A variable-length lightning strike down-conductor is provided that allows for optimized ground path length, where excess down-conductor (grounding) cable is tightly coiled (e.g., as a mono-spiral coil) on a cable reel. Specifically, a primary down-conductor cable from an adjustable-length air terminal is clamped between the air terminal and the cable reel at any intermediate length, and the clamp is shunted to the ground plane (e.g., earth ground) through a second low-impedance down-conductor cable, as opposed to the high-impedance coiled down-conductor cable, thus increasing down-conductor effectiveness by reducing the impedance seen by a lightning strike to reach the ground plane.