Capacitor failure is the number one reason for AC no-cool calls in summer. Here is everything you need to know about sizing and replacing them.
Reading Capacitor Ratings
Every capacitor has three key ratings printed on it:
- **Microfarads (MFD or uF)** — The capacitance value. Must match exactly or be within 5% of the original.
- **Voltage (VAC)** — The voltage rating. You can go higher but never lower. A 440V cap can replace a 370V cap, but not the other way around.
- **Frequency (Hz)** — Almost always 50/60Hz. Match what is on the original.
Single vs Dual Run Capacitors
A **single** capacitor has two terminals and serves one motor (compressor OR fan). A **dual** capacitor has three terminals (C, FAN, HERM) and serves both the compressor and condenser fan motor in one package.
Common Dual Capacitor Sizes
- 35/5 MFD 440V — Very common in 2-3 ton units
- 40/5 MFD 440V — Common in 3-3.5 ton units
- 45/5 MFD 440V — Common in 4-5 ton units
- 55/5 MFD 440V — Larger commercial units
How to Test a Capacitor
Use a multimeter with capacitance mode. Discharge the capacitor first by shorting the terminals with an insulated screwdriver. Read the MFD value. If it is more than 5% below the rated value, replace it.
Common Mistakes
- Installing a lower-voltage capacitor (will fail quickly)
- Swapping the FAN and HERM wires on a dual capacitor (motor will try to run backwards)
- Not discharging before handling (painful shock)
Where to Buy
Search the exact MFD and voltage on PartHawk. We check Johnstone Supply, SupplyHouse, Grainger, Amazon, and more. Prices range from $8 on Amazon to $35 at a supply house — same exact part.