Motor Capacitor Calculator
Calculate Smarter. Work Faster.
Free motor capacitor calculator — estimate start and run capacitor size (µF) for a single-phase motor from HP or kW, voltage and frequency, find a capacitor's real µF from measured volts and amps, and check whether a replacement is suitable.
Motor & Capacitor Details
Pick a tool: estimate capacitor size from motor rating, test a capacitor's actual µF, or check a replacement.
Measure the voltage across the capacitor terminals and the current in one capacitor lead (clamp meter). On a running PSC or CSCR motor, the voltage across the run capacitor is usually higher than the supply voltage.
Enter the single µF value printed on each run capacitor (e.g. 25 µF ±5%).
Typical estimate for common single-phase induction motor designs — not a manufacturer specification. The value on the motor nameplate or the original capacitor label always takes priority.
Enter values and hit calculate
Rough screening reference — not a calculated manufacturer value. Use the nameplate or original capacitor marking first.
Enter values above to see a breakdown.
How Motor Capacitor Sizing Works
A 1 HP capacitor-start capacitor-run motor on a 230 V, 50 Hz supply typically uses a run capacitor of about 18–30 µF and a start capacitor marked around 233–280 µF — that is the kind of starting point this calculator gives when the nameplate is missing. The reason a capacitor is needed at all: a single-phase supply creates a pulsating magnetic field, not a rotating one, so a single-phase induction motor cannot start on its own. Most designs add a second, auxiliary winding placed at an angle to the main winding. A capacitor in series with that auxiliary winding pushes its current ahead of the main-winding current, producing a rotating field that gives starting torque — and, in motors that keep a capacitor in circuit, better running torque, efficiency and power factor.
The correct value is chosen by the motor designer to suit that particular auxiliary winding. That is why the motor nameplate, or the label on the original capacitor, is always the first reference. When both are missing or unreadable, a typical range by horsepower, supply voltage and frequency is a sensible place to start — which is what the Size by HP tool provides.
How the Estimate Is Built
The calculator converts kW to HP (1 HP = 0.746 kW) and picks the next table row at or above your rating, starting from typical ranges for 220–240 V, 60 Hz motors. It then adjusts for your supply. For 110–120 V, this calculator applies illustrative factors of 1.5× for run and 1.9× for start capacitance, reflecting the larger capacitor current needed at half the voltage — these are not design rules, and actual motor designs vary. For 50 Hz, a 1.2× adjustment (the 60 ÷ 50 ratio, from XC = 1 ÷ 2πfC) is used only as a rough screening estimate; actual 50 Hz capacitor values depend on the motor's winding design and must follow the nameplate or manufacturer specification. The run result is shown as a range, together with standard capacitor values that fall inside it; the start result is shown as the nearest standard start-capacitor marking, as a reference range rather than a calculated value.
CSCR wiring: the run capacitor stays in series with the auxiliary winding at all times; the start capacitor adds extra capacitance in parallel for starting and is switched out by the centrifugal switch (or a start relay on many compressors) as the motor nears full speed. Simplified conceptual schematic — overload protection and terminal markings are omitted, and actual motor terminal connections vary by manufacturer and motor design.
Assumptions and Limitations of the Size-by-HP Estimate
The Size by HP result is an estimate from typical ranges, not a sizing calculation. There is no universal formula that gives a single-phase motor’s capacitor from its horsepower: the correct value is set by the turns, wire size and resistance of that motor’s auxiliary winding, which the calculator cannot know. The estimate assumes:
- a conventional single-phase induction motor of the selected type (CSCR, PSC or capacitor-start), within the 1/8–5 HP table range;
- typical ranges compiled for 208–240 V, 60 Hz designs, scaled by illustrative factors for 110–120 V and for 50 Hz — the factors are screening adjustments, not design rules;
- standard capacitor markings as commonly stocked, so the start result is the nearest marked range rather than a computed value.
Do not rely on the estimate for: hermetic compressors and HVAC equipment (use the equipment data plate or wiring diagram), special-purpose or high-efficiency motors, two-value designs with unusual windings, three-phase motors of any kind, or any motor whose original capacitor marking can still be read. If the estimate is used to get a motor running, check the running current against the nameplate full-load current and the capacitor temperature after a period of operation, and replace the capacitor with the specified value as soon as it is known.
