Dual Run vs Single Run Capacitor

One can, two motors

A capacitor gives the motors in your AC a push to start and keeps them running smooth. When it fails, the fan may hum or not spin at all. In many outdoor cooling units, two separate motors need this assistance: the compressor motor and the outdoor fan motor.

A dual run capacitor combines two capacitors inside one single metal can. This design saves space inside the tight electrical cabinet of an air conditioner. It allows the unit to run both the outdoor fan motor and the heavy compressor motor without needing two separate parts.

Terminals: HERM, FAN, and C

The easiest way to tell the difference between these parts is by counting and reading the terminals on top. A dual run capacitor has three terminals, while a single run capacitor has two terminals.

The three terminals on a dual run capacitor are labeled with stamped letters on the metal top:

• C: This stands for Common. It connects to the line voltage coming from the contactor.
• HERM: This stands for hermetic compressor. It connects to the compressor motor winding.
• FAN: This connects directly to the outdoor condenser fan motor.

Single run uses

A single run capacitor supports just one motor. It has only two terminal sets on top. Because it connects to only one motor winding, it does not need separate HERM and FAN labels.

You will often find single run capacitors paired with indoor blower motors inside a furnace or air handler. Some outdoor units also use them if the system was engineered with separate parts for the fan and the compressor from the factory. For example, a single run part might carry a rating such as 10 µF, whereas a dual run part lists two ratings together, such as 40 + 5 µF.

Can you swap one for two singles

A dual run capacitor is electrically equal to two single run capacitors wired together at a common terminal. In an emergency, some technicians might wire two single run capacitors to do the work of one dual run capacitor.

For homeowners, replacing like with like is the recommended choice. Fitting two single capacitors into a space made for one can is difficult. The factory mounting bracket usually holds only one can. Adding a second can requires extra mounting hardware and extra jumper wires. Swapping an old dual run part for a fresh dual run part keeps the installation neat and prevents wiring mistakes.

What to match on your replacement

When you replace any capacitor, check the printed label on the old part. You must match the electrical specifications to keep your equipment running safely.

Follow these matching rules when picking your replacement:

• µF (MFD): Capacitance must be the same as the old part. For a dual run capacitor, both numbers must match, like 40 + 5 µF. A single run unit will have just one number, like 10 µF.
• Volts: The voltage rating must be the same or higher. A 440 V capacitor can replace a 370 V one, but you must never put a 370 V part in place of a 440 V part.
• Shape: Capacitors usually come in round or oval shapes. Round and oval capacitors with the same ratings do the same electrical job. Pick the shape that fits your unit bracket.