Capacitors are used in air conditioners, heat pumps, furnace blowers, refrigeration systems, compressors, pumps, fans, power tools, appliances and many other pieces of motor-driven equipment.

Although many capacitors look nearly identical, their electrical ratings, operating purpose and physical dimensions can be very different. A replacement capacitor should never be selected by appearance alone.

Two silver oval capacitors can have different MFD ratings, voltage ratings, tolerances, terminal arrangements and operating duties. Installing the wrong capacitor may prevent the motor from operating correctly and can contribute to overheating, high current draw or additional equipment damage.

This guide explains the major capacitor types, the numbers printed on a capacitor label, common failure symptoms and the information a parts supplier needs to locate the correct replacement.

What Is a Capacitor?

A capacitor is an electrical component that stores energy in an electric field and releases that energy when required by a circuit.

In motor applications, a capacitor can help create the phase shift and torque required to start the motor or keep it running efficiently. Capacitors are also used in electronic circuits for filtering, timing, voltage smoothing, noise reduction and power-supply stabilization.

Depending on the equipment design, a capacitor may help:

• Start an electric motor
• Support a motor while it continues to operate
• Improve motor efficiency and power factor
• Support compressor operation
• Support condenser fan or blower motor operation
• Smooth voltage in electronic circuits
• Filter electrical noise
• Stabilize a power supply
• Control timing within an electronic circuit
• Protect or support sensitive electronic components

A start capacitor and a run capacitor are designed for different operating duties. They are not automatically interchangeable even when the parts have similar ratings or housings.

What Does “AC Capacitor” Mean?

The phrase AC capacitor is commonly used in two different ways.

Air Conditioner Capacitor

Many customers use “AC capacitor” to describe a capacitor installed in a central air conditioner, heat pump, condenser unit, compressor circuit or outdoor fan motor circuit.

Alternating-Current Capacitor

The term can also describe a capacitor designed to operate in an alternating-current electrical circuit. Motor run capacitors are a common example.

Searching only for “AC capacitor” may return hundreds of incompatible products. A better search includes the complete MFD rating, voltage, capacitor type, original part number or equipment model number.

Start, Run and Dual Run Capacitors

What Is a Hard-Start Kit?

A compressor hard-start kit is not simply a larger run capacitor. It is an application-specific starting device that may contain a start capacitor and relay or an electronic control.

Hard-start components should only be installed when approved for the compressor and equipment application. Installing an unapproved start device can place additional electrical or mechanical stress on the system.

How to Read a Capacitor Label

Before ordering a replacement, photograph every side of the original capacitor. The label may contain several numbers, and the largest or most visible number is not always the equipment manufacturer's service part number.

Important capacitor-label information includes:

• Capacitance: shown in µF, uF, MFD or a dual value such as 45/5 µF
• Voltage rating: commonly shown as VAC
• Tolerance: often shown as a percentage
• Frequency: commonly 50 Hz, 60 Hz or 50/60 Hz
• Operating temperature: shown as a temperature range or climate category
• Manufacturer name: the capacitor brand or original equipment supplier
• Part number: may be an OEM, motor-manufacturer or capacitor-manufacturer number
• Terminal markings: especially important on dual run capacitors
• Certification markings: may identify applicable safety or component certifications
• Physical dimensions: diameter, width, length and height

What Do MFD, µF and uF Mean?

MFD, µF and uF are commonly used to indicate microfarads in motor-capacitor listings.

Examples include:

• 5 MFD
• 7.5 µF
• 10 uF
• 35/5 MFD
• 45+5 µF

The microfarad rating is one of the most important replacement specifications because it determines the capacitor's electrical effect in the motor circuit.

A motor designed for a 10 µF run capacitor should not be assumed to operate correctly with a 7.5 µF or 12.5 µF capacitor. The correct value is determined by the motor or equipment manufacturer, not by choosing the closest capacitor available.

What Does a Dual Rating Mean?

A dual run capacitor contains two capacitance values.

In a typical HVAC application, the larger value may support the compressor circuit while the smaller value supports the condenser fan motor. The wiring diagram, motor information and capacitor markings must still be followed.

Does a Bigger MFD Rating Make the Motor Stronger?

No. A larger MFD value is not an upgrade. Using an incorrect rating can change motor current, operating temperature, torque and overall performance.

Understanding Capacitor Voltage Ratings

The voltage rating indicates the electrical stress the capacitor is designed to withstand under its specified operating conditions.

Motor capacitors are often marked with ratings such as:

• 110 VAC
• 125 VAC
• 250 VAC
• 330 VAC
• 370 VAC
• 440 VAC
• 450 VAC

Do not select a capacitor using only the equipment supply voltage. A 240-volt appliance, motor or air conditioner may use a capacitor with a different component voltage rating.

The capacitance and voltage numbers describe different electrical properties. A capacitor with the correct voltage but the wrong MFD value is not the correct replacement.