Start Capacitor vs Run Capacitor
| Property | Start capacitor | Run capacitor |
|---|---|---|
| Construction | Non-polarised electrolytic, usually in a black plastic case | Metallised polypropylene film, often in a metal or plastic can |
| Duty | Intermittent — in circuit for a second or two per start | Continuous — connected whenever the motor runs |
| Typical value | Tens to hundreds of µF, marked as a range (e.g. 189–227 µF) | About 1.5–80 µF, marked as one value with a tolerance (±5% or ±6%) |
| Typical voltage rating | 110–125 VAC (115 V motors); 250 or 330 VAC (230 V motors) | 250, 370 or 440 VAC |
| Switched out by | Centrifugal switch or start relay at roughly 70–80% of full speed | Not switched — stays in circuit |
| Typical failure signs | Motor hums but won't start; bulged, vented or cracked case | Weak or slow starting, motor runs hot or draws extra current; value drifts low |
The two are not interchangeable. A start capacitor left in circuit continuously will overheat within minutes, and a film run capacitor is far too small in µF to give the starting torque a capacitor-start motor needs.
Testing a Capacitor from Volts and Amps
A capacitor's current is set by its reactance: I = V ÷ XC = 2πfCV. Rearranged, the actual capacitance is C (µF) = I × 106 ÷ (2π × f × V). At 50 Hz this reduces to about 3183 × A ÷ V, and at 60 Hz to about 2652 × A ÷ V. With a running motor, clamp the current in one run-capacitor lead and measure the voltage directly across the capacitor terminals — not at the supply, because in a running PSC or CSCR motor the capacitor voltage is usually higher than line voltage.
Worked example: on a 50 Hz motor the run capacitor carries 1.6 A with 330 V across it. C = 1.6 × 106 ÷ (2π × 50 × 330) = 1,600,000 ÷ 103,673 ≈ 15.4 µF. Against a 16 µF ±6% label that is −3.5%, so the capacitor is within tolerance. The Test µF tool does this calculation and the tolerance check for you.
Live measurements on motor circuits must only be taken by a qualified person using properly rated test instruments and PPE. If you prefer not to measure live, isolate the motor, discharge the capacitor and use a meter with a capacitance range instead.
Worked Example
1 HP CSCR motor, 208–240 V, 50 Hz → 1 HP table row: run 15–25 µF × 1.2 = 18–30 µF (standard values 20, 25, 30 µF); start 208 µF × 1.2 ≈ 250 µF → estimated start-capacitor range ≈ 233–280 µF (nearest standard marking — verify against the motor or capacitor nameplate). Run capacitor 370 or 440 VAC; start capacitor 250 or 330 VAC.
Engineering References
IEC 60252-1 — AC motor capacitors, Part 1: general, performance, testing and rating (the standard covering motor run capacitors, including their rated voltage and capacitance tolerance marking).
IEC 60252-2 — AC motor capacitors, Part 2: motor start capacitors (intermittent-duty start capacitors and their marking).
Motor nameplate data and capacitor manufacturers' datasheets — the authoritative source for the capacitance, tolerance, voltage and duty rating of a specific motor.
The capacitance formula C = I ÷ (2πfV) follows directly from capacitive reactance, XC = 1 ÷ (2πfC). The typical HP ranges and the voltage and frequency factors used in this calculator are compiled estimates for common designs — they are not taken from the standards above.
Sizing results are typical ranges for common single-phase induction motor designs; individual manufacturers' values can differ. Use the nameplate or original-capacitor data wherever it is available, and have live electrical measurements carried out only by a qualified person.
