What does FAA 25.1707 require?

FAA 25.1707 requires every Electrical Wiring Interconnection System on a transport-category aircraft 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. The regulation covers ten subparts (a)–(j), each addressing a different separation scenario.

How is FAA 25.1707 compliance demonstrated?

Through damage assessment — physical testing, simulation, or both — substantiating that a credible EWIS failure cannot create a hazardous condition. The regulation does not adopt fixed separation distances. Industry guidance such as SAE AS50881 provides starting points, but the regulation supersedes industry recommendations and Lectromec testing has shown several AS50881 defaults to be insufficient.

How many subparts does FAA 25.1707 contain?

Ten — subparts (a) through (j). (a) sets the overarching damage standard; (b) covers electromagnetic interference; (c) covers heavy current cables; (d) covers grounding and power source separation; (e)–(h) cover fluid systems; (i) covers mechanical control cables; (j) covers heated equipment.

14 CFR §25.1707 · Seven-part reference

FAA 25.1707 — EWIS System Separation Requirements

A section-by-section technical reference to 14 CFR §25.1707, the federal requirement governing physical and electrical separation of Electrical Wiring Interconnection Systems on transport-category aircraft.

0125.1707(a)

Damage Assessment

FAA 25.1707(a) requires every EWIS to be designed and installed with adequate physical separation — by distance or an equivalent barrier — from other EWIS and aircraft systems, so a single component failure cannot create a hazardous condition. Compliance is shown through physical testing, simulation, or both.

0225.1707(b)

EMI Compliance

FAA 25.1707(b) requires EWIS to be installed so that any electrical interference likely to be present in the airplane cannot produce hazardous effects. Per AC 25.1701-1, designers must consider bus-bar noise, inter-cable coupling, parasitic currents, lightning, static discharge, and dissimilar generator frequencies.

0325.1707(c)

Heavy Current Cables

FAA 25.1707(c) requires heavy-current cables — 16 AWG or larger, or any conductor whose arcing failure could cause significant damage — to be physically separated and electrically isolated so that a fault cannot disable essential-function circuits. Compliance can be shown for the particular case or for the general configuration.

0425.1707(d)

Grounding & Power Separation

FAA 25.1707(d) requires that independent airplane power sources — engines, APU generators, batteries — and their associated EWIS be designed and installed so a fault in one cannot adversely affect another. Independent sources must not share ground terminations, and static grounds must not share locations with independent power source grounds.

0525.1707(e)–(h)

Fluid Line Separation

Subparts (e)–(h) of FAA 25.1707 require EWIS to be adequately separated from fuel, hydraulic, oxygen, water, and waste lines and components, so neither an EWIS failure nor fluid leakage onto EWIS can create a hazardous condition. AS50881 §3.11.11 recommends ≥0.500″ separation, but Lectromec testing has shown tube failure at greater distances.

0625.1707(i)

Control Cable Separation

FAA 25.1707(i) requires EWIS to be separated from flight and other mechanical control cables so that chafing, jamming, or component failure of either cannot create a hazardous condition. Critical clamp markers, jam-prevention clamping, and Common Cause Analysis are the primary design controls.

0725.1707(j)

Heated Equipment Separation

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.

About this reference

This series is derived from Lectromec whitepaper LDC-WP-0013, originally published 2014-09. The web edition was restructured for the 2026-05-14 review to allow each subpart of §25.1707 to be cited independently.

Every section was reviewed for technical accuracy by Lectromec engineering. Where industry guidance diverges from the regulation — most notably AS50881’s 0.500″ fluid-line separation recommendation — the regulation and supporting test data take precedence.