AC Capacitor Calculator – MFD Test & Sizing Tool

To find a run capacitor’s actual capacitance while it’s operating, clamp an amp reading and a voltage reading across the same two terminals (C to HERM, or C to FAN) while the motor runs, then use this AC capacitor calculator with µF = (Amps × 2652) ÷ Volts for 60 Hz systems (US/Canada), or µF = (Amps × 3183) ÷ Volts for 50 Hz systems (UK/Australia/Gulf). Compare the result to the rated µF on the label — most manufacturers allow ±6% tolerance before the capacitor should be replaced. This AC Capacitor Calculator works for both 50 Hz and 60 Hz electrical systems.

Calculator  ·  How It Works  ·  Worked Examples  ·  MFD Reference  ·  Safety Warning  ·  FAQ

Capacitors store an electrical charge even after power is disconnected and can deliver a dangerous or lethal shock. Always discharge a capacitor with an insulated-handle resistor tool before handling it, and never touch the terminals directly. See OSHA’s guidance on capacitor hazards before servicing any capacitor.

Testing a capacitor with the power off only proves it isn’t fully dead — it will not catch a capacitor that has weakened under real operating load, which is the most common failure mode. The under-load method used by this capacitor calculator reads the capacitor’s behaviour while current is actually flowing through it.

With the system running and the electrical panel cover off (leads still connected), clamp your amp meter around only the wire running from the C (common) terminal to the HERM or FAN terminal.

With a multimeter, read the AC voltage across the same two terminals (C to HERM, or C to FAN) while the motor runs.

The formula comes from basic capacitive reactance: for a capacitor on an AC line, current draw I = V × 2πfC, which rearranges to C = I ÷ (2πfV). Converting farads to microfarads and solving the constant at 60 Hz gives 1,000,000 ÷ (2π × 60) ≈ 2,652 — the constant used throughout North America. At 50 Hz (UK, Australia, most of the Gulf), the same math gives 1,000,000 ÷ (2π × 50) ≈ 3,183. This is standard AC circuit theory referenced in motor and capacitor standards published by NEMA (National Electrical Manufacturers Association) for motor-run capacitors.

If you already have a direct µF reading from a capacitance meter with the power off, use the Tolerance Check mode above instead — a healthy capacitor’s under-load result and its power-off reading should agree closely — either way, this AC Capacitor Calculator gives you a fast, accurate result.

Capacitors store energy even after the unit is switched off and the disconnect is pulled. Before touching any capacitor terminal:

• Turn off and lock out power at the disconnect.
• Discharge the capacitor using an insulated-handle resistor tool (never a screwdriver shorting the terminals directly).
• Verify zero voltage with a multimeter before handling.
• Wear appropriate PPE and follow your local electrical code and employer safety procedures.

This calculator is an educational and diagnostic reference tool. It does not replace training, your employer’s lockout/tagout procedure, or local electrical code requirements.

The compressor and condenser fan motor determine capacitor size, not the tonnage of the system — two 3-ton units from different manufacturers can call for different µF. Always match the motor nameplate or the existing dual-capacitor label. The reference table above gives typical ranges only.

Most OEM specs allow ±6%. Some technicians use a tighter ±5%, and some allow ±10% to account for meter error. Use whichever tolerance your manufacturer documentation specifies.

Yes — a higher voltage rating (for example, 440V instead of 370V) is safe to use. Never install a capacitor with a lower voltage rating than the original.

Yes, as long as both single capacitors match or exceed the original’s voltage rating and are wired in parallel from the shared C (common) terminal — see the Dual Split mode above.

Small differences (a few percent) are normal due to meter accuracy, connection resistance, and temperature. A healthy capacitor’s under-load and power-off readings should still land close to the rated value and within tolerance.

Yes, when done correctly with the panel cover removed and clamp-on tools used from outside the live compartment — you are measuring, not touching, live terminals. Always follow standard electrical safety practice and your local code.