Typical Motor Capacitor Ranges by HP, and How to Replace One
Estimated values only — not a sizing standard. The chart below uses the same data and adjustments as the calculator, for 208–240 V motors at both 60 Hz and 50 Hz. Run values are typical ranges; start values are the nearest standard start-capacitor marking. Motors for 110–120 V use larger values — select that voltage in the calculator to see them.
| Motor HP | Run µF (60 Hz) | Run µF (50 Hz) | Start µF (60 Hz) | Start µF (50 Hz) |
|---|---|---|---|---|
| 1/8 HP | 2–4 | 2.4–4.8 | 43–48 | 53–64 |
| 1/6 HP | 3–5 | 3.6–6 | 53–64 | 64–77 |
| 1/4 HP | 4–7.5 | 4.8–9 | 72–88 | 88–106 |
| 1/3 HP | 5–10 | 6–12 | 88–106 | 108–130 |
| 1/2 HP | 7.5–15 | 9–18 | 130–156 | 145–174 |
| 3/4 HP | 10–20 | 12–24 | 161–193 | 189–227 |
| 1 HP | 15–25 | 18–30 | 189–227 | 233–280 |
| 1.5 HP | 20–30 | 24–36 | 216–259 | 270–324 |
| 2 HP | 25–35 | 30–42 | 243–292 | 270–324 |
| 3 HP | 30–45 | 36–54 | 270–324 | 324–389 |
| 5 HP | 40–60 | 48–72 | 340–408 | 400–480 |
These are illustrative typical ranges for 208–240 V single-phase induction motors, not IEC, NEMA or manufacturer sizing tables. Not every motor type uses both capacitors: PSC motors use only the run value, capacitor-start motors only the start value. The motor nameplate and original capacitor specifications take precedence.
Run Capacitor Voltage: 370 V or 440 V?
The voltage rating on a capacitor is a maximum, not an operating value. For the same capacitance and the correct capacitor type, a 440 VAC run capacitor is generally an acceptable replacement for a 370 VAC one, unless the motor or equipment manufacturer specifies otherwise. The higher rating does not change the capacitance value and gives additional voltage margin, but service life also depends on temperature, current, construction and the manufacturer's specifications. The reverse is not safe: fitting a 370 VAC part where 440 VAC was specified can lead to early failure, because the voltage across a run capacitor on a running motor is often well above the supply voltage. The only practical catch with a higher rating is size: 440 VAC cans are often slightly larger, so check that the replacement fits the mounting bracket or terminal box.
Dual Run Capacitors (C, HERM, FAN)
Air-conditioner and heat-pump outdoor units often use a dual run capacitor: two capacitors in one can, sharing a common terminal. C is the common, HERM feeds the hermetic compressor, and FAN feeds the condenser fan motor. It is marked with two values, such as 35/5 µF — the first for the compressor, the second for the fan. A replacement must match both values; a higher voltage rating is generally acceptable. Where the equipment manufacturer or a qualified technician permits substitution, separate correctly rated capacitors can be used to reproduce the required C–HERM and C–FAN values — follow the equipment wiring diagram and the manufacturer's requirements.
Three-Phase Motors: What This Calculator Does Not Cover
Power factor correction at a three-phase motor. Capacitors connected at the terminals of a three-phase induction motor are sized in kVAr, not from a µF-per-HP chart. The limiting factor is self-excitation: if the capacitor kVAr exceeds the motor’s no-load magnetising kVAr, the motor can act as an induction generator after it is switched off, producing overvoltage and a damaging out-of-phase condition on a quick restart. Common practice keeps terminal-connected correction below the no-load magnetising requirement — often expressed as a capacitor current of no more than about 90% of the motor’s no-load current — and follows the motor manufacturer’s maximum kVAr where one is published. Motors that are reversed, plugged, jogged, restarted rapidly or driven by a VFD should not have terminal capacitors at all; central or group correction is used instead, as covered by the APFC calculator.
Running a three-phase motor on a single-phase supply. A static phase-converter arrangement using a capacitor across one winding (often called a Steinmetz connection) can run a small three-phase motor from a single-phase supply, but at a substantially reduced output and with unbalanced winding currents. Capacitor values for that arrangement depend on the motor’s impedance and load and are not produced by this calculator; a purpose-built phase converter or a single-phase-input VFD is usually the better route.
Replacing a Failed Motor Capacitor Safely
- Isolate and lock off the supply. Switching the motor off at a local control is not enough — open the isolator and prove the circuit dead.