Round vs. Oval Capacitors

Motor capacitors are commonly produced in round or oval housings. The shape does not determine the electrical rating, but it can affect installation and mounting.

When comparing a replacement, confirm:

• Overall height
• Can diameter or width
• Mounting-bracket compatibility
• Available clearance around wires and panels
• Terminal orientation
• Number of terminal tabs
• Whether the original mounting strap can be reused

A replacement may be electrically correct but physically too tall, too wide or incorrectly shaped for the original mounting location.

A round capacitor may replace an oval capacitor only when the electrical specifications, application, terminals, dimensions and mounting arrangement are all suitable.

Common AC and Motor Capacitor Sizes

The following are examples of ratings commonly searched by customers. This list does not confirm compatibility with any particular air conditioner, furnace, motor, pump or appliance.

Other dual run capacitors may use a different fan section, including 3, 4, 6, 7.5 or 10 µF. Both capacitance values must be verified.

Signs of a Weak or Failed Capacitor

Capacitor symptoms depend on the equipment and the capacitor's purpose. A motor or compressor can also develop electrical or mechanical faults that create similar symptoms.

Possible capacitor-related symptoms include:

• A compressor or motor that hums but does not start
• An outdoor fan that fails to start or operates inconsistently
• A blower motor that struggles to reach operating speed
• A pump motor that repeatedly trips its overload
• Intermittent starting
• Longer-than-normal startup
• Reduced cooling or airflow
• Short cycling
• Higher motor temperature
• Unexpected shutdowns
• Visible swelling or distortion of the capacitor housing
• Leaking fluid or heavy corrosion
• Burned, loose or overheated terminals

Visual condition alone cannot confirm whether a capacitor is operating correctly. A capacitor may look normal but still test outside its specified capacitance tolerance.

A swollen or leaking capacitor can also be evidence of another problem, including excessive heat, voltage stress, damaged motor windings, failed controls or repeated hard starting.

Why Do Capacitors Fail?

• High ambient temperature
• Poor airflow around electrical components
• Voltage irregularities
• Frequent starting and stopping
• A motor or compressor that is mechanically difficult to start
• Loose, burned or corroded connections
• Incorrect replacement ratings
• Excessive vibration
• Moisture or corrosion
• Normal component aging
• Manufacturing or material defects

Replacing the capacitor may restore operation when it is the only failed component. Repeated capacitor failures can indicate an unresolved motor, compressor, control, wiring or operating-condition problem.

How Is a Capacitor Tested?

A qualified technician can evaluate a capacitor using an appropriate capacitance meter, multimeter or specialized capacitor tester. The measured value is compared with the capacitor's rated capacitance and tolerance.

A complete diagnosis may also include checking:

• Operating voltage
• Motor current
• Terminal and wire condition
• Contactors, relays and switches
• Motor or compressor condition
• Signs of overheating
• The manufacturer wiring diagram
• The condition of the electrical enclosure

Because capacitors can retain hazardous electrical energy, testing should not be attempted without proper training, test equipment and the manufacturer's approved safety procedure.

Where Are Capacitors Used?

Central Air Conditioners and Heat Pumps

Outdoor condenser units commonly use run or dual run capacitors for the compressor and condenser fan motor. Some systems use two separate capacitors instead of one combined dual run capacitor.

Furnace and Air-Handler Blowers

Permanent-split capacitor blower motors may use a separate motor run capacitor. The required value is often printed on the motor label or listed in the equipment service information.

Refrigerators and Freezers

Refrigeration systems may use start capacitors, run capacitors, relays, overload protectors and other start devices. These components must be matched to the compressor and equipment design.

Pool and Spa Pumps

Pool and spa motors may use a start capacitor, run capacitor or both. The motor model, horsepower, voltage and existing capacitor information should be verified before ordering.

Well Pumps and Water Systems

Pump motors may use capacitors inside the motor housing or inside a separate control box. Model number, horsepower, voltage and motor-manufacturer information may all be required.

Washers, Dryers and Other Appliances

Appliance motors, electronic controls and power supplies may contain capacitors. A capacitor used in one washer, dryer, dishwasher or refrigerator should not be selected as a replacement simply because the ratings look similar.

Range Hoods, Exhaust Fans and Ventilation Equipment

Fan motors often use smaller-value run capacitors. These may be rectangular, cylindrical or wire-lead components rather than the larger metal-can style used in central air conditioners.

Microwave Ovens

Variable-Speed and Inverter Equipment

Some newer HVAC and appliance systems use electronically commutated motors, variable-frequency drives or inverter control boards instead of a traditional external motor run capacitor.

Do not assume that every motor problem requires a conventional capacitor. Search using the complete equipment model number and confirm the manufacturer's parts information.

Capacitor vs. Contactor, Relay and Overload

A failed capacitor is only one possible reason that a motor or compressor will not operate.

