AC Capacitor Guide: Air Conditioner Capacitor Selection & Replacement

As a PCB engineer who’s spent fifteen years working with motor control circuits and HVAC systems, I’ve seen plenty of air conditioners fail because of one overlooked component: the AC capacitor. That little cylindrical device sitting in your outdoor condenser unit does more work than most homeowners realize, and when it goes bad, your entire cooling system grinds to a halt.

This guide walks you through everything you need to know about selecting and replacing AC capacitors. Whether you’re a homeowner trying to diagnose a problem, a DIY enthusiast tackling your first HVAC repair, or a technician brushing up on fundamentals, I’ve compiled the practical knowledge that actually matters when you’re standing in front of a non-functioning air conditioner on a 95°F day.

What Is an AC Capacitor and Why Does It Matter?

An AC capacitor is an electrical component that stores and releases energy to help start and run the motors in your air conditioning system. Think of it as a rechargeable power boost that gives your compressor and fan motors the initial jolt they need to overcome inertia and start spinning.

Unlike batteries that provide steady, continuous power, a Capacitor charges up quickly and dumps its stored energy in a powerful burst. This characteristic makes capacitors ideal for motor applications where you need high torque at startup but steady, lower power during operation.

Your air conditioner typically relies on one or more capacitors to perform two critical functions:

Starting the motors: When you turn on your AC, the compressor and fan motors need significantly more power to start moving than they do to keep running. The capacitor provides this extra starting torque by creating a phase shift in the motor’s electromagnetic field, effectively giving the rotor a push in the right direction.

Keeping the motors running efficiently: Once running, the capacitor continues to optimize the phase relationship between voltage and current in the motor windings. This phase correction improves motor efficiency and reduces the current draw from your electrical supply.

Without a functioning capacitor, your AC motor might hum without starting, overheat trying to start, or run inefficiently while consuming excess electricity. In worst-case scenarios, a failed capacitor can damage your compressor, which costs substantially more to replace.

Types of AC Capacitors in Air Conditioning Systems

Understanding the different types of AC capacitors helps you identify what you’re working with and select the correct replacement.

Start Capacitors

Start capacitors provide the initial high-torque boost needed to get your compressor motor spinning from a dead stop. They’re designed for intermittent duty, staying in the circuit only for a few seconds until the motor reaches about 75% of operating speed.

Key characteristics of start capacitors include higher microfarad ratings (typically 70-400 µF or higher), lower voltage ratings (usually 125V, 250V, or 330V), black plastic housings, and connection through a potential relay that takes them out of the circuit once the motor starts.

Start capacitors aren’t continuously energized, so they tend to fail less frequently than run capacitors. However, when they do fail, your compressor won’t start at all.

Run Capacitors

Run capacitors remain in the circuit continuously during motor operation. They maintain optimal phase angle between voltage and current, improving motor efficiency and performance throughout the cooling cycle.

Run capacitors typically have lower microfarad ratings (2.5-80 µF), higher voltage ratings (370V or 440V AC), metal (aluminum or steel) housings, and oval or round shapes.

Because run capacitors operate continuously, they experience more wear and tend to fail more frequently than start capacitors. A failing run capacitor might let your motor start but cause it to run hot, inefficiently, or with reduced cooling capacity.

Dual Run Capacitors

Most residential air conditioners use dual run capacitors, which combine two run capacitors into a single package. This design saves space and simplifies wiring by providing separate capacitance for both the compressor motor and the condenser fan motor.

Dual capacitors have three terminals marked as C (Common), HERM or COMP (for the hermetic compressor), and FAN (for the fan motor). You’ll see two microfarad ratings on the label, such as 40/5 µF, where the larger value powers the compressor and the smaller value powers the fan.

