated from heated equipment, hot-air ducts, and lines so neither EWIS failure nor heat or hot-air leakage creates a hazardous condition. Three heat inputs must be assessed: equipment-generated heat, ambient zone temperature, and the harness's own resistive heating — with bundle derating applied.
“EWIS must be designed and installed with adequate physical separation between the EWIS components and heated equipment, hot air ducts, and lines, so that: (1) An EWIS component failure will not create a hazardous condition. (2) Any hot air leakage or heat generated onto EWIS components will not create a hazardous condition.”
Three heat inputs must be summed. (1) Generated heat from nearby equipment and air ducts can radiate onto the harness; even when the wire itself is rated to 150°C or more, ancillary components — splices, heat-shrink tube, clamps, harness sleeving — may not be. (2) Ambient zone temperature affects every component in the volume and must be combined with radiant input. (3) The harness's own resistive heating raises conductor temperature above ambient and is a function of the wires in the bundle and the load on each.
Ampacity is the rated current-carrying capacity at a given temperature. AS50881 provides the formula and charts 1–5 used to estimate harness ampacity across mixed-gauge bundles. The estimate is decades old but remains a defensible first-order calculation when no measured data exists.
A worked example: a harness rated to 150°C in a zone whose flight-time ambient is 50°C has a 100°C headroom for resistive rise. Using the AS50881 charts, ampacity comes out to approximately 40 A. Route the same harness near a hot-air duct adding 20°C of radiant heat to the surface, and ampacity drops by roughly 10% to 36 A. The remediation is one of: larger gauge, re-route, or a protective sleeve.
Derating is the variable most often missed in EWIS thermal compliance. Designers correctly check wire temperature class, then forget that bundle population and radiant heat from a nearby duct can knock 10–20% off the rated current. The fix is to make derating a checklist item alongside ampacity, not a follow-up calculation.
Frequently asked
How is wire bundle derating calculated for FAA 25.1707(j)?
Use the AS50881 ampacity formula and charts 1–5. The derating factor depends on the number of wires in the bundle and the maximum allowable temperature rise above ambient. Add any radiant heat from nearby ducts to ambient, then recalculate ampacity against the new effective ambient.
What heat sources must be assessed under 25.1707(j)?
Three: (1) heat generated by nearby equipment and hot-air ducts; (2) ambient zone temperature during flight; (3) resistive heating from current carried by the harness itself. All three combine to set the harness's effective operating temperature.