If you are replacing a failed motor capacitor, the correct microfarad (µF) size depends strictly on the motor’s horsepower, line voltage, and whether the circuit requires a start or run capacitor. For a standard 1/2 HP, 230V single-phase motor, you typically need a 5–7.5 µF run capacitor and a 54–65 µF start capacitor. Guessing the value based on physical size or using a generic "universal" kit without checking the µF rating will result in high winding temperatures, failed starts, or exploded electrolytic casings.
Single-Phase Motor Capacitor Sizing Chart (NEMA & IEC Baseline)
How to read this table: First, determine which column applies to your installation. The Run Capacitor column applies to circuits where the capacitor remains energized continuously while the motor spins (wired between the start and run windings). The Start Capacitor column applies to circuits where the capacitor is only energized for a few seconds during startup before a centrifugal switch or potential relay drops it out of the circuit. Locate your motor’s Horsepower (HP) and nominal Voltage. The values below represent standard nominal ranges based on NEMA MG 1 guidelines and IEC 60252 baseline specifications for fractional and integral horsepower single-phase induction motors.
| Motor HP | Line Voltage | Run Capacitor (µF) | Start Capacitor (µF) | Typical Start Voltage Rating |
|---|---|---|---|---|
| 1/3 HP | 115V | 5 – 10 µF | 88 – 108 µF | 125VAC / 250VAC |
| 1/2 HP | 115V | 10 – 15 µF | 108 – 130 µF | 125VAC / 250VAC |
| 1/2 HP | 230V | 5 – 7.5 µF | 54 – 65 µF | 250VAC / 330VAC |
| 3/4 HP | 230V | 7.5 – 10 µF | 88 – 108 µF | 250VAC / 330VAC |
| 1 HP | 230V | 10 – 15 µF | 108 – 130 µF | 250VAC / 330VAC |
| 1.5 HP | 230V | 15 – 20 µF | 162 – 194 µF | 330VAC |
| 2 HP | 230V | 15 – 20 µF | 216 – 259 µF | 330VAC |
| 3 HP | 230V | 20 – 30 µF | 324 – 389 µF | 330VAC |
| 5 HP | 230V | 30 – 40 µF | 430 – 516 µF | 330VAC |
Applying Derating Factors and Installation Variables
The baseline chart above assumes a standard 60Hz (or 50Hz equivalent) sinusoidal supply, an ambient temperature of 40°C or less, and standard intermittent duty for start circuits. Real-world installations require derating adjustments to prevent premature dielectric breakdown.
Temperature and Duty Cycle Derating
Run capacitors use metallized polypropylene film and are rated for continuous duty. However, if your installation environment exceeds 70°C (common inside unventilated compressor shrouds or attic-mounted air handlers), you must use a capacitor rated for 105°C rather than the standard 70°C. Operating a 70°C-rated cap at 90°C will halve its expected lifespan due to accelerated dielectric evaporation.
Start capacitors use non-polarized electrolytic chemistry. They are strictly rated for intermittent duty—typically a maximum of 3 seconds per start, and no more than 20 starts per hour. If your application involves heavy inertia loads, frequent jog cycles, or potential relay failures that keep the start winding engaged for >5 seconds, the electrolyte will boil. In high-duty-cycle applications, you must derate by moving to a much larger physical start capacitor case or redesigning the circuit to use a continuous-duty film run capacitor.
Voltage Derating for Electrolytic Start Caps
Unlike film run capacitors, electrolytic start capacitors are highly sensitive to overvoltage. Never apply more than 110% of the start capacitor's rated AC voltage. If you are measuring 252V on a 230V nominal line, do not use a 250VAC-rated start capacitor; step up to a 330VAC unit. Run capacitors (film) are more forgiving, but you should still maintain at least a 20% voltage headroom above your maximum measured line voltage.
What This Capacitor Sizing Chart Cannot Tell You
While the chart provides the electrical baseline, it cannot dictate mechanical or proprietary OEM constraints. Keep these blind spots in mind before ordering replacement parts:
- Physical Case Dimensions: A 45µF / 370V run capacitor can be manufactured in a 2-inch diameter oval case or a 1.5-inch diameter round case. The chart does not account for the mounting bracket clearances inside your specific HVAC condenser or bench motor bell housing.
- Terminal Configurations: You must verify whether the OEM cap uses 1/4-inch push-on spade terminals, 3/16-inch spades, or screw-down studs. Mismatching terminals leads to loose crimps, arcing, and melted wiring harnesses.
- Proprietary OEM Tuning: Manufacturers like Copeland or Baldor sometimes tune their start windings to a highly specific µF value (e.g., exactly 88µF) rather than a broad range. While a generic 88-108µF start cap will spin the motor, it may draw 15% higher locked-rotor amperage (LRA) than the OEM spec, causing nuisance breaker trips on marginal supply transformers.
- 50Hz vs 60Hz Shifts: Capacitive reactance ($X_c$) is inversely proportional to frequency. If you are running a 60Hz motor on a 50Hz supply (or vice versa via a VFD), the effective starting torque and phase shift angle will change. The µF values in this chart are optimized for the motor's nameplate Hz rating.
Frequently Asked Questions
Can I use a higher microfarad (µF) run capacitor than the chart specifies?
No. Run capacitors dictate the phase shift angle between the start and run windings. If you install a 15µF cap where a 7.5µF cap is specified, you will over-energize the start winding during continuous operation. This causes the start winding to overheat, degrading the copper enamel insulation and eventually shorting the winding to the stator core. Always match the run capacitor µF to within ±5% of the OEM nameplate specification. You can go higher on the voltage rating (e.g., using a 440VAC cap instead of a 370VAC cap), but never on the µF rating.
How do I calculate start capacitor size if my motor HP isn't on the chart?
If you are sizing a capacitor for a custom-wound motor or an unmarked import motor, you can calculate the required start capacitance using the empirical formula derived from electrical machine theory:
C (µF) = (2652 × I_start) / V_cap
Where I_start is the measured current through the start winding during the locked-rotor phase (in Amps), and V_cap is the voltage drop across the capacitor during startup. For a quick bench estimate without clamping the start winding, assume the start capacitor µF should be roughly 12 to 15 times the run capacitor µF.
Does the voltage rating on a capacitor sizing chart need to match exactly?
The voltage rating is a maximum ceiling, not a target operating voltage. You can always substitute a capacitor with a higher voltage rating (e.g., replacing a 370VAC run cap with a 440VAC unit). The higher-rated unit will simply run cooler and last longer. However, you must never substitute a lower voltage rating. Installing a 250VAC start cap on a 230V line that experiences a 10% utility swell (253V) will cause the dielectric oxide layer inside the electrolytic cap to puncture, resulting in a catastrophic venting or explosion.
Why did my replacement start capacitor explode after one week?
Start capacitors explode when the electrolyte boils and generates gas faster than the pressure relief vent can release it. This is almost always caused by one of three installation errors: 1. Stuck Centrifugal Switch: The mechanical switch inside the motor failed to open, leaving the start cap in the circuit continuously. 2. Welded Potential Relay: The contacts on your solid-state or potential relay fused together, failing to drop the cap out of the circuit. 3. Short Cycling: The system is short-cycling (starting more than 20 times an hour), exceeding the cap's thermal recovery time. Always test the potential relay or centrifugal switch with a multimeter when replacing a blown start capacitor.