The following table summarizes the key differences between these capacitor types:

Feature	Start Capacitor	Run Capacitor	Dual Run Capacitor
Duty Cycle	Intermittent (few seconds)	Continuous	Continuous
Typical µF Range	70-400+ µF	2.5-80 µF	Varies (e.g., 40/5 µF)
Voltage Ratings	125V, 250V, 330V	370V, 440V AC	370V, 440V AC
Housing Material	Black plastic	Aluminum or steel	Aluminum or steel
Shape	Round	Round or oval	Round or oval
Number of Terminals	2	2	3 (C, HERM, FAN)

Understanding AC Capacitor Specifications

Selecting the right replacement capacitor requires matching several critical specifications. Getting these wrong can damage your equipment or create safety hazards.

Capacitance Rating (Microfarads)

The microfarad (µF) rating, sometimes shown as MFD, indicates how much electrical charge the capacitor can store. This is the most critical specification to match when selecting a replacement.

Always use a capacitor with the same microfarad rating as the original. Using a higher or lower value changes the motor’s operating characteristics and can cause overheating and premature motor failure, increased noise and vibration, reduced cooling efficiency, and higher energy consumption.

Most run capacitors have a tolerance of ±5% to ±10%. If your capacitor measures more than 10% below its rated value, it needs replacement. For example, a 45 µF capacitor should measure between 40.5 µF and 49.5 µF to be considered within acceptable tolerance.

Voltage Rating

The voltage rating indicates the maximum voltage the capacitor can safely handle. Run capacitors are typically rated at 370 VAC or 440 VAC.

An important rule applies here: you can use a capacitor with a higher voltage rating than required, but never use one with a lower voltage rating. Using a 440V capacitor in a 370V application is fine and may actually extend the capacitor’s life. Using a 370V capacitor in a 440V system will cause premature failure and potential hazards.

Voltage affects capacitor life exponentially. Studies show that operating a capacitor just 10 volts above its rating can reduce lifespan by 20%, while operating 30 volts above can cut life by 75%.

Frequency Rating

AC capacitors are rated for specific operating frequencies, typically 50/60 Hz for residential applications. This matches standard power grid frequencies worldwide. Make sure any replacement capacitor is rated for your local power frequency.

Terminal Type and Configuration

Capacitors use different terminal styles for connections. The most common include push-on tab terminals (usually ¼” spade connectors), screw terminals, and wire leads.

Match the terminal type to your existing wiring to avoid adapter hassles. While you can use crimp-on adapters to convert between styles, maintaining the same terminal type ensures secure connections and easier installation.

Physical Dimensions

While electrical specifications take priority, physical size matters for installation. Capacitors must fit within the compartment space in your outdoor unit. You can usually substitute oval capacitors for round ones (and vice versa) if space permits, but verify clearance before purchasing.

The following table summarizes key specifications and their matching requirements:

Specification	Must Match Exactly?	Can Go Higher?	Notes
Capacitance (µF)	Yes	No	Match within ±5-10% tolerance
Voltage (VAC)	Minimum required	Yes (recommended)	Higher voltage extends life
Frequency (Hz)	Yes	N/A	50/60 Hz standard
Terminal Type	Preferred	N/A	Adapters available if needed
Physical Size	Must fit	N/A	Verify clearance

Signs Your AC Capacitor Is Failing

Recognizing the symptoms of a bad AC capacitor early can prevent more expensive damage to your compressor or fan motors. Here are the warning signs I’ve learned to watch for:

Performance Symptoms

AC won’t start: If your outdoor unit hums but doesn’t start, the capacitor likely can’t deliver the starting boost your motors need. You might hear clicking from the contactor but see no fan movement.

Hard starting or delayed startup: The motor struggles to start, sometimes taking several attempts or cycling the thermostat multiple times before the unit runs.

Weak airflow or warm air: A failing run capacitor reduces motor efficiency. Your fan might spin slowly, or the compressor might not run properly, resulting in reduced cooling.

Short cycling: The air conditioner starts and stops frequently, running for only brief periods before shutting down. This strains the entire system and increases energy consumption.

Rising energy bills: As capacitor efficiency drops, motors draw more current to compensate. A sudden increase in your electric bill during cooling season often indicates capacitor problems.

Physical Signs

Bulging or swollen top: Internal pressure buildup causes the capacitor’s metal case to bulge, especially at the top where the safety vent is located. Any bulging means immediate replacement is needed.

