Capacitor Variance Checker — pass, replace soon, or fail in seconds.
Enter a capacitor's rated and measured microfarads and get a verdict against its tolerance. Dual-run sections are checked independently, and a running-test mode works from amps and volts. No account, nothing uploaded, nothing stored.
Beta. This tool is live for real-world testing. Check a real capacitor and tell us what to fix — nothing is uploaded or stored.
⚠️ Safety first — read before you test
Lockout / tagout: turn off power at the breaker and verify no voltage is present before you touch anything.
Discharge the capacitor through a 20 kΩ 2 W resistor for about 5 seconds before disconnecting wires or measuring.
Do NOT short the terminals with a screwdriver. That can cause arc flash, weld the contacts, and damage the capacitor's dielectric.
Capacitors hold lethal charge after power is disconnected. Treat every capacitor as charged until you have discharged it yourself.
How it works: pick run or start, enter the rating from the capacitor's label and your meter reading. The tool shows the acceptable range, the variance, and a verdict. A reading below rating is the common failure — capacitance drops as run capacitors wear. A reading above rating is unusual and usually means the wrong capacitor or a meter problem: double-check the label and the meter before trusting it.
1 · The capacitor
Run capacitors are typically ±5% or ±6% — use the tolerance printed on the label when it is shown.
2 · How did you test it?
Running capacitance (60 Hz single-phase): µF = amps × 2652 ÷ volts. The result is then checked against the same tolerance bands as a meter reading.
3 · Verdict
Enter values above and the verdict appears here.
Replacing the capacitor
Microfarads must match exactly. A 35 µF does not substitute for a 40 µF — their tolerance bands do not even overlap (35 µF ±6% tops out at 37.1; 40 µF ±6% starts at 37.6).
Voltage rating must be equal or higher. A 440 V capacitor safely replaces a 370 V one — never the reverse.
For a dual-run capacitor, replace the whole capacitor if any section fails, and plan replacement if any section reads Replace Soon.
Guidance, not a diagnosis. A capacitor can pass its µF check and still fail under load, and a motor can fail with a good capacitor. If the motor still struggles after a verified-good capacitor — slow start, humming, tripping on overload — look past the capacitor (bearings, windings, voltage supply) before condemning anything else. Sources: see SOURCES.md in this tool's folder.
