Run Capacitor Calculator

What the run capacitor calculator does

Single-phase motors in air conditioners, heat pumps, fans, and pumps use a run capacitor to shift the current in the second winding so the motor starts and runs smoothly. Capacitors lose capacitance as they age and heat up, which makes the motor run hot, draw more current, and fail to start. A capacitor under load passes a current proportional to its capacitance, the voltage, and the frequency, so measuring current and voltage reveals its real value without removing it. The run capacitor calculator turns those readings into microfarads and compares them with the label.

Capacitor test formula

The reactance of a capacitor is X_C = 1 ÷ (2π f C), and Ohm's law gives I = V ÷ X_C. Solving for C:

The calculator then compares the result with the rated value and its tolerance band. For the sizing option it uses the common rule for the Steinmetz connection, about 70 µF of run capacitance per kW of motor power at 230 V and 50 Hz, scaled for other voltages and frequencies, and doubles it for an optional start capacitor that a switch or relay removes once the motor is up to speed.

Run capacitor calculator example

An air conditioner compressor with a 40 µF ±6% run capacitor. With the unit running, the start winding draws 5.2 A and the capacitor sees 350 V:

That is 1.5% below the rating and inside the 37.6 to 42.4 µF band, so the capacitor is fine. A reading of 34 µF would be 15% low and the capacitor should be replaced. To run a 1.1 kW three-phase motor on a 230 V single-phase supply, the sizing option gives about 77 µF of run capacitance.

How to use the capacitor uf calculator

With the unit running, clamp an ammeter around the wire from the capacitor terminal to the start winding (the HERM or FAN terminal) and measure the voltage across the capacitor terminals, from HERM or FAN to C. Enter both readings, the supply frequency, the rated capacitance from the label, and its tolerance, usually ±5 or ±6%. To run a three-phase motor on single phase, choose the sizing option and enter the motor power and supply voltage. Press Calculate.

Reading the run capacitor test result

A capacitor within its tolerance band is healthy. A reading below the band means it has lost capacitance and should be replaced with the same µF and an equal or higher voltage rating; a reading far above the band usually points to a wrong measurement. Do not replace a run capacitor with a larger µF value to make a motor start: too much capacitance overheats the start winding. A swollen top or leaking oil means the capacitor has failed whatever the reading.

When to use an ac capacitor calculator

• Testing an AC or heat pump capacitor during service
• Checking a fan or blower motor that runs hot or slow
• Diagnosing a compressor that hums but does not start
• Sizing a capacitor for a three-phase motor on single phase
• Confirming a replacement capacitor before fitting it

Common run capacitor calculator mistakes

• Not discharging the capacitor. Discharge it before touching the terminals.
• Measuring the wrong wire. Clamp the capacitor lead, not the common line.
• Using the supply voltage. Measure across the capacitor; it is higher.
• Picking the wrong frequency. 50 Hz and 60 Hz give different constants.

Tips for a run capacitor test

• Switch off and discharge before you connect test leads.
• Replace with the same µF and equal or higher voltage.
• Keep a spare dual capacitor for common AC units.

Related motor tools

The capacitor works with the motor windings. The motor FLA calculator gives the expected running current, the motor starting current calculator estimates the inrush, and the capacitive reactance calculator shows the reactance of any capacitor.

Dual run capacitor and how to test both sides

Most outdoor AC units use one dual run capacitor with three terminals: C (common), HERM for the compressor, and FAN for the condenser fan, for example 40 + 5 µF. Test each side on its own: the compressor current with the voltage from HERM to C, then the fan current with the voltage from FAN to C. The fan side is small, so a clamp meter with 0.01 A resolution helps. Compare the running current of the compressor with its nameplate and the motor FLA calculator; high current together with a weak capacitor is a classic sign that the capacitor is the cause.

Motor run capacitor size for single-phase use

A three-phase motor can run on single phase with a capacitor across one winding, the Steinmetz connection. The motor must be connected for the supply voltage, usually delta for 230 V on a 230/400 V motor, and it delivers only about 70% of its rated power. Large or heavily loaded motors need a start capacitor as well, because the starting torque without it is low and the motor draws a high starting current for longer, which the motor starting current calculator estimates. The capacitor reactance at the supply frequency follows from the capacitive reactance calculator.

Capacitor test chart: µF from amps and volts

Capacitor for 3 phase motor on single phase (230 V, 50 Hz)

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