Defrost Breaker Trip on a Single-Phase Freezer
A 208 V single-phase walk-in freezer has a 2 HP condensing unit with RLA 12.5 A, two 2,500 W defrost heaters, evaporator fans, and a 30 A two-pole breaker. The freezer runs normally in cooling, but the breaker trips about 8 minutes into defrost. A technician measures 208 V at the panel and 24 V at the control circuit. During defrost, the heater current is about 20 A and the compressor current is 0 A. Near the end of the timed defrost period, the compressor starts while one heater bank is still energized. The defrost contactor contacts are pitted and the timer is a mechanical type. The evaporator fan is off during defrost. The customer reports the problem began after a power surge.
1. Using the readings, explain why the breaker trips only at the end of defrost. Include the total current if the compressor starts with one heater bank energized.
Read the worked answer
Two 2,500 W heaters at 208 V draw about 18 A total because each heater's resistance is fixed and it produces less wattage at 208 V than at 240 V. The measured 20 A is close enough that the heaters themselves are not the main surprise. The trip occurs when the compressor starts while one heater bank is still energized: roughly 20 A of heat plus 12.5 A compressor RLA is about 32.5 A, already above the 30 A breaker, and compressor starting inrush adds more. Pitted defrost contactor contacts can weld or stick and keep a heater on after the timer ends. The breaker is doing its job; it is not simply undersized.
2. What safety and control checks would you perform before replacing the breaker with a larger size?
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Before changing the breaker size, verify the defrost sequence: the compressor and heaters must not run together. Inspect the defrost timer, contactor, and wiring for a stuck contact or miswire; ohm the heaters and check for grounds; measure current on each heater bank and the compressor. Replace the pitted contactor and any damaged timer. Do not upsize the breaker to mask a control failure, because the conductors and heaters may not be protected. Confirm the heater circuit is de-energized at defrost termination.
3. Describe the correct defrost sequence and the final verification after repair.
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Correct sequence: pump down or stop the compressor, keep evaporator fans off, energize the heaters, terminate defrost on coil temperature or time, then de-energize heaters before restarting the compressor and fans. Verify with a full forced defrost: measure heater current, confirm compressor is off during heat, confirm all heater contacts open at termination, then observe compressor start and fan delay. Check breaker and conductor temperature, and record readings. The repair is complete only when repeated defrost cycles no longer approach the breaker rating.