Also in: Disaster Response and Recovery
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
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeSizes 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 running at 120 V, well inside a 20 A circuit's 16 A continuous limit, but a cold start at 3x draws 21 A and trips it (Raychem's table stops 5 W/ft cable at 129 ft on 20 A for a -20 F start), so check the manufacturer's maximum circuit length; a 400 ft run is 18.3 A even running and must split. The required W/ft comes from the pipe's insulated heat loss; the manufacturer's tables govern.
Formula or tablecable_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; start_amps = amps x start_factor; breaker_ok = amps <= 0.8 x breaker_a and start_amps <= breaker_a.
Source and editionHeat-trace sizing identity by name (cable length with allowances; wattage and current from the rated W/ft); first-principles electrical arithmetic.
Free accessThe cable-length and circuit arithmetic is public first-principles; the required W/ft and max circuit length come from the manufacturer's design tables.
What governsEstimate. AHJ and licensed professional govern.
API field namespipe_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