High Amps and a Tripping Breaker on a 208/230 V Split
Service call on a 3-ton R-410A split, 208/230 V single phase. Nameplate: RLA 16.7 A, LRA 96 A, MCA 21.5 A, MOP 35 A. Field wiring is #10 stranded copper in PVC, 118 ft one-way from the panel, protected by a 40 A breaker with a non-fused disconnect at the condenser. With the compressor running: 232 V at the panel, 214 V at the contactor, 24 A draw after 8 minutes; suction 105 psig, liquid line 350 psig; outdoor ambient 108°F; return air 78/64, supply 63/58; superheat 26°F; subcooling essentially 0°F; piston metering. The condenser coil is visibly loaded with dust and cottonwood fuzz. Windings are balanced and insulation resistance is over 100 MΩ. The breaker trips after 10–12 minutes.
1. Rank the faults you see and justify each with the readings. Which one is the root cause and which are contributors?
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Primary: the system is undercharged. Superheat of 26°F with a piston at 78/64 return is far above target, and subcooling near 0°F confirms it. There is also a leak to find - do not just add gas.
Contributor: a fouled condenser. Liquid pressure of 350 psig corresponds to roughly 107°F saturation against a 108°F ambient - the coil is rejecting almost nothing. That drives head pressure, compression ratio, and amp draw upward.
Contributor: excessive voltage drop. Expected drop on 118 ft of #10 at 24 A is about 2 × 118 ft × 24 A × 1.21 Ω/1000 ft ≈ 7 V. You measured 18 V. That extra ~11 V is being dropped across a termination, breaker stab, or splice - a real defect to fix even though 214 V is still inside the unit's 197–253 V utilization range.
Violation: the 40 A breaker exceeds the nameplate MOP of 35 A. Overcurrent protection must not exceed the manufacturer's marked maximum.
The 24 A draw is a symptom (high head plus high superheat from low charge), not a failing compressor - balanced windings and good insulation resistance support that.
2. Before you add any refrigerant, what electrical checks do you make and what does the code require for the conductor and the breaker?
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Electrical checks, in order:
- Tightness of line and load terminations at the breaker, disconnect, contactor, and any splice; look for heat discoloration. Measure voltage drop across each connection under load - anything over about 1–2 V at a termination is a defect.
- Verify conductor ampacity and sizing. Minimum circuit ampacity is 1.25 × 16.7 = 20.9 A; #10 copper at 75°C (35 A) satisfies the branch-circuit conductor requirement, so the wire gauge is fine.
- Verify the overcurrent device against the nameplate. MOP is 35 A; a 40 A breaker is not permitted. Replace with a 35 A (or smaller) device sized within the nameplate limits.
- Confirm the disconnect is correctly rated and that the equipment grounding conductor is continuous and properly bonded.
- Confirm the compressor is not single-phasing or short-cycling on a control - check the run capacitor microfarads against rating while you are in the box.
Only after terminations are repaired and voltage at the contactor under load is confirmed should you move to the refrigeration side. Never use a larger breaker to 'solve' a tripping problem.
3. Lay out your repair sequence and the documentation you leave behind. How do you know when you are done?
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Sequence:
- Kill and lock out power. Repair the high-resistance connection and retorque all terminations; replace the 40 A breaker with a 35 A device.
- Clean the condenser coil (chemical or coil cleaner plus rinse, protect the controls) and confirm the fan is moving design air.
- Leak search - electronic detector and/or nitrogen pressurization with trace gas, focusing on the evaporator, braze joints, and Schrader ports. Repair, evacuate to 500 microns and hold.
- Weigh in the charge to the nameplate amount, adjusted for line-set length per the manufacturer, then confirm superheat against the charging chart at the actual indoor wet bulb and outdoor dry bulb.
Acceptance criteria: amp draw back within nameplate RLA (roughly 16–17 A at these conditions, well under 24 A), superheat on chart, subcooling per spec, contactor voltage drop repaired, no breaker trip through a 30-minute loaded run plus a restart cycle.
Document the before/after readings, the breaker change, the leak location and repair method, the weight of refrigerant added, and the voltage-drop repair on the invoice - and get the customer's signature.