The Chattering Contactor and the 26-Volt Drop
Callback on a 2.5-ton split in a Slidell condo. Complaint: it cools for five to seven minutes, then nothing, and the breaker in the outdoor panel trips. Nameplate: RLA 16.4, LRA 98, 30-A breaker, 10 AWG, 65-ft run. With the unit running: 240 V at the panel, 214 V at the contactor line side, 24 V at the transformer secondary open-circuit but 15.8 V at the contactor coil while energized, and the contactor chatters when the thermostat calls. Compressor windings: C-R 1.4 Ω, C-S 2.1 Ω, R-S 3.5 Ω. Run capacitor rated 45/5 µF, measured 36.0/2.9 µF. Compressor amps climb from 12.4 A to 19.1 A over about four minutes while head pressure climbs 250 → 330 psig; the condenser coil is clean and the condenser fan is running. The homeowner mentions another company replaced the breaker last summer.
1. Separate causes from symptoms in this data set, and rank what you would investigate first.
Read the worked answer
Rank by what explains the most measurements:
1. Supply voltage drop (cause, primary). 240 V open vs 214 V under load is a 10.8% drop - far beyond normal contactor and conductor loss. Look at the breaker lugs, bus stab, neutral/ground bar, and the 10 AWG run for a loose termination, corrosion, or an undersized/overlength feeder. A high-resistance connection heats under load, the voltage sags, current rises, and eventually the breaker trips on thermal. The replacement breaker last summer is a clue that somebody already treated a symptom.
2. Run capacitor (cause, contributing). 36.0 vs 45 µF is 20% low; 2.9 vs 5 µF is 42% low. Low capacitance cuts starting torque and raises running current - this is measurable and correctable.
3. Compressor (symptom). C-R + C-S = 3.5 Ω, matching R-S. Windings are balanced; no short or open. Do not condemn the compressor.
4. Control voltage sag (symptom). 15.8 V at the coil is a consequence of the sagging supply and a marginal transformer; the chatter is the contactor dropping out, not a bad thermostat.
5. Amps climbing with head pressure (symptom, watch it). Rising head at a clean coil with the fan running suggests overcharge or non-condensables; pump-down and weigh-in will tell you.
2. Explain the breaker trip. Why didn't replacing the breaker fix it, and what do you check before condemning another one?
Read the worked answer
A 30-A breaker on a 16.4 RLA compressor should never see 30 A in normal running. It tripped anyway, so either the current actually exceeded the trip curve or the breaker is being heated by something upstream. Both are consistent with the 26 V drop.
Checklist before you replace anything:
- Clamp the actual current at the breaker, not at the contactor, with a recording meter. The 19.1 A you read at the unit may not be the peak at the panel during a start or a stall.
- Thermal-scan or feel the breaker and bus stab after a run cycle. A warm stab or discolored bus is the finding; a warm breaker body alone can be a tired breaker.
- Torque the terminations to the panel's labeled values, and verify the 10 AWG over 65 ft is adequate for the minimum circuit ampacity on the nameplate.
- Verify the breaker's rating and type match the panel's listing - a wrong-class breaker in an older panel is a real and common problem.
Replacing the breaker without correcting the voltage drop just resets the clock. The honest write-up is: supply-side connection/conductor deficiency, plus a failed run capacitor, both of which drive the overcurrent condition.
3. Write the repair order you would leave. What gets replaced, what gets corrected, and what gets verified?
Read the worked answer
Repair order:
Correct:
- Locate and repair the high-resistance connection causing the 26 V drop - re-terminate, replace damaged conductors, torque to spec. Re-measure under load and confirm within 3% of the panel voltage.
- Verify panel bus and breaker; replace only with the listed type and correct rating.
Replace:
- Run capacitor with a 45/5 µF rated part, and verify measured µF within the label tolerance before it leaves the truck.
- Any pitted contactor contacts; a chattering contactor has already arced.
Then, and only then, evaluate the refrigerant side: the amps climbing with head pressure at a clean coil suggests an overcharge from a previous 'top-off.' Recover, evacuate, and weigh in to nameplate plus line-set allowance; verify subcooling and superheat.
Verify and document: running amps at the panel, voltage at the contactor under load, capacitor µF, delta T, subcooling, superheat, and a complete run cycle past the previous failure point. Leave the recording-meter printout or the numbers in the ticket. When the homeowner calls back in August, that paperwork is the difference between a warranty call and an argument.