A motor capacitor is not a component you can choose from a formula. It is a mains-voltage part whose capacitance, voltage rating and duty class are fixed by the motor manufacturer and printed on the motor nameplate or on the capacitor itself. Fitting the wrong value or the wrong voltage rating causes the capacitor to overheat, and an overheated capacitor vents or ruptures. That is why this page takes the nameplate figures as inputs and does not publish a table of values to choose from.
What it does instead is show you the electrical consequences of the figures you already have: the current the capacitor branch draws, its reactance at your supply frequency, the reactive power it handles, and the energy it holds at peak voltage. Arb Digital built the page this way deliberately, because those are questions arithmetic can answer honestly. Which capacitor belongs in a given motor is a question only the nameplate and the manufacturer's data can answer, and the work of fitting it belongs to a qualified electrician or appliance technician.
What This Motor Capacitor Calculator Does
Enter the capacitance and voltage rating marked on the part, along with your supply voltage and frequency, and the calculator returns the current flowing in the capacitor branch, the capacitive reactance, the reactive power and the energy stored at the peak of the AC cycle. If you have taken a reading with a capacitance meter, entering it alongside the tolerance printed on the part shows how far the measurement sits from the marked value in percentage terms.
Every one of those outputs is a consequence of the numbers you supplied. None of them is a judgement about whether the capacitor is the right one for the motor, and this page does not make that judgement in either direction. A measurement that falls outside the marked tolerance is a measurement result, and what it means for that motor is a matter for someone who can see the machine.
Three related tools cover neighbouring ground, and the boundaries matter. The capacitance calculator derives capacitance from plate geometry and dielectric constant — it is about the physics of a capacitor, not about selecting a motor part. The capacitor charge time calculator handles DC charging through a resistor and the time constant that governs it. The capacitor size calculator sizes DC smoothing and reservoir capacitance from a ripple specification, and it states explicitly that it does not size motor-run or motor-start capacitors. None of the three, and not this page either, selects a component for a live appliance.
How to Use It
- Read the capacitance and voltage rating off the part or the motor nameplate. Both are printed on a motor capacitor, together with the duty class and tolerance.
- Select the duty class. Run and start capacitors are different components; the selector records which one you are looking at.
- Enter the supply voltage and frequency. Reactance depends directly on frequency, so a 50 Hz figure and a 60 Hz figure are not interchangeable.
- Optionally add a meter reading from a discharged, disconnected capacitor, with the tolerance marked on the part, to see the percentage deviation.
- Read the branch current and reactive power. These describe the electrical loading of the capacitor circuit at the figures you entered.
The Formula: How It's Calculated
A capacitor's opposition to alternating current is its reactance, which falls as frequency or capacitance rises:
XC = 1 ÷ (2πfC), so the branch current is I = V ÷ XC = 2πfCV
An ideal capacitor consumes no real power, because its current leads the voltage by 90 degrees and the product averages to zero over a cycle. What it handles is reactive power, Q = V² ÷ XC in volt-amperes reactive. The energy held at the peak of the cycle is U = ½CVpeak², with Vpeak = √2 × Vrms for a sinusoidal supply. The phase relationship behind all of this, and the power-factor arithmetic that follows from it, is set out in OpenStax University Physics Volume 2, section 15.4 on power in an AC circuit.
A worked example using the defaults on this page: a 35 µF capacitor on a 240 V, 60 Hz supply. The reactance is 1 ÷ (2π × 60 × 35 × 10−6) = 75.79 Ω. The branch current is 240 ÷ 75.79 = 3.167 A, and the reactive power is 240 × 3.167 = 760.0 VAR. Peak voltage is 240√2 = 339.4 V, so the stored energy at the peak is ½ × 35 × 10−6 × 339.4² = 2.016 joules. On a 50 Hz supply the same capacitor has a reactance of 90.9 Ω and draws about 17% less current, which is exactly why frequency is part of the specification.
Run and Start Capacitors Are Different Duty Classes
They look similar and they are not interchangeable. A run capacitor is in circuit the entire time the motor runs, so it is built for continuous duty: an oil-filled or metallised-film construction designed to dissipate its losses indefinitely, typically in the low tens of microfarads. A start capacitor is switched into circuit for a second or two during starting and then removed by a centrifugal switch or a relay. It is built for intermittent duty, usually with an electrolytic construction, and it carries a much larger capacitance in a smaller case precisely because it is never expected to stay in circuit.
Fitting a start capacitor where a run capacitor belongs leaves an intermittent-duty part energised continuously, and it will overheat. Fitting a run capacitor where a start capacitor belongs leaves the motor without the starting torque it was designed around. Both substitutions are wrong for reasons that have nothing to do with the microfarad number matching, which is why capacitance alone never defines the part. The duty class, the voltage rating, the tolerance and the temperature class are all part of the specification, and the ratings and marking requirements for motors and their components are set out in standards such as NEMA MG 1, Motors and Generators.
Why the Voltage Rating Is Not Negotiable
The voltage rating printed on a motor capacitor is not the supply voltage. It is the maximum continuous AC voltage the dielectric is built to withstand, and in a motor circuit the voltage across the capacitor can be considerably higher than the supply, because the capacitor sits in a resonant branch with the auxiliary winding. A capacitor marked 440 V is commonly fitted to a 240 V motor for exactly that reason.
