Capacitor (Start/Run)

Definition

A residential AC has two motors that need help starting and running: the compressor motor and the condenser fan motor. Capacitors provide that help. They store electrical charge and discharge it in precisely-timed pulses, which lets a single-phase AC motor produce enough starting torque and maintain a clean phase relationship during normal run.

Most Austin-area condensers use a dual run capacitor — a single component with two capacitance values that serves both motors. Ratings are in microfarads (μF) and voltage (typically 370V or 440V). Common dual-run sizes: 35/5 μF (3-ton AC) or 45/5 μF (4-ton AC).

Failure modes: capacitors bulge, leak dielectric oil, or simply drop below their rated capacitance. A capacitor at 60% of rated value can still start the motor sometimes but will fail under heat and load — which is exactly when the AC needs to start.

Diagnosis is a 5-minute test with a multimeter set to capacitance. Replacement is a $250-450 service call (parts + labor). It's the cheapest and most common AC repair we do.

Why it matters in Austin

Austin afternoons in July and August routinely push the outdoor condenser cabinet past 130°F — direct sun, dark color, hot ground. Capacitors are rated for high temperatures but they degrade with cumulative thermal stress. By year 5-7, the rated capacitance has often dropped 10-15%. By year 10, you're in fail territory.

The classic symptom: AC stops cooling on the hottest afternoon of the year, condenser fan won't spin, you hear a humming compressor. That's a failed start/run capacitor 8 times out of 10. We carry common sizes on the truck so most calls are same-day fixes.