Run Capacitor Check Calculator
Work out a run capacitor’s actual microfarads from the current through it and the voltage across it, and see whether it is within its rated tolerance.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
Outside tolerance
- Measured capacitance
- 8.6µF
- Difference from rating
- 7.54%
8.6 µF measured vs 8 µF rated (7.5 %) — outside ±6 %.
- Capacitors store charge. Measurements under power must be made by a qualified technician following the equipment manufacturer’s safety procedures; discharge a capacitor before handling it.
- Constant 10⁶ ÷ (2π f)
- 2,653
Show the working
- Capacitive reactance X = V ÷ I, and X = 1 ÷ (2π f C)so C = I ÷ (2π f V)
- C (µF) = I × 10⁶ ÷ (2π f V)1.2 × 10⁶ ÷ (2π × 60 × 370) = 8.603 µF
The formula
A capacitor’s opposition to AC current, its reactance, is 1 ÷ (2π × frequency × capacitance). Measuring the current through the capacitor and the voltage across it while the motor runs gives the reactance, and therefore the capacitance:
µF = amps × 1,000,000 ÷ (2π × frequency × volts), which is 2,652.6 × amps ÷ volts at 60 Hz.
Compare the result with the rating on the capacitor label and its tolerance, often ±5 % or ±6 %.
Safety
Capacitors store a charge even after power is removed, and this test is done with the equipment energised. It should be done only by a qualified technician following the manufacturer’s procedures. Discharge a capacitor safely before touching or removing it.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Current through the capacitor
- 1.2 A
- Voltage across the capacitor
- 370 V
- Supply frequency
- 60 Hz
- Rated capacitance
- 8 µF
- Rated tolerance (±)
- 6 %
Result
- Measured capacitance
- 8.6 µF
- Difference from rating
- 7.54 %
- Constant 10⁶ ÷ (2π f)
- 2,653
8.6 µF measured vs 8 µF rated (7.5 %) — outside ±6 %.
Assumptions
- Sinusoidal supply; the capacitor’s current and voltage are measured at the same time while running.
Limitations
- A meter’s capacitance function, used with power off and the capacitor discharged, is the direct test; this method checks it in operation.
Frequently asked questions
What does a weak capacitor do?
A run capacitor below its rating reduces the motor’s starting and running torque and efficiency and can make it run hot. Compressors and fan motors may hum, start slowly or trip on overload.
Can I test with power off?
Yes — many meters have a capacitance function. Discharge the capacitor and disconnect it first. That is the more common and safer test.
What if I am on 50 Hz power?
Enter 50 Hz; the constant changes to 3,183 × amps ÷ volts.
Sources
- University Physics Volume 2, §15.2 Simple AC Circuits — OpenStax, Rice University (CC BY 4.0). Capacitive reactance X_C = 1 ÷ (2π f C), so C = I ÷ (2π f V).
Spotted an error? Report a problem with this calculator — see our corrections policy.