Motor Power from Run Cap Calculator
Enter the run capacitor value, voltage and frequency to estimate the motor power in W, kW and HP.
Step-by-step
- Enter valid values to begin.
Estimate only. P = C × 2π f V² / 10&sup6; gives the reactive volt-amperes of the run capacitor, which on a PSC motor is typically close to its rated output in watts. Designers trade capacitance against efficiency, so expect ±25%. Never size breakers or overloads from this result; use the motor nameplate and NEC Article 430.
Motor Power from Run Capacitor: Reading a Motor Through Its Can
Core Engineering Principles
A run capacitor is a phase-shifting device, and it sits across the auxiliary winding the whole time the motor turns. The reactive volt-amperes it supplies are Q = 2πfCV², and on a permanent split capacitor motor that number lands very close to the motor’s rated output in watts. That’s why the old-timers can look at a 25 µF can on a 230 V, 60 Hz machine and say “about two thirds of a horsepower” without touching a meter. It works because the manufacturer picked the capacitance to give a nearly circular rotating field at full load, and the field only goes circular when the capacitor carries roughly the load’s share of volt-amperes.
Treat the answer as an estimate, not a nameplate. A designer is free to trade capacitance against efficiency, so the real ratio of capacitor VA to shaft watts wanders by a quarter either way. Use the plain formula when you are checking a nameplate for sanity, and the efficiency-adjusted option when you want a figure that matches the run capacitor sizing page. Never size a replacement from a power you inferred from a can that may have drifted; a tired one can lose 10 to 20% of its microfarads before it looks swollen.
Padj = P / η (matches C = P·η / 2πfV²)
HP = P / 746 • IC = 2π f C V
NEC & Standard References
NEMA MG-1 Part 1 covers single-phase motor ratings and the definitions of permanent split capacitor and capacitor-start types. IEC 60252-1 sets the requirements for AC motor capacitors, including tolerance (commonly ±5% or ±10% on a run capacitor) and the self-healing metallised film construction. UL 810 covers capacitors in North America. For the motor itself, NEC Article 430 sizes conductors and protection from the nameplate current or the tables and never from a capacitor rating. Confirm the adopted edition before applying any of these.1. 2π × 60 = 376.99 rad/s.
2. V² = 52,900.
3. P = 25 × 376.99 × 52,900 / 106 = 498.6 VA, which is about 0.67 HP (0.50 kW).
4. Capacitor current = 376.99 × 25 × 10−6 × 230 = 2.17 A.
5. Look for a ½ HP or ¾ HP rating before you order parts.
- A capacitor is not a wattmeter. The result is an order-of-magnitude check. Don’t size breakers or overloads from it.
- Frequency matters. The same can on 50 Hz gives 17% less VA than on 60 Hz, so a 60 Hz motor run on 50 Hz mains is undercompensated.
- Voltage is squared. A 10% sag drops the capacitor’s VA by almost 19%, and the field turns elliptical, so the motor hums and runs hot.
- Measure before you trust the label. Meter the capacitor out of circuit with a proper LCR meter; a drifted can will give a falsely low motor size.
- Discharge first. A run capacitor holds charge after power-off. Short it with a 20 kΩ resistor before touching the terminals.