Free Electrical Tool

Capacitor Calculator for Fan Motors — Run Capacitor µF & Voltage (PSC)

The run capacitor is the most-replaced part on a single-phase AC fan. This tool gives the µF and voltage rating for a permanent-split-capacitor (PSC) motor from its nameplate power, checks an existing capacitor from the auxiliary-winding current, and lists the symptoms that mean it is time to replace it.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-23

Run Capacitor Size

Rule-of-thumb sizing for PSC fan motors; nameplate or manufacturer data always overrides. Capacitors are not user-serviceable on EC/DC fans.

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

How run capacitors work on PSC fan motors

A permanent-split-capacitor motor has two windings 90° apart. The capacitor shifts the phase of the current in the auxiliary winding so the two windings create a rotating field — that is what gives the motor its direction and its torque. Unlike a capacitor-start motor, the capacitor stays in circuit all the time, so it must be an AC-rated, continuous-duty (film, self-healing) type, never an electrolytic start capacitor.

From measurements C [µF] = I_aux ÷ (2π × f × U_cap) × 10⁶
Capacitive reactance X_c = 1 ÷ (2π f C)
Rule of thumb (PSC fan, 230 V 50 Hz) C ≈ 1.0–1.5 µF per 10 W input
Voltage scaling C ∝ 1 / U² // 115 V motor needs ~4× the µF of its 230 V twin
Voltage rating ≥ 1.25 × voltage measured across the capacitor (often 1.3–1.8 × supply)

Typical values for fan motors

Fan / motor	230 V 50 Hz	115 V 60 Hz
Shaded-pole motor (any size)	no capacitor	no capacitor
Compact axial 120–200 mm, 15–40 W	1–2 µF	4–6 µF
External-rotor axial 250–300 mm, 60–120 W	2–4 µF	7.5–12.5 µF
Axial 350–450 mm, 150–300 W	4–8 µF	15–25 µF
Condenser fan 500–630 mm, 400–800 W	8–14 µF	25–45 µF
Forward-curved blower 1/4–1/2 HP	6–12 µF	15–40 µF
Cross-flow / range hood motor 40–100 W	2–4 µF	5–10 µF
Capacitor-run motor 1–2.2 kW	25–60 µF	—

Voltage ratings: 450 V AC for 230 V mains, 370 V AC (or 440 V) for 115 V motors in North America; 500 V for 277 V. Class of operation S2 (10,000 h) or better; 85 °C minimum, 105 °C near hot motors.

Frequently Asked Questions

How do I know what size capacitor my fan motor needs?

Read the motor nameplate or the old capacitor — it states µF and voltage. If unreadable, estimate ≈ 1–1.5 µF per 10 W at 230 V (4× that at 115 V) and confirm by measuring: the fan should reach rated rpm and draw nameplate current.

Can I use a bigger capacitor on my fan motor?

Stay within ±5–10 % of the nameplate value. A larger capacitor increases auxiliary-winding current and heating and can shorten motor life; it does not make the fan "stronger". A higher voltage rating, on the other hand, is always acceptable.

What are the symptoms of a bad fan capacitor?

The fan hums but will not start (and runs if you spin the blade), runs slowly or in the wrong direction, the motor runs hot, the can is swollen or leaking, or the breaker trips. A capacitor that measures more than 10 % below its rating should be replaced.

Do EC or DC fans have a run capacitor?

No. EC and brushless DC fans use electronic commutation; their internal electrolytic capacitors are part of the drive electronics and are not field-replaceable. Replacing an ageing PSC fan with an EC fan eliminates the capacitor failure mode entirely.

Why is the voltage across the capacitor higher than the supply?

The capacitor and the auxiliary winding form a resonant-like circuit; the voltage across the capacitor is typically 1.3–1.8 times the supply voltage. That is why a 230 V motor needs a 450 V capacitor and why measuring it requires care.

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This calculator is free to reference in articles, forums, specifications and course material. Please credit it with a link:

<a href="https://www.longwellfans.com/resources/capacitor-sizing-calculator/">Fan Motor Capacitor Calculator (µF & Voltage)</a> by LONGWELL Fans

APA: LONGWELL Fans. (2026). Fan Motor Capacitor Calculator (µF & Voltage). https://www.longwellfans.com/resources/capacitor-sizing-calculator/

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