Leaking fluid: Electrolyte leaking from the capacitor indicates internal failure. The oily substance you see is the dielectric fluid that should remain sealed inside.

Burn marks or melting: Scorch marks on the capacitor or surrounding wiring indicate overheating and potential fire hazard. Replace immediately and inspect related components.

Physical cracks: Any cracks in the capacitor housing compromise its integrity and safety. Don’t attempt to continue using a cracked capacitor.

Auditory Signs

Humming without starting: A loud hum from the outdoor unit without motor rotation typically indicates the capacitor can’t provide starting torque.

Clicking sounds: Repeated clicking from the contactor while the motor fails to start suggests capacitor failure. The contactor is trying to start the motor, but the capacitor can’t deliver the needed power.

How to Test an AC Capacitor

Testing your AC capacitor requires basic electrical safety knowledge and a multimeter with capacitance testing capability. If you’re uncomfortable working with electrical components, call a professional. Capacitors store dangerous charges even when power is off.

Safety Precautions

Before testing any capacitor, shut off power at the circuit breaker and verify power is off with a non-contact voltage tester. Discharge the capacitor using an insulated screwdriver to short across terminals. Wait 30 seconds and discharge again to be sure. Wear insulated gloves and safety glasses and never touch terminals with bare hands.

Testing Procedure

Step 1: Disconnect power at the breaker and verify it’s off at the unit.

Step 2: Remove the access panel from your outdoor condensing unit. The capacitor is usually located near the compressor and looks like a small cylinder with wires connected to the top.

Step 3: Photograph or label the wire connections before disconnecting anything. Note which wires connect to C, HERM, and FAN terminals.

Step 4: Discharge the capacitor by placing an insulated screwdriver across the terminals. For dual capacitors, bridge C to FAN and C to HERM separately.

Step 5: Disconnect the wires from the capacitor terminals using needle-nose pliers.

Step 6: Set your multimeter to capacitance mode (look for the µF or MFD symbol).

Step 7: For a single capacitor, place one probe on each terminal. For a dual capacitor, test each section separately: place one probe on C and one on FAN, record the reading, then place one probe on C and one on HERM, and record that reading.

Step 8: Compare readings to the rated values on the capacitor label. Readings within ±10% indicate a good capacitor. Readings more than 10% low indicate a failing capacitor that needs replacement.

Interpreting Test Results

Reading	Diagnosis	Action Required
Within ±6% of rated value	Capacitor is good	No action needed
6-10% below rated value	Capacitor is marginal	Plan replacement soon
More than 10% below rated	Capacitor is failing	Replace immediately
Zero or very low reading	Capacitor is shorted	Replace immediately
OL (overload) reading	Capacitor is open	Replace immediately

How to Replace an AC Capacitor

If your testing confirms a bad capacitor, replacement is straightforward for anyone comfortable with basic electrical work. The entire process takes 30-60 minutes.

Tools and Materials Needed

You’ll need a replacement capacitor with matching specifications, an insulated screwdriver, needle-nose pliers, a socket wrench or nut driver (typically 5/16″ or 1/4″), a digital camera or phone for photos, a marker or labels for wires, and safety glasses and insulated gloves.

Step-by-Step Replacement Process

1. Ensure safety first. Turn off power at the breaker. Verify power is off. Remove the access panel.

2. Document the existing setup. Take clear photos of all wire connections. Note which wires go to which terminals (C, HERM, FAN). Record the capacitor’s specifications from the label.

3. Discharge and remove the old capacitor. Discharge the capacitor using an insulated screwdriver. Disconnect the wires from each terminal using pliers. Remove the mounting bracket screw holding the capacitor in place. Lift the old capacitor out of the unit.

4. Install the new capacitor. Verify the new capacitor matches required specifications (µF and minimum voltage). Position the new capacitor in the mounting bracket. Secure with the mounting screw. Reconnect wires to the correct terminals according to your photos. Ensure all connections are tight and secure.

5. Test the installation. Replace the access panel. Restore power at the breaker. Set your thermostat to call for cooling. Observe the unit starting. Listen for normal operation sounds. Verify cool air at supply registers within 15-20 minutes.

