Motor Capacitor Sizing Calculator

Start capacitor µF = 2 652 × HP / V at 60 Hz (230V) or 5 304 × HP / V at 60 Hz (120V). These are HVAC industry constants. A 1/2 HP 230V motor: µF = 2652 × 0.5 / 230 ≈ 5.76 µF — practical start caps are 80–100 µF because start caps deliver energy, not phase shift. The constant alone gives only a guide.

Start capacitors are non-polarized electrolytic, designed for intermittent duty only — they cool between starts. Their job is to deliver a burst of energy to the motor's start winding, creating starting torque. Run capacitors are film or metallized polypropylene, rated for continuous duty — they stay in the start winding circuit the whole time the motor runs, providing permanent phase shift for improved torque and efficiency.

A dual-value capacitor combines a start cap and a run cap in one can — common in residential HVAC condensers. The can label reads '35 + 5' or '45 + 7.5': first number is the start section µF, second is the run section µF. They share a common terminal but are electrically independent sections.

The 2652 and 5304 constants come from the reactance formula rearranged to solve for capacitance from current. C = I / (2πfV). A 230V motor draws roughly half the current of an equivalent 120V motor, so the constant doubles when voltage halves — 230V uses 2652, 120V uses 5304.

Always use an equal-or-higher voltage rating. The motor operates at line voltage, but capacitors experience voltage up to 1.5× line voltage during startup transients. 230V line motors use 250–330V capacitors. 120V line motors typically use 250V caps. If in doubt, go higher — never lower.

Three failure modes. Too small: motor hums but won't start, or starts slowly and overheats. Too large: motor starts fine but runs hot and draws high amps, damaging the start winding. Wrong voltage: cap fails prematurely or vents violently. Always match both µF and voltage to the motor nameplate.

Use the HVAC under-load test — clamp the start winding lead, measure volts across the cap terminals while the motor runs, compute µF = 2652 × A ÷ V at 60 Hz. Graded: within ±5% = good, 5–10% off = marginal, beyond ±10% = replace. The dedicated under-load calculator on this site runs this formula for you.

Probably. A dead start capacitor is the #1 cause of a motor that hums but does not start. Check it with a capacitance meter or the under-load test. If the cap reads good, investigate the start winding for open circuits, the centrifugal switch (if present), and the main winding for burned connections.