Run & Start Capacitor Reference
Run versus start capacitors for HVAC motors — what each does, their microfarad and voltage ranges, tolerances, and how they are switched. Run capacitors stay in the circuit to keep the motor efficient; start capacitors give a brief torque kick at startup and are dropped out by a relay. Values are manufacturer-typical — match the µF and voltage to the equipment.
Run vs start capacitors
| Attribute | Run capacitor | Start capacitor |
|---|---|---|
| Function | Energized continuously — improves running efficiency and torque | In circuit only briefly at start for high starting torque, then switched out |
| Capacitance (typical) | ~2–80 µF | ~25 to 400+ µF (electrolytic, banded) |
| Voltage rating (typical) | 370 or 440 VAC | 110–330 VAC (125, 165, 250, 330 common) |
| Tolerance | ±6% (some ±5%) | Looser — roughly a ±20% band |
| Construction | Metallized film, oil-filled, continuous duty | Electrolytic, short-duty only |
| Switched out by | Never — stays in circuit | A potential (voltage) relay, ~75% of run speed |
| Form | Often a dual round cap: HERM + FAN + C terminals | Single value, separate can, wired through the relay |
Two capacitors, two jobs
A single-phase motor needs help both to start and to run efficiently, and the two capacitors split that work. The start capacitor delivers a large phase shift for a big burst of starting torque — but it is an electrolytic type that would overheat if left energized, so a potential (voltage) relay senses the motor coming up to speed and switches it out at around 75% of run speed. The run capacitor is a rugged film type that stays in the circuit continuously, trimming current and boosting running torque and efficiency. Many condensing units use a single dual round capacitor with HERM, FAN, and C terminals to run both the compressor and the condenser fan.
When you replace one, the µF value is non-negotiable — it must match the spec — while the voltage rating only has to be equal or higher. A capacitor that has drifted out of tolerance, shorted, opened, or bulged is the single most common failure on a no-cool call, which is why a µF meter is a first-line diagnostic tool.
Common questions
What is the difference between a run and a start capacitor?
A run capacitor stays in the circuit the whole time the motor runs, improving efficiency and torque — it is a continuous-duty film capacitor, typically 2–80 µF at 370 or 440 volts. A start capacitor is only in the circuit for the first moment of startup, giving a big torque boost, then a potential relay switches it out — it is a short-duty electrolytic type with much higher capacitance, from tens up to 400+ µF.
Can I replace a 370V capacitor with a 440V one?
Yes. The printed voltage is a maximum-not-to-exceed rating, so a 440V capacitor can replace a 370V one and gain safety margin — but you cannot go the other way and put a 370V cap where 440V is specified. The microfarad (µF) value, however, must always match; many capacitors are stamped "370/440V" to settle the question.
How do I test a capacitor?
Discharge it, take it out of the circuit, and measure capacitance with a meter set to µF. Compare the reading to the printed value and its tolerance — run capacitors are typically ±6% — and replace it if the reading falls outside. A capacitor can also fail by shorting, going open, or bulging, so a swollen top is an automatic replacement even if it still reads.