HVAC capacitor size calculator

What a run capacitor actually does in an AC system

A run capacitor stores and releases electrical energy that gives the compressor and condenser fan motor the phase-shifted current they need to spin smoothly. Without the capacitor, the motors get a single sine wave of current that cannot produce starting torque, so they hum, draw locked-rotor amps, and either burn out windings within minutes or trip the breaker. A weak capacitor produces the same symptoms in slow motion: motors that run hot, draw more current than rated, and fail prematurely.

Capacitors are the single most failure-prone component on a residential AC unit. They sit in summer heat, take electrical surges, and degrade chemically every year. A typical run capacitor lasts 7 to 12 years. Half of all "AC not cooling" service calls on systems older than 5 years trace back to a capacitor that has dropped below tolerance, even when the system still seems to be starting and running.

Run capacitor MFD by AC tonnage

The MFD rating depends primarily on the compressor size, with a small fan-motor rating attached for dual-run capacitors. Typical residential values:

• 1.5 ton: 25 to 30 MFD compressor, 5 MFD fan
• 2 ton: 35 MFD compressor, 5 MFD fan
• 2.5 ton: 35 to 40 MFD compressor, 5 MFD fan
• 3 ton: 40 to 45 MFD compressor, 5 MFD fan
• 3.5 ton: 45 to 50 MFD compressor, 5 MFD fan
• 4 ton: 55 MFD compressor, 5 MFD fan
• 5 ton: 60 to 70 MFD compressor, 5 MFD fan

These values are typical, not absolute. The actual MFD for any specific unit is stamped on the original capacitor and on the unit's wiring diagram. Always match the original rating exactly within ±6 percent. Going larger than rated will start the compressor harder but also wears the start windings faster. Going smaller will fail to start the compressor reliably.

The MFD calculation formula every electrician should know

When the original capacitor is unreadable and the unit nameplate is gone, you can compute the correct MFD by measuring the start winding current and the voltage between the START and RUN terminals while the motor is operating. The formula is:

MFD = (Start winding amps × 2652) / Capacitor voltage

The constant 2652 comes from the capacitive reactance constant for 60 Hz US power. At 50 Hz (Europe, most of Asia), use 3183 instead. Capacitor voltage in this formula is NOT the line voltage and NOT the capacitor's voltage rating. It is the voltage measured between the S (start) and R (run) terminals while the motor runs at steady-state, typically 200 to 280V.

Worked example: you measure 6.0 amps on the start wire and 240V between S and R. MFD = (6.0 × 2652) / 240 = 66.3 MFD. That motor needs roughly a 65 MFD capacitor. Choose the nearest standard size (typically 60 or 70 MFD).

How to test a run capacitor and read the result

Run capacitors fail two ways: they drift below their rated MFD over time (most common) or they short internally and read open or wildly wrong (less common but catastrophic). Both cases require replacement. The test procedure:

• Turn off and lock out power to the outdoor unit at the disconnect.
• Discharge the capacitor with an insulated resistor or a properly-rated discharge tool. A charged capacitor can deliver a lethal shock minutes after power is removed.
• Remove the wiring from the capacitor terminals (label them first if not color-coded).
• Set a multimeter to capacitance measurement (the µF or MFD scale on any HVAC multimeter).
• For a dual capacitor, test HERM to C for the compressor side, then FAN to C for the fan side. Both readings should fall within ±6% of the stamped values.
• Compare to the nameplate: in tolerance (±6%) means keep it. More than 6% low means weak and degrading. More than 15% low means failed and unable to start the motor reliably.

A weak capacitor will often still let the system run, but it forces the motor to draw 10 to 25 percent more current than rated. That extra current shows up as higher utility bills, hotter motor windings, and faster failure of the compressor or fan motor. Replace weak capacitors at the next service call, not when the system finally fails on the hottest day of the year.

Dual run vs single run capacitor configurations

Most residential AC condensers use a dual-run capacitor: one physical can with three terminals (HERM for compressor, FAN for fan motor, C for common from the contactor) that serves both motors. Dual capacitors are slightly cheaper and easier to wire than two separate capacitors, which is why manufacturers default to them.

Some installations use two single-run capacitors instead: one for the compressor (typically 35 to 70 MFD) and one for the fan motor (typically 5 to 10 MFD). This is common on older equipment, on heat pumps with separate metering for the reversing valve, and on commercial equipment where motors are sized very differently from typical residential. The MFD ratings stay the same as the dual-cap equivalent; the only difference is two cans instead of one.

A failed dual-run capacitor is sometimes replaced with two singles when the original style is hard to source. This is fine electrically as long as the MFD values match, but it requires re-routing the wiring. Get the right replacement from a supply house before improvising.

Voltage rating rules: 370V vs 440V replacement

AC capacitors come in two standard voltage ratings: 370 VAC and 440 VAC. The voltage rating is the maximum voltage the capacitor can safely handle, not the operating voltage. The actual voltage across the capacitor while running is usually 240 to 280V, well below either rating.

The rule is straightforward and matters for safety: you can ALWAYS replace a 370V capacitor with a 440V capacitor. You can NEVER replace a 440V capacitor with a 370V capacitor. Going up in voltage rating is fine because the capacitor has more headroom against transient voltage spikes. Going down in voltage rating puts the capacitor at risk of breakdown during normal operation, leading to a shorted capacitor that can damage the compressor or start a fire.

Many supply houses now stock 440V capacitors exclusively for this reason. If your original was 370V and the replacement on the truck is 440V, install it without hesitation. The MFD rating is what matters for performance; the voltage rating is what matters for safety.

Hard start kits: when adding capacitance helps

A hard start kit is a separate device (typically a start capacitor plus potential relay or PTCR) added to a single-phase compressor to boost starting torque. It does not replace the run capacitor; it supplements it during the first half-second of each start cycle, then drops out. Hard start kits help in three situations:

• Older R-22 compressors that have lost some starting torque from age.
• Compressors on low utility voltage (under 220V at the unit during peak demand) that struggle to overcome short-cycle pressure imbalance.
• Long line set installs where pressure equalization between off-cycles is slow and the compressor restarts under partial load.

Hard start kits do not fix a failing run capacitor. They are a separate add-on for starting performance. If the run capacitor is weak, replace the run capacitor first, then evaluate whether a hard start kit is still needed. Most modern variable-speed compressors do not benefit from hard start kits; consult the manufacturer.

Safety: capacitors store dangerous voltage even when power is off

A run capacitor can hold a charge for minutes after power is removed, sometimes exceeding 400V. That is more than enough to cause cardiac arrest if both terminals are touched simultaneously. Always discharge before handling using a 20kΩ resistor with insulated leads held across the terminals for 5 seconds. Insulated discharge tools designed for HVAC capacitors cost under $20 and are the right tool for the job. If you are not a licensed electrician or trained HVAC technician, hire one.