Fitting a part with a lower voltage rating than the one specified stresses the dielectric beyond its design. The failure mode is not a graceful loss of performance: the dielectric degrades, losses rise, the capacitor heats, and it eventually vents or ruptures. Modern capacitors include a pressure-interrupter feature intended to disconnect the element before the case fails, but that is a last-resort protection, not a licence to under-rate. A capacitor with a higher voltage rating than specified is a different question again, and one that depends on physical fit, duty class and the manufacturer's guidance rather than on any calculation available here.
A Start Capacitor Stays Charged After Power Off
This is the part that injures people. A capacitor stores energy, and switching off the supply does not remove it. A start capacitor in particular can hold a substantial charge for a long time after the motor stops, because the circuit that would otherwise bleed it away has been opened by the starting switch. Many capacitors are fitted with a bleeder resistor precisely to drain that charge, and a failed bleeder leaves the part charged indefinitely with no external sign at all.
The energy figure in the grid gives a sense of scale: a few joules is enough to cause a painful shock and a reflex reaction, and a reflex reaction next to moving machinery is the real hazard. This is why any work on a motor capacitor begins with isolation, lock-out and a verified discharge, in that order, using a method appropriate to the part rather than shorting the terminals with a screwdriver. Isolation and lock-out procedures in the United States are governed by the OSHA standard on the control of hazardous energy, 29 CFR 1910.147, and equivalent regulations apply in most jurisdictions. Doing this work without the training and the equipment is not a shortcut — it is the part of the job that belongs to a qualified electrician or appliance technician.
What a Capacitance Measurement Does and Does Not Tell You
Capacitors age. Metallised-film run capacitors lose capacitance slowly as the metallisation self-heals around dielectric faults, and electrolytic start capacitors lose it as the electrolyte dries. A measurement below the marked value with the marked tolerance applied is therefore a common and meaningful finding, and the deviation figure in the grid quantifies it.
What it does not tell you is whether the capacitor is the correct part, whether the motor's problem lies elsewhere, or whether the deviation you are seeing is the cause of a symptom or a consequence of one. Equivalent series resistance, insulation condition and the state of the starting switch are all invisible to a capacitance reading. A capacitor also has to be discharged and disconnected from the circuit before it can be measured at all, because a meter reading taken in-circuit is measuring the whole network. The motor's mechanical side — whether it is producing the torque it should — is a separate question handled by the motor torque calculator, and the supply-side loading by the kVA calculator and the power factor calculator.
Arb Digital builds calculators that are honest about what they can and cannot answer, which is what makes them worth linking to. Browse the library, or tell us what your audience needs.
Browse All Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Substituting a start capacitor for a run capacitor, or the reverse — they are different duty classes and constructions, and matching the microfarad number does not make them equivalent.
- Fitting a part with a lower voltage rating than the one specified — the rating is a dielectric limit, and under-rating leads to overheating, venting or rupture.
- Assuming a capacitor is dead once the power is off — a start capacitor can hold charge indefinitely if its bleeder resistor has failed.
- Measuring capacitance in circuit — the reading includes the rest of the network, so the part must be discharged and disconnected first.
- Treating a capacitance reading as a diagnosis — it says nothing about equivalent series resistance, insulation, the starting switch or the winding.
Related Free Tools From Arb Digital
For the physics of capacitance itself, use the capacitance calculator; for DC charging behaviour, the capacitor charge time calculator; for smoothing and ripple in a DC supply, the capacitor size calculator; and for stored energy in its own right, the capacitor energy calculator. On the motor and supply side, see the motor torque calculator, the power factor calculator, the kVA calculator and the Ohm's law calculator. Everything else is in the free online tools hub.
Frequently Asked Questions
No. The capacitance depends on the winding design, not on horsepower alone, and two motors of identical rating from different manufacturers can specify different capacitors. The value comes from the motor nameplate, the original part or the manufacturer's data, and this page takes those figures as inputs rather than producing them.
No. They are different duty classes. A run capacitor is built for continuous operation and a start capacitor for a few seconds at a time, with different constructions and different failure behaviour. Matching the microfarad rating does not make one suitable in place of the other, and either substitution causes damage.
Yes, and a start capacitor in particular can hold charge for a long time, because the starting switch opens the circuit that would otherwise bleed it away. Bleeder resistors are fitted for this reason and can fail silently. Isolation, lock-out and a verified discharge come before any contact with the terminals.
Because the voltage across the capacitor is not the supply voltage. The capacitor sits in a branch with the auxiliary winding, and the voltage developed across it can be considerably higher than the supply. The marked rating is a dielectric limit for the part, and it is specified by the motor manufacturer.
The auxiliary winding current and its phase shift move away from the design point. Depending on the direction of the error, that shows up as reduced starting torque, higher running current, extra heating in the windings or the capacitor, or a motor that hums without starting. The correct value is the one the manufacturer specifies.
Yes, directly. Reactance is inversely proportional to frequency, so the same capacitor draws about 20% more current at 60 Hz than at 50 Hz for the same voltage. That is one reason a motor specified for one supply frequency cannot simply be moved to another.
It means the measurement differs from the marking by the percentage shown, and nothing more. Capacitors do lose capacitance as they age, but a reading says nothing about equivalent series resistance, insulation condition or the rest of the motor circuit. Interpreting it, and deciding what to do, is work for a qualified electrician or appliance technician.
This tool is provided for educational and engineering-estimate use only. It reports the electrical consequences of the figures you enter and does not select, specify, approve or verify any component. It cannot tell you that a capacitor value or voltage rating is correct or safe for your motor: the motor nameplate and the manufacturer's data govern that, and installing, testing or replacing a mains-voltage motor capacitor is work for a qualified electrician or appliance technician, carried out with the supply isolated, locked out and the capacitor verified discharged.
