An 80 m freeze-protection circuit on a 114 mm insulated pipe sizes cleanly for heater output and breaker rating — but adding the heating cable's own resistance reveals a voltage drop that blows past any reasonable limit.
| Pipe outer diameter / insulation thickness | 114 mm / 50 mm (k=0.04 W/m·K, mineral wool) |
| Maintain temperature / minimum ambient | 10°C / -10°C |
| Design factor | 1.3 (IEEE 515 practice margin) |
| Selected heater output | 20 W/m |
| Circuit length / voltage | 80 m / 230 V |
| Breaker rating | 16 A |
The initial pass above skipped the voltage-drop check because no cable resistance was entered — a self-regulating or constant-wattage cable's real resistance per meter is needed to check whether the far end of an 80 m run still receives enough voltage to deliver its rated output.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Heater output vs. required W/m | heaterOutput ≥ requiredWPerM | 20 W/m ≥ 10.38 W/m | ✓ PASS |
| Circuit current vs. breaker rating | ≤ 16 A | 6.96 A | ✓ PASS |
| Voltage drop (with 0.05 Ω/m cable resistance) | typically ≤ 5-10% for heat-tracing circuits | 12.10% | ✗ FAIL |
Key insight: A heat-tracing circuit can pass its power-output sizing and breaker check while still failing on voltage drop, because those are governed by entirely different physics — output sizing depends on the required W/m vs. the heater's rating, while voltage drop depends on cable resistance accumulating over the full run length. Leaving the cable-resistance input at zero silently skips this check altogether, which is exactly why it's worth deliberately entering a manufacturer's real resistance-per-meter figure rather than accepting the default of 0.
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Open Heat Tracing Circuit Sizing calculator →The main levers are shortening the circuit run (splitting one long circuit into two shorter ones fed from separate breakers), increasing the supply voltage to the heat-tracing circuit if the installation allows it, or selecting a heating cable with lower resistance per meter for the same output — splitting into shorter circuits is often the simplest fix since it directly reduces the total resistance the current has to travel through.
Self-regulating cables behave somewhat differently from constant-wattage cables because their output already varies with temperature along the cable's length, but they still rely on adequate voltage reaching every point along the circuit to produce their rated output — a large voltage drop reduces the power delivered at the far end of either cable type, so the voltage-drop check remains relevant regardless of which heating cable technology is used, even though the exact resistance-per-meter figure and its behavior differ.