Selection Guide
What Size Capacitor Do I Need?
The fastest safe method is: match the original µF value, keep the same capacitor type, and use the same or higher voltage rating. Then verify temperature, ESR/ripple requirements, and physical fit.
Quick Selection Rules
Match capacitance (µF) first for replacements.
Use the same capacitor type (run, start, electrolytic, film, etc.).
Use equal or higher voltage rating, never lower.
Check polarity and mounting before installation.
For power supplies, verify ESR and ripple-current suitability.
For high-heat areas, choose higher temperature ratings.
What Must Match by Application
| Application | Must Match | Safe Upgrade Options |
|---|---|---|
| Motor Run (HVAC / pumps / compressors) | µF value (very close), AC voltage class, continuous-duty type | Higher voltage rating if physical fit is acceptable |
| Motor Start | Start-cap µF range and duty type (intermittent start use) | Higher voltage rating if dimensions fit and specs match |
| Power Supply Filtering | Capacitance, polarity, minimum voltage, temperature rating | Lower ESR series, higher temperature rating, higher voltage |
| Audio / Signal Coupling | Capacitance and dielectric behavior for the design intent | Tighter tolerance or higher voltage where appropriate |
If You Are Replacing a Failed Capacitor
- 1. Read the original label: capacitance, voltage, temperature, and type.
- 2. Confirm whether it is polarized and ensure orientation is correct.
- 3. Match the µF value and select equal/higher voltage.
- 4. Verify mechanical fit: diameter, height, lead spacing, and mounting style.
- 5. Test and verify with meter readings before returning equipment to service.
If you are sizing for safety-critical, medical, aerospace, or high-energy systems, validate against applicable standards and manufacturer datasheets before installation.
Motor Capacitor Sizing by Horsepower
For HVAC and pump motor capacitors, the required run capacitor value is determined by the motor manufacturer and printed on the nameplate. However, if the nameplate is illegible or the original capacitor label is damaged, these approximate ranges by motor horsepower can help identify the correct value:
| Motor HP | Typical Run Cap (µF) | Typical Start Cap (µF) | Common Application |
|---|---|---|---|
| 1/4 HP | 5 – 10 µF | 88 – 108 µF | Fan motors, small blowers |
| 1/3 HP | 7.5 – 15 µF | 88 – 130 µF | Furnace blowers, small pumps |
| 1/2 HP | 10 – 20 µF | 108 – 145 µF | Pool pumps, well pumps |
| 1 HP | 15 – 25 µF | 145 – 175 µF | Compressors, larger pumps |
| 2 HP | 25 – 35 µF | 189 – 227 µF | AC compressors, deep well pumps |
| 3 HP | 30 – 45 µF | 233 – 280 µF | Large AC units, industrial pumps |
| 5 HP | 45 – 60 µF | 270 – 324 µF | Commercial compressors, large pumps |
Important: These are approximate ranges. Always use the exact value from the motor nameplate when available. Using an incorrect capacitor value can cause motor overheating, reduced efficiency, or failure to start. See our motor capacitor troubleshooting guide for detailed diagnostics.
Power Supply Capacitor Sizing
For switch-mode power supply (SMPS) output filtering, capacitor sizing involves balancing ripple voltage, holdup time, and ESR requirements:
- Ripple voltage — The output capacitance must be large enough to keep output voltage ripple within specification. The basic relationship is: C ≥ Iload / (fsw × ΔV), where fsw is the switching frequency and ΔV is the allowable ripple voltage.
- Holdup time — If the power supply must maintain output through brief input interruptions, the capacitance must store enough energy: C ≥ 2 × P × thold / (Vmax² − Vmin²), where thold is the required holdup time and Vmax/Vmin are the DC bus voltage limits.
- ESR contribution — In modern high-frequency SMPS, ESR often dominates the ripple voltage more than capacitance. The ESR component of ripple equals Iripple × ESR. Low-ESR capacitors (polymer or multi-capacitor paralleling) may be needed even when capacitance is adequate.
Voltage Rating Selection
Choosing the right voltage rating involves more than just exceeding the circuit voltage:
- Minimum derating — For general-purpose applications, select a voltage rating at least 20% above the maximum expected circuit voltage. For aerospace and medical applications, 50% derating is standard practice.
- Transient voltage — Consider voltage spikes during startup, load switching, and fault conditions. Motor circuits can see voltage transients 2-3× the steady-state value during starts.
- AC ripple on DC — In filtered DC circuits, the peak voltage equals the DC value plus the AC ripple peak. A 400V DC bus with 20V peak ripple sees 420V at the capacitor.
- Higher voltage = longer life — Operating a capacitor well below its rated voltage reduces electrical stress on the dielectric, extending service life. A 450V capacitor on a 300V bus will last significantly longer than a 350V capacitor on the same bus.
Sizing Capacitors for Common Repairs
- HVAC capacitor replacement — Match the µF and voltage from the unit nameplate or existing capacitor label. For AC units, the compressor run capacitor (typically 25-60µF) and fan motor capacitor (typically 3-10µF) are often combined in a single dual-run capacitor. Read our HVAC capacitor guide for brand-specific sizing.
- TV/monitor power supply repair — Identify the failed capacitor by visual inspection (bulging, leaking). Match the capacitance, voltage, and diameter. When replacing, consider upgrading to a 105°C rated, low-ESR series for longer life.
- Amplifier recapping — For audio amplifier restoration, match all capacitor values to the schematic. Power supply filter capacitors can be upgraded to higher voltage ratings and lower ESR types. Signal path coupling capacitors should use high-quality film types.
- VFD DC link replacement — Match the exact capacitance, voltage, ripple current rating, and physical dimensions (snap-in, screw-terminal, or stud mount). See our VFD capacitor replacement guide for brand-specific specifications.
- Obsolete capacitor replacement — When the original part is discontinued, use our cross-reference tool to find form-fit-function replacements. Our technical team can verify compatibility for critical applications.