Freeze-Protection Heat-Trace Cable and Circuit
Sizes heat-trace for freeze protection: cable = pipe x (1 + allowance) + valves x allowance; watts = rated W/ft x cable; amps = watts / voltage. A 150 ft line with one valve on a 5 W/ft cable needs 168 ft drawing 840 W - 7.0 A at 120 V, fine on a 20 A circuit - but a 400 ft run hits 18.3 A, over the 80% continuous limit, so it splits across two circuits. The required W/ft comes from insulation-heat-loss; the manufacturer's tables govern.
Formula and source
cable_ft = pipe_ft x (1 + allowance_pct/100) + num_valves x valve_allow_ft; watts = rated_w_per_ft x cable_ft; amps = watts / voltage; breaker_ok = amps <= 0.8 x breaker_a.
Heat-trace sizing identity by name (cable length with allowances; wattage and current from the rated W/ft); first-principles electrical arithmetic.
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