Cold-start current and why circuit sizing must use the cable’s startup specifications.
A self-regulating heating cable has a heating core with a positive temperature coefficient of resistance: its resistance increases as the core warms. A cold cable therefore draws more current when first energized than it does after warming under the same supply voltage.
The current settles as the cable and its surroundings warm. Do not assume one startup duration or current multiplier applies to every cable and installation.
Use the maximum circuit-length and breaker-sizing information for the exact cable, supply voltage and minimum startup temperature. The WarmlyYours roof-and-gutter cable manual explicitly bases maximum circuit lengths on startup load; steady-state amps per foot depend on cable temperature. Normal operating current alone is not enough to size the circuit.
Sources: Heat Trace: Safety and Longevity of Self-Regulating Heating Cables (PDF page 8, printed page 4) and WarmlyYours roof-and-gutter cable manual (PDF page 6).
