Motor Power from Start Cap Calculator
Enter the start capacitor value, voltage and frequency to estimate the motor power and the likely range.
Step-by-step
- Enter valid values to begin.
Estimate only. P = Cstart × 2π f V² / (3.5 × 10&sup6;). The 3.5× start-to-run ratio really varies from about 3.0 to 5.0, so the band shown is more honest than the single figure. A replaced or aged start can gives a poor estimate; use the nameplate where it exists.
Motor Power from Start Capacitor: Working Backwards from a Can
Core Engineering Principles
When a motor arrives without a nameplate, the start capacitor is often the only legible clue. We know the starting capacitor is roughly 3.5 times the run value, and we know the run value is close to the motor’s output in volt-amperes divided by the squared voltage and the angular frequency. Put the two together and you get P = C × 2πfV² / (3.5 × 10⁶). The estimate is only as good as the 3.5 multiple, which truly floats between 3.0 and 5.0, so the answer is a band, not a point.
That band matters. At 88 µF on 230 V and 60 Hz, a 3.0 multiple gives 585 W and a 5.0 multiple gives 351 W. We therefore show the nominal figure and both ends, and we suggest you pick the nearest standard frame. Cross-check against the winding resistances, the frame size and the physical rotor before trusting the number. A start can is also the part most likely to have been replaced with whatever the shop had on the shelf, so the capacitor in the motor may not be the one the factory fitted. Use the estimate for choosing a replacement motor or a gearbox, not for sizing the circuit breaker.
Pmin = C × 2πfV² / (5.0 × 106) • Pmax = C × 2πfV² / (3.0 × 106)
HP = P / 746
NEC & Standard References
NEMA MG-1 Part 1 gives the standard single-phase horsepower ratings the answer should be matched to, from fractional up to 10 HP. IEC 60252-1 covers motor capacitor tolerance, which means the labelled value can differ noticeably from the real one. NEC 430.6 says branch-circuit sizing comes from the full-load current tables, not from the nameplate, and certainly not from an estimate like this one. Confirm the adopted edition before applying any of these.1. 2π × 60 × 230² = 376.99 × 52,900 = 19.94 × 10⁶, so each µF is worth 19.94 VA.
2. P = 88 × 19.94 / 3.5 = 501 W, about 0.67 HP.
3. At 3.0× the answer is 585 W (0.78 HP). At 5.0× it is 351 W (0.47 HP).
4. The nearest standard motor is ¾ HP, and the likely nameplate is 0.5 to 0.75 HP. A can of 88 µF is a normal fit for ⅔ HP.
- The multiple is not fixed. A hard-starting compressor motor might use 5× while a belt-driven fan uses 3×. Present the answer as a range.
- Check the voltage on the can. A 125 V start capacitor on a 230 V motor is a common wrong replacement and fails fast.
- Capacitance drifts. An old start can may have lost a third of its capacity, which makes the estimated power too low.
- Start capacitors are not run capacitors. Don’t apply the run capacitor formula to the start value.
- Discharge before measuring. The can holds a charge that can bite. Use a bleed resistor, never a screwdriver.