AC Amp Draw High But Cooling Fine Decision Tree

Why this matters

A system that cools well but draws current above RLA is failing slowly, not failing now. The customer hasn't called yet because the house is cold. You found this on a tune-up. The cause is one of four things, all of them progressive: weak capacitor, dirty condenser coil, restricted indoor airflow, or a compressor that's putting in extra work for the same job. Catching it at tune-up before it becomes a callback or a compressor replacement is the entire business of preventive maintenance.

Symptom presentation

• Compressor running.
• Supply air cold at the register; cooling complaint absent.
• Clamp meter on the compressor leg reads above RLA on the data plate.
• May or may not coincide with high head pressure, low subcooling, or unusual sounds.

Quick checks (under 2 min)

Step 1: Confirm the amp reading. Use a clamp-on AC ammeter on the common leg of the compressor (the line out of the contactor going into C terminal). Compare against RLA on the data plate. Note the ambient temperature - RLA is rated at design conditions, and a hot day can pull amps slightly above RLA on a healthy system if pressures are at the high end of normal.

Step 2: Take basic refrigeration readings:

• Suction pressure / temperature, calculate superheat or subcool depending on TXV/EEV vs piston.
• Discharge pressure / temperature.
• Liquid line temperature, ambient air temperature, return air dry-bulb and wet-bulb at the indoor coil.

Step 3: Read the run capacitor on the HERM terminal. Compare microfarads to OEM spec.

Step 4: Visual on the condenser coil. Look at the inside surface (the side facing the fan, not the side facing you) - this is where the dirt accumulates.

Isolation tree

Branch 1: Weak run capacitor. Reads below 90% of rated microfarads but still functional. The compressor pulls higher run current because the phase shift is wrong. Replace capacitor, retest amp draw. This is the single most common cause and the fastest fix.

Branch 2: Dirty condenser coil. Higher head pressure forces the compressor to do more work per BTU rejected. Amps follow head. Chemical-clean the coil with appropriate cleaner (alkaline for aluminum, neutral pH for microchannel), rinse, retest pressures and amps.

Branch 3: Restricted indoor airflow. Loaded filter, dirty coil, undersized return, closed registers - all reduce evaporator load and raise indoor coil temperature. The compressor runs longer per BTU removed; in some failure modes amps climb because of high return temperature and the resulting high suction. Address indoor airflow first; if amps drop, you've found it.

Branch 4: Overcharge. Subcooling above OEM spec, head high, suction normal or slightly high. Common after a previous "topping off." Recover refrigerant in 4-oz increments, retest, document. Amps drop with corrected charge.

Branch 5: Non-condensable contamination. Standing pressure test (system off 30 minutes, ambient measured, P/T chart comparison) shows pressure 10 psig or more above saturated for ambient. Recover, evacuate to 500 microns, hold 10 minutes, recharge by weight. Skipping this on a brazed-in change-out is how non-condensables get into the system in the first place.

Branch 6: Failing compressor mechanical efficiency. Subcool and superheat in spec, charge correct, head and suction nominal, but amps elevated. The compressor is doing more work per pound of refrigerant moved (valves leaking by, scroll set wearing). Confirm with a benchmark:

• Compare to compressor data plate RLA at rated conditions.
• Calculate volumetric efficiency from pressures and amps if you have the time and the compressor data.
• Look at compressor cooling: the suction-cooled hermetic should be cool to slightly cool to the touch on the body. Hot body + high amps = inefficient compressor. Replacement is the only fix. Document and discuss with the customer.

Branch 7: Voltage low at the compressor. Low utility voltage under load (below 200 VAC) makes the compressor draw more amps to maintain torque. Read voltage at the compressor terminals during run:

• Below 200 VAC = upstream problem (utility, panel, conductor sizing, loose connection at disconnect or contactor).
• Borderline (200 to 220 VAC) under load = check feeder conductors and termination torque.

Branch 8: Wrong-rated capacitor or wrong fan capacitor installed. On a previous service call somebody put in a microfarad value off-spec. The compressor runs but pulls high amps. Read the data plate carefully and match.

Branch 9: Wrong contactor sizing or burned contacts dropping voltage at the load side. Burned contactor terminals reduce delivered voltage at the compressor; high amps result. Inspect, replace contactor if pads are pitted or burned.

Confirming the diagnosis

After repair, log on the invoice:

• Compressor amps at steady state.
• Suction pressure / temperature.
• Discharge pressure / temperature.
• Subcool.
• Superheat (TXV) or evap delta T (piston).
• Voltage at compressor terminals during run.

Amps within 10% of RLA at the ambient measured. If amps still high but system is at peak summer ambient on a properly serviced system, document the conditions and recheck in cooler weather before committing to a compressor replacement.

Repair / remediation

• Capacitor replacement: OEM microfarad rating, equal-or-higher voltage rating.
• Coil cleaning: chemical wash, fin straightening, ground clear.
• Charge correction: weighed recovery, weighed recharge per data plate.
• Drier replacement: any time the system is opened.
• Contactor: match coil voltage and FLA rating.
• Compressor replacement: weighed-in charge, new drier (suction-side drier on burnout systems), 500-micron vacuum, 10-minute hold.

Why catch this at maintenance

A system running 10 to 15% above RLA today is heading toward a compressor failure inside 2 to 3 cooling seasons. Caught at tune-up and fixed with a $40 capacitor and a coil clean, the customer keeps the system 5 to 7 more years on the original compressor. Missed and reported to the customer as "running great" - the compressor dies in August and you've lost the maintenance relationship.

References

• AHRI Standard 210/240 - Performance Rating of Unitary Air-Conditioning Equipment
• ACCA Standard 5 - HVAC Quality Installation Specification
• ASHRAE Handbook - HVAC Systems and Equipment
• OEM data plates and service manuals - RLA, FLA, LRA per model