AC Capacitor Selection Guide

When shopping for a replacement AC capacitor, follow this checklist to ensure compatibility:

Match capacitance exactly: Use the same microfarad rating as the original. Don’t deviate from this specification.

Match or exceed voltage: Use the same voltage rating or higher. Never use a lower-rated capacitor.

Verify frequency compatibility: Ensure 50/60 Hz rating matches your local power.

Check physical fit: Measure the space available and verify the replacement will fit.

Consider quality: Buy from reputable suppliers. Cheap capacitors often fail prematurely.

Common AC Capacitor Sizes

The following table lists typical capacitor ratings for common HVAC applications:

Application	Typical Capacitance	Voltage Rating
Condenser fan motor	3-10 µF	370/440 VAC
Compressor (1.5-2 ton)	30-45 µF	370/440 VAC
Compressor (3-5 ton)	45-60 µF	370/440 VAC
Dual capacitor (typical)	35/5 to 60/10 µF	370/440 VAC
Blower motor	5-15 µF	370/440 VAC

AC Capacitor Replacement Costs

Understanding typical costs helps you budget for repairs and evaluate whether professional service makes sense for your situation.

Cost Component	DIY	Professional
Capacitor part	$9-$45	$15-$75
Labor	$0	$60-$200
Service call fee	N/A	$50-$100
Total Cost	$9-$45	$120-$300

Most repairs average around $170-$200 for professional service, with the capacitor itself being the least expensive component. If you’re comfortable with electrical work, DIY replacement offers significant savings. However, professionals provide warranty protection, proper diagnosis of related issues, and safety assurance.

Why AC Capacitors Fail

Understanding failure causes helps you prevent future problems and make informed replacement decisions.

Heat exposure: Capacitors degrade faster in high-temperature environments. Outdoor condensing units experience extreme temperatures, especially in southern climates. Keep vegetation trimmed away from units to improve airflow and cooling.

Age and wear: Like all electrical components, capacitors have finite lifespans. Most quality run capacitors last 30,000-60,000 operating hours, roughly 10-20 years under normal use. Start capacitors typically last longer due to intermittent duty.

Power surges: Voltage spikes from lightning strikes or grid fluctuations can damage capacitors. Whole-house surge protectors and dedicated HVAC surge protection devices help prevent this damage.

Voltage stress: Operating above rated voltage dramatically reduces capacitor life. Electrical supply problems, undersized wiring, or improper capacitor selection can all cause voltage stress.

Manufacturing defects: Lower-quality capacitors may use inferior materials or have construction flaws that cause premature failure. Buying from reputable suppliers reduces this risk.

Motor problems: Failed or failing motors can overload capacitors with excessive current, causing secondary capacitor failure. If your capacitor fails repeatedly, investigate the motor.

Frequently Asked Questions About AC Capacitors

Can I use a capacitor with a higher microfarad rating?

No. You must match the microfarad rating exactly. A higher value changes the motor’s phase angle and operating characteristics, potentially causing overheating, reduced efficiency, and premature motor failure. The only specification you can safely exceed is voltage rating.

How long do AC capacitors typically last?

Quality run capacitors typically last 10-20 years or 30,000-60,000 operating hours. However, heat exposure, power quality issues, and heavy usage can shorten this lifespan significantly. Capacitors in hot climates or systems that run frequently may need replacement every 5-10 years.

Can I run my AC with a bad capacitor?

You should not run your AC with a bad capacitor. The motor may fail to start, overheat trying to start, or run inefficiently. Continued operation with a failing capacitor can damage your compressor motor, which costs far more to replace than a capacitor. Turn off the system and replace the capacitor before operating.

What’s the difference between 370V and 440V capacitors?

Both voltage ratings indicate the maximum safe operating voltage. A 440V capacitor can be used in place of a 370V capacitor (and may last longer due to reduced voltage stress), but a 370V capacitor should not be used in a system designed for 440V. When in doubt, choose the higher voltage rating.

Why does my new capacitor keep failing?