Replacing a capacitor without confirming the cause may delay the correct repair or contribute to another component failure.

How to Find the Correct Replacement Capacitor

Method 1: Search the Original Part Number

Enter every letter, number, dash and suffix printed on the original capacitor. Capacitor manufacturers and equipment manufacturers may update old part numbers, so a current replacement may carry a different number.

Method 2: Search the Equipment Model Number

The complete equipment model number is often the best way to confirm the capacitor specified for an air conditioner, furnace, appliance, pump or other machine.

Look for the model number on the equipment identification plate. Do not use a partial family number when a complete suffix is available.

Method 3: Search the Motor Model Number

Replacement motors may require a different capacitor than the motor originally installed at the factory. If the motor has already been changed, use the capacitor rating approved for the currently installed motor.

Method 4: Search the Complete Electrical Rating

Include all relevant information:

• Capacitance or MFD rating
• Voltage rating
• Start, run or dual run type
• Round or oval shape
• Approximate dimensions
• Terminal markings
• Equipment application

Common Capacitor-Buying Mistakes

Ordering by Appearance

Many capacitors use similar metal housings. Compare the complete label, not just the shape, colour or terminal arrangement.

Matching Only One Number

A dual run capacitor has two capacitance values. Both values must be verified.

Confusing MFD With Voltage

Capacitance and voltage are different specifications. Matching one number does not compensate for a mismatch in the other.

Using the Outdoor Model Number for an Indoor Blower Part

Split HVAC systems normally have separate indoor and outdoor model numbers. Use the model number from the section containing the capacitor.

Using the Original Motor Specification After the Motor Was Replaced

A universal or replacement motor may require a different capacitor from the original factory motor. Follow the currently installed motor's approved specification.

Assuming a Start and Run Capacitor Are Interchangeable

Start and run capacitors are designed for different operating duties. Use the type specified by the equipment or motor manufacturer.

Ignoring Physical Dimensions

Confirm that the replacement can be securely mounted without contacting wiring, panels or moving components.

Frequently Asked Questions About AC and Motor Capacitors

Is MFD the same as µF?

In motor-capacitor listings, MFD, µF and uF are commonly used to represent microfarads. Always confirm the complete label and application.

What does 35/5 mean on an AC capacitor?

It normally identifies a dual capacitor containing a 35 µF section and a 5 µF section. The voltage, tolerance and equipment application must still be checked.

Can I replace a 40/5 capacitor with a 45/5?

Not unless the motor or equipment manufacturer specifically approves that value. The larger capacitance section is different and may change compressor operation.

Can I replace a 5 µF capacitor with a 7.5 µF capacitor?

Do not make that substitution unless it is specifically approved for the motor. Run-capacitor capacitance should match the value specified by the motor or equipment manufacturer.

Is a 370 VAC capacitor the same as a 440 VAC capacitor?

No. The voltage ratings are different component specifications. Use the approved equipment, motor or replacement-part information to confirm the required rating.

Can a round capacitor replace an oval capacitor?

Only when the electrical specifications, application, terminals, dimensions and mounting arrangement have all been confirmed.

Can a capacitor be bad without bulging?

Yes. A capacitor can lose capacitance or develop an internal fault without obvious external damage.

Will a bad capacitor stop an AC compressor from starting?

A weak or failed capacitor can contribute to a compressor that hums, struggles to start or does not start. Similar symptoms can also be caused by wiring, contactor, voltage, control, overload or compressor problems.

Why does my AC fan run but the compressor does not?

One section of a dual run capacitor may fail while the other section continues operating. The cause could also be another compressor, electrical or control problem.

Why does the compressor run but the outdoor fan does not?

The fan section of the dual capacitor may be weak or failed, but the fan motor, wiring, control and mechanical condition should also be checked.

Can I determine the capacitor size from the air conditioner's tonnage?

Not reliably. Two systems with the same nominal cooling capacity may use different compressors, fan motors and capacitor ratings.

Does every air conditioner use a dual run capacitor?

No. Some systems use separate capacitors, electronically commutated motors, variable-speed drives or inverter electronics.

Should a capacitor be replaced when the motor is replaced?

Follow the replacement motor's instructions. A replacement motor may require a specific capacitor even when the old motor used a different value.

How do I know which terminal is which on a dual capacitor?

Dual run capacitors are commonly marked C, FAN and HERM. The equipment wiring diagram and manufacturer's service procedures must be followed. Do not rely only on terminal position or wire colour.

What does HERM mean on a capacitor?

HERM commonly identifies the hermetic compressor section of a dual run capacitor.

What does C mean on a dual capacitor?

C commonly identifies the common terminal. Correct wiring should be confirmed using the equipment diagram.

What information should I provide when requesting a capacitor?

Provide the full capacitor label, equipment brand, complete model number, motor number, MFD rating, voltage, capacitor type, shape and dimensions.