- Discharge the capacitor. A capacitor can hold a dangerous charge after power is removed. Discharge it through an appropriately rated discharge resistor following the equipment manufacturer's or your site's service procedure — do not short the terminals — then verify zero voltage with a properly rated meter before touching.
- Record the wiring and the label. Photograph the connections and note the µF value, voltage rating, tolerance and whether it is a start or run type.
- Choose the replacement. Same type, same µF (same range for a start capacitor), equal or higher voltage rating, and a size that fits. The Replacement Check tool above runs the basic numerical check.
- If a start capacitor failed, check what switches it. A centrifugal switch or start relay that sticks closed keeps the start capacitor in circuit and destroys it — a new capacitor will fail the same way unless the switch is fixed.
- Test after fitting. Run the motor, check that it starts cleanly, and compare the running current with the nameplate full-load current.
For wider motor care — bearings, insulation resistance, alignment and vibration — see the Electric Motor Maintenance Guide.
Common Mistakes When Sizing or Replacing Motor Capacitors
1. Sizing from a chart when the nameplate is readable. Charts and calculators give typical ranges across many designs. The nameplate or original capacitor label gives the value for your motor — always use it when you have it.
2. Using a start capacitor in a run position, or the reverse. Electrolytic start capacitors are built for a few seconds of use per start and overheat if left in circuit. Film run capacitors are too small in µF to start a capacitor-start motor. Match the type as well as the value.
3. Fitting a lower voltage rating than the original. Equal or higher is fine; lower is not. Remember that the voltage across a run capacitor on a running motor is often higher than the supply voltage.
4. "Upsizing" a run capacitor for more torque. Extra run capacitance raises auxiliary-winding current and heating. The motor may seem to start better but run hotter, shortening winding life.
5. Using a 60 Hz chart for a 50 Hz motor (or the reverse). Capacitive reactance depends on frequency, so a 50 Hz design is often estimated to need roughly 20% more capacitance than a comparable 60 Hz design — a screening ratio, since the actual value depends on how the winding was designed for its frequency. Many published charts are for 60 Hz only.
6. Handling a capacitor without discharging it. Motor capacitors can hold a charge after the supply is off. Discharge through a resistor and verify with a meter before touching the terminals.
7. Replacing a start capacitor without checking the start switch or relay. A switch that sticks closed is a common reason start capacitors fail. Replacing only the capacitor leads to a repeat failure.
8. Confusing motor capacitors with power factor correction capacitors. Motor capacitors are part of the motor's own starting and running circuit and are sized in µF for its winding. Power factor correction capacitors are connected across the supply and sized in kVAr — a different job entirely. On a three-phase motor, terminal-connected correction must also stay below the motor’s no-load magnetising kVAr to avoid self-excitation after switch-off.
Frequently Asked Questions
How do I find the right capacitor size for a single-phase motor? +
Check the motor nameplate or the label on the old capacitor first — they give the exact µF value and voltage rating the manufacturer designed for. If both are unreadable, use a typical range based on HP, supply voltage and frequency, as this calculator gives, then confirm with the motor manufacturer or by checking the running current against the nameplate after fitting.
What size run and start capacitor does a 1 HP motor need? +
It depends on the design, but for a 1 HP capacitor-start capacitor-run motor on 208–240 V at 60 Hz, run capacitors are typically around 15–25 µF and start capacitors around 189–227 µF. On 50 Hz supplies, values roughly 20% higher are typical (about 18–30 µF run and a 233–280 µF start marking), because capacitive reactance rises as frequency falls. Motors for 110–120 V use noticeably larger capacitors. The nameplate value always takes priority over these typical ranges.
What capacitor size do I need for a 1.5 HP motor? +
For a 1.5 HP capacitor-start capacitor-run motor on 208–240 V, typical values are a run capacitor of about 20–30 µF with a 216–259 µF start capacitor at 60 Hz, or about 24–36 µF run with a 270–324 µF start capacitor at 50 Hz. These are illustrative typical ranges, not a manufacturer specification — the value on the motor nameplate or the original capacitor always takes priority.
