How to Test a Run / Start Capacitor
General field practice for qualified electricians. The equipment manufacturer's instructions, your adopted code edition, and the AHJ govern where they differ.
⚠ Capacitors hold a charge after power is off. Turn off and lock out the disconnect, verify zero voltage, then discharge the capacitor across its terminals (each section of a dual cap) with an insulated discharge tool or a resistor (for example about 20 kΩ, 5 W) before you touch the terminals. Don't short the terminals with a screwdriver.
1 · Identify the capacitor
- Rating: capacitance in µF (MFD) with a tolerance (often ±5% or ±6%), and an AC voltage rating (commonly 370 or 440 VAC). Replace with the same µF and an equal or higher voltage rating.
- Dual run capacitors have three terminals: C (common), HERM (compressor), and FAN. Each section is rated separately, for example 40 + 5 µF.
- Start capacitors (electrolytic, black plastic case) are intermittent-duty and have a much wider tolerance range printed on the case (for example 88–108 µF).
2 · Visual check
- Bulged or domed top, oil leaking, cracked case, or burned terminals: replace it.
- A capacitor can look normal and still be weak. Always measure it.
3 · Meter test (power off)
- Lock out, verify, and discharge (see the warning above). Photograph or label the wires, then remove them.
- Set the meter to capacitance (µF). Measure C–HERM and C–FAN on a dual cap, or across the two terminals of a single cap.
- Compare to the label using the tolerance. Use the checker below.
- Also check each terminal to the metal can. It should read open. Any continuity to the case means the capacitor has failed.
4 · Under-load test (running)
Use this when you can't remove the capacitor, or to confirm it under operating conditions. Work live only with the PPE and procedures your arc-flash and shock risk assessment requires.
- With the unit running, clamp the wire from the capacitor's HERM (or FAN) terminal. That is the start-winding current through the capacitor, in amps.
- Measure voltage across the same capacitor terminals (C to HERM, or C to FAN). It is usually higher than line voltage because of the motor's back EMF.
- Calculate: µF = 2652 × A ÷ V at 60 Hz (or 3183 × A ÷ V at 50 Hz). The constant comes from C = I ÷ (2πfV) × 10⁶.
5 · Capacitor calculator
6 · What the result means
- Within tolerance: the capacitor is OK. Keep looking for the problem elsewhere (contactor, motor windings, control).
- Low: the capacitor is weak. Replace it. Motors on a weak cap run hot, draw high amps, or won't start.
- Zero or open: the capacitor has failed open. A motor on a shorted cap can trip the breaker or hum without starting.
- If a new capacitor fails quickly, check for high voltage, a failing motor, or a mismatched replacement.
Reference only — not engineering, legal, or code-compliance advice. Results are estimates for qualified persons and must be verified against the NEC edition adopted where the work is done (your selected edition), manufacturer instructions, and the local AHJ. Use is subject to the Terms of Use & Disclaimer. — Morningstar Electric LLC