Capacitor Size Calculator for Motors — Start & Run Capacitors
Size the correct start capacitor and run capacitor for your single-phase motor from its horsepower (HP) or kilowatts (kW). Below you will also find a run capacitor µF chart by HP, a start capacitor sizing guide, voltage-rating rules, and safe replacement steps.
Motor Capacitor Size Calculator
Motor Capacitor Size Chart (µF by HP)
Industry-standard reference values for single-phase motors at ~230 V / 60 Hz. For sizing between rows, the calculator interpolates.
| Motor HP | kW | Start capacitor (µF) | Run capacitor (µF) |
|---|---|---|---|
| ¼ | 0.19 | 59–71 | 5 |
| ⅓ | 0.25 | 70–84 | 7.5 |
| ½ | 0.37 | 88–108 | 10 |
| ¾ | 0.56 | 108–130 | 12.5 |
| 1 | 0.75 | 145–174 | 15 |
| 1½ | 1.12 | 161–193 | 20 |
| 2 | 1.49 | 189–227 | 25 |
| 3 | 2.24 | 270–324 | 40 |
| 5 | 3.73 | 378–454 | 55 |
| 7½ | 5.60 | 540–648 | 80 |
| 10 | 7.46 | 800–960 | 100 |
Source: compiled from common NEMA single-phase motor capacitor application charts. Always confirm against your motor's nameplate or manual — OEM spec wins over any generic chart.
How the sizing works
- Run capacitor: stays in circuit and corrects phase angle. Rule of thumb is roughly 8–15 µF per HP for capacitor-run motors, increasing with load. Going too large causes excess current and overheating; too small causes poor torque and hum.
- Start capacitor: briefly in circuit during starting, typically 2–4× the run value (or per the chart above). Undersizing causes failed starts; gross oversizing stresses windings.
- Voltage rating rule: capacitor rated VAC must be ≥ the supply voltage, with margin: use ≥ 370 VAC for 208/230 V supplies, ≥ 250 VAC for 115 V, and ≥ 600 VAC for 460 V motors. A higher VAC rating is always acceptable; a lower one is not.
- Capacitance tolerance: replacements should match within ±10% (run) or stay inside the start range. µF must match — physical size does not matter.
Frequently asked questions
How do I size a start capacitor for a motor?
Use the motor's HP and the chart above (e.g. a 1 HP motor typically needs 145–174 µF). Alternatively, start capacitors are usually 2–4× the run capacitor value. The safest source is the motor nameplate or manual; this calculator gives the standard industry recommendation when the nameplate is unreadable.
How do I size a run capacitor for a motor?
Match the original µF within ±10%. As a rule of thumb, capacitor-run single-phase motors need roughly 8–15 µF per HP — for example 15 µF for 1 HP, 40 µF for 3 HP, 55 µF for 5 HP. The run capacitor voltage rating must be ≥ the supply voltage (370+ VAC for 230 V supplies).
Can I use a higher µF capacitor than specified?
For start capacitors, a value within the specified range is fine — slightly higher is tolerable but don't double it. For run capacitors, stick within ±10% of the specified µF; a much larger run capacitor raises winding current and can burn out the motor.
Can I use a higher voltage rating capacitor?
Yes — the voltage rating is a minimum. A 440 VAC capacitor can replace a 370 VAC one of the same µF, but never the reverse. Higher VAC is always safe; lower VAC is dangerous.
Do 3-phase motors need start or run capacitors?
Normally no. Three-phase motors running on true 3-phase power need no capacitors. A capacitor is only needed when running a 3φ motor from single-phase supply via a static phase converter — select the "phase converter" option above for that case.
What is the difference between a start and a run capacitor?
A start capacitor gives high starting torque for a few seconds and then drops out via a centrifugal switch or relay. A run capacitor stays energized the whole time the motor runs, improving efficiency and torque. They are not interchangeable: start caps are electrolytic with a short duty cycle; run caps are oil-filled for continuous duty.
What capacitor size for a 1 HP motor?
A typical 1 HP single-phase motor uses a 145–174 µF start capacitor and a 15 µF run capacitor at 230 V / 60 Hz.
What capacitor size for a 2 HP motor?
A typical 2 HP single-phase motor uses a 189–227 µF start capacitor and a 25 µF run capacitor at 230 V / 60 Hz.
Does capacitor size change between 50 Hz and 60 Hz?
Slightly. At 50 Hz, motor current is a little higher for the same power, so run capacitor values are often ~10–20% higher than 60 Hz charts. This calculator flags the 50 Hz case; always verify with the motor manufacturer for 50 Hz duty.
How can I tell if my motor capacitor is bad?
Symptoms: humming but not starting, slow start, tripped breakers, or a bulged/leaking can. Confirm with a multimeter that has a capacitance mode — after safely discharging, a reading far below the marked µF (or open/short) means replacement. Replace like-for-like µF.
How do I safely discharge a motor capacitor?
Turn power off and lock it out, then discharge across the terminals with an insulated resistor (e.g. 20 kΩ, 5 W) for several seconds and verify with a meter. Never short terminals with a screwdriver — the arc can damage the capacitor and injure you.
What is a dual run capacitor?
A dual run capacitor is one can containing two capacitors (e.g. 40 + 5 µF) sharing a common terminal, common in air conditioners for compressor + fan. Size each section like a normal run capacitor; the common terminal must be wired correctly.
Can I replace a start capacitor with a run capacitor of the same µF?
No. Start capacitors are electrolytic and rated for intermittent duty only — a run-rated capacitor used as a start cap is oversized physically and wasteful, and a start-rated capacitor used as a run cap will overheat and fail, possibly explosively. Match the duty type.
Why did my motor capacitor fail?
Common causes: age/heat drying out the dielectric, voltage spikes above rating, running with a stuck centrifugal switch (start caps), or wrong µF from a previous replacement. Chronic failure points to an underlying motor or supply issue — have it checked rather than just swapping cans.
Do ceiling fans and compressors use the same sizing rules?
Ceiling fans use small permanent-split run capacitors (typically 1–6 µF) chosen for speed control, not the HP chart above. Compressors (HVAC) use run capacitors sized by the manufacturer (e.g. 25–80 µF) — always replace with the exact marked µF. The HP chart here is for general-purpose single-phase motors.
Is this calculator a substitute for the motor nameplate?
No. The nameplate or OEM manual is authoritative. This tool gives standard industry recommendations for when the plate is missing or unreadable, and for estimating replacements. For critical or large motors, confirm with the manufacturer or a technician.
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