What is a start capacitor?
A start capacitor gives a single-phase motor the extra torque it needs to break away from a standstill, then leaves the circuit. It is a much larger capacitance than a run capacitor — typically 70–200 µF against a run cap's 5–50 — held in circuit for about a second by a potential relay, which drops it out once the motor is turning. It is built for that second and no longer.
Common questions
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Can I use a start capacitor instead of a run capacitor?
No, in either direction, and the failure is not subtle. A start capacitor left in circuit continuously will overheat and vent within hours — they are not rated for continuous duty at any capacitance. A run capacitor in a start capacitor's place cannot supply the inrush torque, so the compressor stalls on its locked-rotor current and trips the breaker.
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How do I know if my unit even has one?
Most residential condensers do NOT. A standard single-stage unit starts on its run capacitor alone. A start capacitor appears where the compressor needs help — a hard-starting scroll, a long line set, or a unit fed by a weak service — and it is always paired with a potential relay, so if you cannot see a relay you almost certainly do not have one.
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Why did somebody add one to my unit?
Usually as a field fix for a compressor that had begun to draw hard on startup — a hard-start kit is a start capacitor and a relay sold together. It is a legitimate repair, but it is also worth knowing what it means: a compressor that needed help to start is a compressor that is telling you something about its remaining life.