Motor Capacitor Size Calculator — Run & Start Capacitor Sizing
Size run capacitors from measured motor no-load current, size start capacitors with the 2–4× rule, pick the right voltage rating, and design a discharge bleed resistor. Works for 50 Hz and 60 Hz single-phase motors (PSC, capacitor-start, capacitor-start/run).
Educational tool. Capacitors store a dangerous electrical charge — always discharge before touching, and have a qualified technician do mains work you are not trained for.
Run capacitor size from no-load current
Enter the motor's no-load current (clamp meter, motor uncoupled from its load), line voltage, and frequency.
Calculated run capacitance:
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Rule of thumb: a run capacitor's µF must match the motor design (±5–10% tolerance). The VAC rating may be higher, never lower.
Source: Comar Condensatori motor-capacitor catalog (manufacturer datasheet) — the standard industry formula, equivalent to Xc = 1/(2πfC) with Ic = V/Xc. At 60 Hz this reduces to the well-known “2652 rule”: C ≈ 2652.58 × I / V.
Start capacitor sizing (2–4× rule)
Start capacitors are electrolytic, short-duty only (disconnected by a centrifugal switch or relay after ~1–2 s). Size them from the run capacitor value.
Rule-of-thumb band (2×–4× run):
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Start-capacitor voltage class must be ≥ line voltage (standard classes: 125 / 165 / 250 / 330 V — 250 V is the usual choice for 220–240 V motors). Never run a start capacitor continuously.
Motor capacitor voltage rating & discharge (bleeder) resistor
Picking the correct motor capacitor voltage rating matters as much as the µF value — underrated capacitors fail violently.
Stored energy for reference: E = ½CV² (see our capacitor energy calculator). Even after the bleed resistor has done its job, always verify 0 V with a meter before touching terminals.
Typical motor capacitor chart (by HP)
Typical values for single-phase capacitor motors at ~230 V (1hp, 2hp, 3hp, 5hp and 10hp motors, 50hz and 60hz). 60 Hz run values follow the standard NEMA application table; 50 Hz run values are ≈20% higher (from the frequency term in the formula). Start ranges are standard EIA RS-463 MFD ranges. Always prefer the motor nameplate.
| Motor HP | Run µF (60 Hz) | Run µF (50 Hz ≈) | Start µF range |
|---|---|---|---|
| 1/4 | 5 | 6 | 59–71 |
| 1/3 | 7.5 | 9 | 70–84 |
| 1/2 | 10 | 12 | 88–108 |
| 3/4 | 12.5 | 15 | 108–130 |
| 1 | 15 | 18 | 145–174 |
| 1½ | 20 | 24 | 161–193 |
| 2 | 25 | 30 | 189–227 |
| 3 | 40 | 48 | 270–324 |
| 5 | 55 | 66 | 378–454 |
| 7½ | 80 | 96 | 540–648 |
| 10 | 100 | 120 | 800–960 |
HVAC note: fan motors and compressors often use dual run capacitors (e.g. 40+5 µF) — one can serving two motors. Size each section independently with the run calculator above. Reading this chart answers common questions like "what size capacitor for a 1 HP motor" (≈15 µF run, 145–174 µF start at 60 Hz) or "what size capacitor for a 2 HP motor" (≈25 µF run, 189–227 µF start).
How motor capacitor sizing works
1. Run capacitor — the physics
Why does a single phase motor need a capacitor? A single-phase supply produces a pulsating, not rotating, magnetic field, so the motor cannot start or develop full torque on its own. The capacitor feeds a second (auxiliary) winding with a phase-shifted current, creating the rotating field the rotor needs.
A permanent-split-capacitor (PSC) motor feeds its auxiliary winding through the run capacitor. The capacitor's reactance Xc = 1/(2πfC) sets the auxiliary-winding current Ic = V/Xc, which creates the phase shift the motor needs for torque. Rearranging gives the sizing formula:
Measure I with a clamp meter on the uncoupled (no-load) motor — nameplate full-load current is the wrong number here. Because f sits in the denominator, a 50 Hz supply needs ~20% more µF than 60 Hz for the same current.
2. Start capacitor — the 2–4× rule
Capacitor-start motors need a much larger phase shift for a second or two at startup, so the start capacitor is 2–4× the run value, then a centrifugal switch takes it out of circuit. Because they only see brief duty, start capacitors are non-polarized electrolytics in standard MFD ranges (EIA RS-463), not continuous-duty film types.
3. Voltage ratings
µF must match; VAC may be higher, never lower. Size run capacitors at ≥ 1.5× line voltage (370 VAC minimum, 440 VAC preferred, for 220–240 V systems) because the auxiliary winding can see voltages above line. Start-capacitor classes (125/165/250/330 V) need only cover line voltage briefly — 250 V is standard for 230 V motors.
4. Discharge safety
A capacitor is ~99.3% discharged after five time constants (t = 5RC). A bleeder resistor sized R = t/(5C) across the terminals makes the capacitor safe to handle after the target time — but always verify 0 V with a meter before touching, since bleeders can fail open.
Worked example
A 230 V / 50 Hz motor draws 2.0 A uncoupled (about 15 microfarads per hp at 60 Hz — that is how many microfarads per hp a typical motor needs; see the chart): C = 2 × 10⁶/(2π × 50 × 230) ≈ 27.7 µF → nearest standard 30 µF run capacitor, 370 VAC (440 VAC preferred). Start: 2–4× → 60–120 µF band → nearest standard EIA range containing 3× (90 µF) is 80–100 µF, 250 V class.
Frequently asked questions
Related motor capacitor sizing topics
This page covers the searches people run when a single-phase motor will not start or a capacitor has failed: single phase motor capacitor calculator, electric motor capacitor calculator, PSC motor capacitor size, capacitor start capacitor run sizing, dual run capacitor size (such as 40+5 µF), HVAC, compressor, fan motor, well pump and pool pump capacitor sizing, 370V vs 440V run capacitor choice, start capacitor voltage rating, how to safely discharge a motor capacitor, and bleed resistor sizing. Use the calculators above for your exact numbers — the chart is only a starting point when the nameplate is unreadable.
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