Freeze-Protection Heat-Trace Cable and Circuit
The heat-trace cable and wattage a run needs to stay above freezing.
Example
You enter
- Pipe run length (ft) 150
- Support / spiral allowance (percent) 10
- Valves and flanges (count) 1
- Cable allowance per valve (ft) 3
- Cable rated wattage (W/ft) 5
- Supply voltage (V) 120
- Circuit breaker rating (A) 20
You get
- Heat-trace cable 168
- Watts 840
- Circuit load 7
Details, formula, and sources
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 the pipe's insulated heat loss; the manufacturer's tables govern.
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.
The cable-length and circuit arithmetic is public first-principles; the required W/ft and max circuit length come from the manufacturer's design tables.
Estimate. AHJ and licensed professional govern.
Field names used by the API: pipe_ft, allowance_pct, num_valves, valve_allow_ft, rated_w_per_ft, voltage, breaker_a, cable_ft, watts, amps
- Rated W/ft the cable rating, at or above the pipe heat loss from insulation-heat-lossheat-trace manufacturer
- Continuous limit load must stay under 80% of the breaker (NEC continuous load); a cold start draws 2-3xNEC / manufacturer