How do I calculate motor capacitor size in µF? +
The exact value depends on the design of the motor's auxiliary winding, so it can only be calculated from the manufacturer's design data — which is why the nameplate value is the reference. Without it, estimate from a typical range by HP, supply voltage and frequency, as this calculator does. To find the actual µF of a capacitor already fitted, use C (µF) = I × 10⁶ ÷ (2π × f × V) with the measured capacitor current and voltage.
Is the HP-based capacitor size an exact value for my motor? +
No. It is an estimate from typical ranges for common single-phase motor designs. The correct capacitance is set by the design of the motor’s auxiliary winding, so the value on the nameplate, the original capacitor or the equipment wiring diagram always governs. Use the estimate only to get an unmarked motor running, then check running current and capacitor temperature and fit the specified value as soon as it is known.
What is the difference between a start capacitor and a run capacitor? +
A start capacitor is a high-value electrolytic capacitor rated for intermittent duty; it is in circuit for only a second or two during starting and is then disconnected by a centrifugal switch or start relay. A run capacitor is a lower-value film capacitor rated for continuous duty; it stays connected while the motor runs to improve running torque, efficiency and power factor. The two are not interchangeable.
Can I use a 440 V capacitor instead of a 370 V capacitor? +
Generally yes, as long as the µF value, the type (run or start) and the physical fit are the same, and the equipment manufacturer does not specify otherwise. The voltage rating is a maximum, so a higher rating adds margin without changing the capacitance value. Never go the other way — a lower voltage rating than the original can fail early.
How close does the replacement µF value need to be? +
For run capacitors, match the original value as closely as possible — most are labelled ±5% or ±6%, and a value outside that tolerance changes the auxiliary-winding current and can make the motor run hotter. Start capacitors are marked as a range, such as 189–227 µF, so choose a replacement with the same marked range or one that overlaps it closely.
How do I test a motor run capacitor with a clamp meter? +
With the motor running, measure the current in one capacitor lead and the voltage directly across the capacitor terminals, then calculate C (µF) = I × 10⁶ ÷ (2π × f × V). At 50 Hz this is about 3183 × A ÷ V, and at 60 Hz about 2652 × A ÷ V. Compare the result with the rated µF. Live measurements should only be taken by a qualified person using properly rated meters and PPE.
When should a motor capacitor be replaced? +
Replace it if the case is bulging, cracked or leaking, if it measures outside its rated tolerance, or if the motor hums without starting, starts slowly, or runs hot with a higher-than-normal current. A run capacitor that has drifted well below its rating — for example more than about 10% low — is a common cause of weak starting and overheating.
What happens if I use the wrong size capacitor on a motor? +
Too little capacitance reduces starting torque, so the motor may hum, start slowly or trip its overload. Too much run capacitance increases auxiliary-winding current and heating, which can shorten winding life even if the motor seems to run normally. An oversized start capacitor can also overheat if the start switch is slow to open.
Why do 50 Hz motors need more µF than 60 Hz motors? +
A capacitor's reactance is Xc = 1 ÷ (2π × f × C), so at 50 Hz the same capacitor has about 20% more reactance than at 60 Hz. To give the auxiliary winding a similar current and phase shift, 50 Hz designs typically use roughly 20% more capacitance. This calculator applies that 60 ÷ 50 ratio to its 60 Hz typical ranges as an estimate; actual 50 Hz values depend on the motor's winding design and the manufacturer.
Is a motor capacitor the same as a power factor correction capacitor? +
No. A motor start or run capacitor is part of a single-phase motor’s own starting and running circuit and is sized in µF for that winding. Power factor correction capacitors are connected across the supply to offset the reactive power of inductive loads and are sized in kVAr — use the APFC calculator for that. Where correction capacitors are connected directly at a three-phase motor’s terminals, their kVAr must stay below the motor’s no-load magnetising kVAr (often expressed as a capacitor current of no more than about 90% of no-load current) to avoid self-excitation after switch-off.
Can I size a capacitor to run a three-phase motor on single-phase power? +
Not with this calculator. A capacitor phase-shifting arrangement (a Steinmetz connection) can run a small three-phase motor from single-phase supply, but only at a substantially reduced output and with unbalanced winding currents, and the capacitor value depends on the motor’s impedance and load. A purpose-built phase converter or a single-phase-input VFD is usually the better choice.