Repeated capacitor failure usually indicates an underlying problem. Common causes include motor issues drawing excessive current, power supply voltage problems, poor ventilation causing heat buildup, incorrect capacitor specifications, or low-quality replacement parts. Have a technician diagnose the root cause before replacing another capacitor.

Useful Resources for AC Capacitor Replacement

The following resources provide additional technical information and parts sourcing:

Technical References:

• HVAC School (hvacrschool.com): Technical training resources for HVAC professionals
• TEMCO Industrial (temcoindustrial.com): Detailed capacitor selection guides and specifications
• iFixit (ifixit.com): Step-by-step repair guides with photos

Parts Suppliers:

• Grainger (grainger.com): Industrial-grade HVAC components
• RepairClinic (repairclinic.com): Part lookup by model number
• Amazon (amazon.com): Wide selection with reviews
• Local HVAC supply houses: Often best prices for professionals

Safety Information:

• OSHA Electrical Safety Guidelines
• National Electrical Code (NEC) requirements for HVAC equipment

Preventive Maintenance for Longer Capacitor Life

Taking proactive steps to protect your AC capacitor extends its service life and prevents inconvenient failures during peak cooling season.

Keep Your Condenser Clean

Debris, dirt, and vegetation restrict airflow around your outdoor unit, causing higher operating temperatures. Capacitors degrade faster in hot environments. Trim bushes and plants to maintain at least two feet of clearance around all sides of the condenser. Rinse the condenser coils with a garden hose annually to remove accumulated dust and debris.

Schedule Annual Professional Maintenance

Professional HVAC technicians test capacitor health during routine maintenance visits. They measure capacitance values and can identify components that are marginal but haven’t yet failed. Catching a weakening capacitor early lets you schedule replacement at your convenience rather than during an emergency.

During maintenance visits, technicians also check for loose electrical connections, corroded terminals, and other conditions that stress capacitors. Addressing these issues prevents secondary failures.

Install Surge Protection

Power surges from lightning strikes and grid fluctuations damage capacitors and other sensitive HVAC components. A whole-house surge protector at your electrical panel provides baseline protection. For additional security, consider a dedicated HVAC surge protector installed at your outdoor unit.

Monitor System Performance

Pay attention to how your air conditioner sounds and performs. Changes in startup behavior, unusual noises, or reduced cooling capacity often indicate developing problems. Addressing issues early, before complete failure, prevents the cascade of damage that occurs when a failing capacitor stresses other components.

When to Call a Professional

While capacitor replacement is manageable for experienced DIYers, certain situations call for professional expertise:

Multiple component failures: If you’ve had repeated capacitor failures or notice other problems alongside capacitor issues, a professional can diagnose underlying causes you might miss.

Warranty considerations: DIY repairs may void manufacturer warranties on your HVAC equipment. Check your warranty terms before attempting repairs.

Complex systems: Some HVAC systems have multiple capacitors, hard-start kits, or non-standard configurations that complicate replacement.

Safety concerns: If you’re uncomfortable working with high-voltage electrical components, the peace of mind from professional service is worth the cost.

Time constraints: When you need your AC working immediately during extreme heat, a professional typically completes repairs faster and can source parts same-day.

Final Thoughts on AC Capacitor Maintenance

The AC capacitor might be one of the least expensive components in your air conditioning system, but its proper function is essential for reliable cooling. By understanding how these components work, recognizing failure symptoms early, and knowing how to test and replace them correctly, you can avoid the discomfort and expense of unexpected AC failures.

Regular maintenance extends capacitor life. Have your HVAC system inspected annually, preferably in spring before cooling season begins. During these inspections, technicians test capacitors and can identify weakening components before they fail completely.

When replacement becomes necessary, take time to select the correct specifications and quality components. The few extra dollars spent on a quality capacitor pays dividends in reliability and longevity. And always prioritize safety when working with electrical components. Capacitors store dangerous charges and demand respect.

Your air conditioner works hard to keep you comfortable. Understanding and maintaining its components, especially the humble capacitor, ensures it can continue doing that job reliably for years to come.