When replacing a failed single-phase motor capacitor, guessing the microfarad (µF) rating will result in poor starting torque, overheated windings, or a tripped breaker. As a direct baseline: a standard 1 HP, 120V/240V single-phase motor typically requires a 324–389 µF start capacitor and a 12–15 µF run capacitor. However, "size" in this context refers to both the electrical capacitance and the physical can dimensions required to fit inside the motor housing.
This reference guide provides the industry-standard capacitance values per horsepower, explains how to read the derating columns, and details the physical constraints the electrical chart cannot show. Safety Note: Always de-energize the motor, lock out the disconnect, and discharge the capacitor terminals with a 20k-ohm, 5-watt resistor before handling. Stored charge can be lethal.
The Motor Capacitor Size Chart (µF per HP)
How to read this table: The chart below maps fractional and integral horsepower (HP) ratings to the required microfarad (µF) ranges for both start and run capacitors. The values assume a standard 60Hz AC supply, a 40°C maximum ambient temperature, and standard NEMA (National Electrical Manufacturers Association) single-phase induction motor designs. The voltage rating (VAC) listed is the minimum required; you can always use a higher voltage-rated capacitor, but never a lower one. Source data aligns with NEMA MG-1 standards for single-phase motors and standard manufacturer application guides from Cornell Dubilier (CDE).
• 1/2 HP: Start 189-227 µF | Run 7-10 µF
• 3/4 HP: Start 243-292 µF | Run 10-15 µF
• 1 HP: Start 324-389 µF | Run 12-15 µF
• 2 HP: Start 648-778 µF | Run 20-25 µF
| Motor HP | Start Capacitor (µF) | Start Min VAC | Run Capacitor (µF) | Run Min VAC (120V Line) | Run Min VAC (240V Line) |
|---|---|---|---|---|---|
| 1/4 HP | 108 – 130 | 125V | 4 – 5 | 200V | 370V |
| 1/3 HP | 130 – 156 | 125V | 5 – 7 | 200V | 370V |
| 1/2 HP | 189 – 227 | 125V | 7 – 10 | 200V | 370V |
| 3/4 HP | 243 – 292 | 125V | 10 – 15 | 200V | 370V |
| 1 HP | 324 – 389 | 125V | 12 – 15 | 200V | 370V |
| 1.5 HP | 486 – 583 | 125V | 15 – 20 | 200V | 440V |
| 2 HP | 648 – 778 | 125V | 20 – 25 | 250V | 440V |
| 3 HP | 800 – 1000 | 125V | 30 – 40 | 330V | 440V |
| 5 HP | 1000 – 1300 | 125V | 45 – 55 | 330V | 440V |
Which Column Applies & How Derating Modifies Values
The most common mistake when reading a capacitor size chart is selecting the wrong voltage column or ignoring environmental derating. Here is how to apply the data to your specific installation.
Which Column Applies to Your Installation?
Start vs. Run: Start capacitors (electrolytic) are designed for intermittent duty (typically less than 3 seconds per start, max 20 starts per hour). Run capacitors (metallized polypropylene) are rated for continuous duty. Never swap them. If your motor uses a centrifugal switch to cut out the start winding, you need the Start column. If the auxiliary winding stays energized while the motor runs, you need the Run column.
Voltage Columns: The "Min VAC" columns represent the minimum voltage rating printed on the can. For a 240V line, the run capacitor must be rated for at least 370V (often labeled 370VAC or 440VAC). Using a 250VAC run capacitor on a 240V line will result in dielectric breakdown and a shorted can within weeks due to voltage spikes during motor operation.
How Derating Rows Modify the Base Value
The chart assumes a 40°C ambient environment. If your motor operates in a high-heat environment (e.g., inside an unventilated compressor shroud or a hot attic), you must apply temperature derating to the voltage rating, not the µF value. For every 10°C rise above 40°C, the expected lifespan of a run capacitor is halved. If ambient exceeds 55°C, you must step up to a capacitor rated for 105°C (rather than the standard 70°C or 85°C) to prevent the dielectric fluid from expanding and rupturing the pressure interrupter.
Additionally, altitude derating applies above 3,300 feet (1,000 meters). Thinner air reduces cooling efficiency, requiring you to treat the motor as if it were one HP size larger, which means moving down one row in the chart to select a capacitor with a higher thermal mass and current-handling capability.
Physical Can Sizes: What the Chart Cannot Tell You
An electrical size chart is useless if the replacement capacitor physically will not fit inside the motor's capacitor housing. The µF and VAC ratings do not dictate physical dimensions; manufacturer case codes do.
• Physical Dimensions: Diameter (inches/mm) and height.
• Terminal Type: 1/4" push-on spade, 1/8" push-on spade, or screw terminals.
• ESR and Ripple Current: Critical for VFD (Variable Frequency Drive) applications, but irrelevant for standard across-the-line AC motor starting.
• Case Shape: Round (cylindrical) vs. Oval (for tight mounting brackets).
For standard fractional HP motors, run capacitors typically come in round cylindrical cans ranging from 1.4" to 2.0" in diameter, with heights between 2.0" and 3.5". Start capacitors are almost exclusively round, black phenolic or plastic cases, typically 2.16" in diameter with heights varying from 3.4" to 4.5" depending on the µF rating. Always measure the internal depth of your motor's capacitor cup before ordering. If you are upgrading from a 370V to a 440V run capacitor for better longevity, be aware that the 440V can will likely be 0.2" to 0.4" taller due to the thicker dielectric film required.
Frequently Asked Questions
Can I use a higher µF capacitor than the chart recommends?
For start capacitors, you can safely go up to 20% higher on the µF rating to increase starting torque in hard-start applications (like a compressor with high head pressure), provided you do not exceed the maximum start winding current limit. For run capacitors, absolutely not. A higher µF run capacitor will shift the phase angle of the auxiliary winding, causing it to draw excessive current, overheat, and burn out the winding. Always match run capacitors within ±5% of the original specification.
What size capacitor do I need for a 1 HP air compressor motor?
A typical 1 HP, 240V air compressor motor requires a start capacitor in the 324–389 µF range (125VAC minimum) and a run capacitor in the 12–15 µF range (370VAC or 440VAC). Because compressors start under load, many 1 HP compressors also utilize a hard-start relay kit paired with the start capacitor to disconnect it from the circuit precisely at 75% of synchronous speed.
Does the physical can size affect the microfarad rating?
Not directly, but they are correlated. Higher µF ratings and higher voltage ratings require more surface area of the internal metallized film or electrolytic foil. Therefore, a 50 µF / 440V run capacitor will physically be larger than a 10 µF / 370V run capacitor. However, two 15 µF / 370V capacitors from different manufacturers might have slightly different can dimensions based on the thickness of the dielectric film and the potting compound used inside the case.
How do I calculate the exact run capacitor size if my motor isn't on the chart?
If you are rewinding a motor or working with a non-standard 50Hz/60Hz hybrid, you can calculate the required run capacitance using the auxiliary winding current. Measure the current ($I$) flowing through the auxiliary winding while the motor is running without a capacitor, then apply the formula: C (µF) = (2652 × I) / V (for 60Hz systems), where $V$ is the applied line voltage. For a deeper dive into the phase-shift mathematics behind this formula, refer to the All About Circuits AC textbook chapter on capacitors. This ensures the auxiliary winding current leads the main winding current by exactly the electrical degrees required for optimal rotating magnetic field generation.
