Compressor Tripping on Overload: Reading the Evidence
A 3-ton R-410A split system, nameplate RLA 16.0 A, LRA 96 A, 208/230 V single phase. The compressor trips its internal overload after 6–8 minutes; the condenser fan keeps running. Readings: 241 V at the panel, 229 V at the contactor under load; compressor amps 14.0 at start, climbing to 19.2 just before the trip; suction 118 psig (40°F saturation); liquid line 385 psig (113°F saturation); outdoor ambient 88°F. The condenser coil is clean on both faces, but the unit sits in a 3-foot-wide alcove with a solid wood fence 12 inches from the fan discharge. Run capacitor marked 45/5 µF ±6%, measured 41.0 and 4.8 µF. Winding resistance: C–R 1.2 Ω, C–S 1.9 Ω, R–S 3.1 Ω; insulation to ground 50 MΩ.
1. Evaluate the capacitor and the winding readings. What have you ruled out, and what is still in play?
Read the worked answer
Capacitor: rated 45 µF, measured 41.0 µF. That is (45 − 41)/45 = 8.9% low, outside the ±6% marking. The 5 µF fan section measures 4.8 µF (4% low), which is acceptable. A weak run capacitor reduces starting/run torque, drives current up, and is a legitimate contributor here - it is also the cheapest thing on the truck, so it gets replaced regardless.
Windings: C–R 1.2 Ω + C–S 1.9 Ω = 3.1 Ω, which equals the measured R–S of 3.1 Ω. The main and start windings add up to the run-to-start reading, so there is no short between windings and no open. Insulation to ground at 50 MΩ is healthy (well above the usual 1 MΩ minimum).
Ruled out: shorted windings, open windings, short to ground. Still in play: high head pressure/mechanical load, low voltage, the weak capacitor, and - if all of those clear - a mechanically failing compressor.
2. What explains the 385 psig head and the rising amp draw, and what would you do first? What should the readings do afterward?
Read the worked answer
At 88°F ambient, 385 psig is about 113°F saturation, a 25°F condensing split over ambient. That is high; the coil is clean, so the cause is not dirt. The unit is boxed in a 3-foot alcove with a fence 12 inches off the fan discharge - that is a classic hot air recirculation arrangement. Discharge air bounces off the fence and back through the coil, raising condensing temperature, head pressure, and compressor amps, which is exactly the pattern of a unit that runs fine for several minutes and then trips as the alcove heat-soaks.
Order of work:
- Replace the out-of-tolerance capacitor and confirm the contactor and connections are sound.
- Correct the discharge - reposition or remove the fence panel, add a deflector, or relocate the unit so the fan discharge has manufacturer-specified clearance. Verify against the installation manual, not guesswork.
- Re-check: head pressure should fall toward a 10–20°F split over ambient and amps should settle below RLA. Suction at 40°F saturation is in range for a 75°F indoor coil if superheat is correct; verify superheat too.
If amps still climb above RLA with normal head pressure and correct voltage, the compressor is the remaining suspect.
3. The customer asks whether the voltage is "bad enough to matter." Compute the drop and explain how you would report it.
Read the worked answer
Drop = 241 V − 229 V = 12 V. As a percentage: 12 ÷ 241 = ≈5.0% from the panel to the unit under load.
The recognized guidance (NEC informational notes, and manufacturer literature) is ≤3% on the branch circuit and ≤5% total including the feeder. Here the panel-to-unit measurement is the branch circuit, and it is already at 5% - beyond the 3% guideline for that segment alone, with the feeder drop not yet accounted for. The 229 V is also only 220 V-equivalent against a 230 V nominal, and starting voltage sag on a compressor with LRA 96 A will be far worse.
Report it as: measured 12 V/5.0% drop on the branch circuit under load; this is outside the 3% branch guideline and contributes to high running amps and hard starting. Recommend an electrician verify conductor size, terminations, and breaker connections before or alongside the mechanical repair. Do not simply "turn up" anything - record the readings in the work order so the recommendation is documented.