Auxiliary Heat That Trips the Breaker

For resistance heat, current = watts / volts. At the nameplate rating: 15,000 / 240 = 62.5 A. At the measured 238 V, the elements draw slightly less: 15,000 x (238/240) squared / 238, or more simply about 61 A - which is exactly what the technician measured. So the heat kit is drawing its normal, expected current, balanced across both legs, with element resistances within tolerance and insulation resistance high enough to rule out a ground fault.

That changes the direction of the diagnosis. A failed or shorted element would show imbalanced current, low resistance, or low megohm readings; a failing blower or airflow problem would trip the high-temperature limit rather than the breaker. Here the load is healthy and the breaker trips after 10-15 minutes - the classic signature of a thermal overload rather than an instantaneous short. The equipment is doing what it was designed to do; the circuit supplying it is not sized for that equipment. The next step is to compare the circuit's ampacity and overcurrent protection against the nameplate minimum circuit ampacity and maximum overcurrent protection, not to replace parts on the heat kit.

Nameplate: MCA 78 A, maximum overcurrent protection 80 A. The circuit should be 4 AWG copper (rated 85 A at 75°C, adequate for the 78 A MCA after adjustment) with an 80 A breaker. Installed: 6 AWG copper NM-B on a 60 A breaker.

Two separate defects: (1) 6 AWG copper NM-B is limited to the 60°C column, which is 55 A, so the 60 A breaker already exceeds the conductor's ampacity - the wire is not protected against overload and is a fire hazard in the panel and along the run; (2) the circuit is undersized for the connected load, so a 60 A breaker carrying 61 A for 10-15 minutes will eventually trip thermally, exactly as reported. The 80 A maximum overcurrent protection is a ceiling, not a target; the governing number for the conductor is the 78 A MCA.

Consequences: chronic nuisance trips (the owner loses auxiliary heat during the coldest weather, and each trip imposes a hard restart on the compressor and controls), possible damage to breaker termins from heat, and an electrical fire risk if the conductor is asked to carry the load continuously. This is a reportable safety defect regardless of whether it has tripped yet.

Distinguishing tests: a grounded or shorted element trips the breaker instantly or within seconds, and shows low insulation resistance (megohm test) or a large current imbalance between legs; here insulation resistance is over 50 megohms and current is balanced at the expected value. A failing breaker typically trips below its rating, shows heat damage or a loose connection, and may trip with no load; the fix is to test the circuit under load with a clamp meter at the breaker and check terminal torque and temperature, then compare the trip behavior against the connected load. Because the measured load (61 A) is above the installed protection (60 A) and above the conductor ampacity (55 A), the undersized circuit explains the trips without needing a defective component. An electrician should confirm with a load calculation and a panel inspection.

Report language: 'Air handler is equipped with a 15 kW electric heat kit with a nameplate minimum circuit ampacity of 78 A and maximum overcurrent protection of 80 A. The installed branch circuit is 6 AWG copper NM-B protected by a 60 A breaker. Conductor ampacity (55 A) is below the connected load and the breaker rating is below the equipment MCA. Auxiliary heat operation is reported to trip the breaker after 10-15 minutes. This is a fire and no-heat hazard; evaluation and correction by a licensed electrician, coordinated with an HVAC contractor to confirm the correct heat kit and circuit, is recommended before further use.' Do not reset the breaker repeatedly or recommend simply installing a larger breaker, since the wire would remain undersized.