Roof Heat Cable Calculator (Length, Watts, Circuit & Cost to Run)

How it works

loop height = overhang + 6″ (≥ 12″) · eave cable = eave length × 2.0 (12″ loops) / 2.5 (18″) / 3.2 (24″) / 4.0 (30″+) · gutter = its length; downspouts = 2 × length each · total × 1.05 + 6 ft leads · watts = ft × 5–8 W/ft; amps = W ÷ V · season kWh = W × hours ÷ 1000 (× 0.7 self-regulating)

De-icing cable works by keeping melt channels open where snowmelt would otherwise refreeze — the unheated overhang, the gutter and the downspout — so the length follows from tracing those paths: a zigzag along the eave tall enough to reach past the warm wall line, a straight run through each gutter, and a doubled run down every downspout. Watts per foot then give the electrical load and the circuits it needs, and hours of operation, which a sensor controller cuts dramatically, give the cost of a winter. It is a treatment for the symptom; the ice dam itself is made by heat leaking from the house into the attic.

Worked example

Sixty feet of eave with a 12-inch overhang needs 18-inch loops — about 150 ft of cable in the zigzag — plus 60 ft through the gutter and 72 ft in three 12-ft downspouts: about 302 ft with leads and waste. Self-regulating cable at 7 W/ft draws about 2,115 W (17.6 A at 120 V, with a cold-start inrush near 44 A), so three 20 A GFCI circuits at 150 ft each; energised 600 hours on a thermostat it uses roughly 890 kWh, about $140 a winter — a snow sensor would cut that by half or more.

Frequently asked questions