Heavy Current Cable Separation under FAA 25.1707(c)
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.
“Wires and cables carrying heavy current, and their associated EWIS components, must be designed and installed to ensure adequate physical separation and electrical isolation so that damage to circuits associated with essential functions will be minimized under fault conditions.”
The supplemental documentation pulls forward the requirements that previously lived in §25.1353 and adds the supporting EWIS components. A heavy current cable is defined as 16 AWG or larger, or any conductor whose arcing failure would produce significant damage to nearby systems.
Two framing questions precede a heavy-cable assessment. First, is the goal to qualify the particular installation or to qualify the general case? Particular-case testing is narrower and usually cheaper, but the result applies only to the specific configuration. General-case testing produces a broader recommendation but typically forces a larger keep-out distance than any single installation requires.
Second, what is in the surrounding area? The assessment radius for nearby wiring is often greater than for equipment, because adjacent harnesses present larger, longer targets for an arc plume. In a representative installation with 4 AWG three-phase feeders running 2.0″ from a jacketed, shielded twisted-pair signal harness, breach and arc initiation are most likely at the clamps; the susceptibility of the signal harness becomes the limiting case.
Treating heavy-current 'separation' as a fixed inches-of-clearance number misses the failure mode. Arcing initiates where the harness is mechanically loaded — at the clamps — and propagates through the weakest adjacent target, not the closest one. The assessment radius and the test article have to be chosen against that physics, not against AS50881's default geometry.
Frequently asked
What qualifies as a 'heavy current cable' under 25.1707(c)?
A conductor of 16 AWG or larger, or any conductor whose arcing failure could cause significant damage to nearby systems. The latter clause means that a smaller conductor on a high-energy bus can still fall under 25.1707(c).
Should 25.1707(c) testing target the particular case or the general case?
Particular-case testing is preferred when the installation is fixed and certifying a specific configuration is the goal — it is cheaper and the result is directly defensible. General-case testing is appropriate when the same separation rule must apply across many installations and a conservative bound is acceptable.