What is a run capacitor?
A run capacitor stores and releases energy to keep an HVAC motor — a compressor or a fan — running efficiently. It provides a constant phase-shifted voltage to a secondary motor winding, which is what gives a single-phase motor something to push against. Unlike a start capacitor, which assists only for the first second and then drops out, a run capacitor is in the circuit every moment the motor turns.
Common questions
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How is it different from a start capacitor?
A run cap operates continuously; a start cap engages briefly at startup and then drops out of the circuit. They are not interchangeable in either direction: a start capacitor is built for a few seconds of very high capacitance and will cook if left in circuit, and a run capacitor cannot deliver the starting torque a hard-starting compressor needs. Many condensers use a dual run capacitor to serve both the compressor and the fan from one can.
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What µF do I need?
Exactly what is stamped on the can you are replacing — a capacitor is matched to its motor's windings, and "close" is a motor running hot for the rest of its life. The voltage rating is the one number you may go UP on: 440V safely replaces 370V because the rating is a ceiling, not a target. Never the reverse.
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What are the signs it's failing?
The classic one: the compressor hums but the condenser fan does not spin, and a nudge on the blade with a stick starts it turning. Others are a hard start followed by normal running, a unit tripping its breaker on startup, and a bulged or leaking can top. A flat top proves nothing — capacitors fail electrically long before they fail visibly, so the only real test is a microfarad reading.
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How long should one last?
Manufacturers rate them around 60,000 hours, which is roughly ten cooling seasons — but that figure assumes the rated ambient temperature, and the inside of a condenser cabinet in an Ohio August is well past it. Five to eight years is the realistic field life, and they fail earlier in direct sun or with restricted airflow around the unit.