35/5 MFD Capacitor: Air Conditioner Specifications and Technical Guide

When your air conditioning system starts struggling to kick on or makes unusual humming sounds, there’s a good chance the dual-run capacitor is the culprit. As someone who’s spent years designing and troubleshooting power circuits, I can tell you that the 35 5 capacitor is one of the most commonly replaced components in residential HVAC systems. This guide breaks down everything you need to know about these critical components from an engineering perspective.

Understanding the 35 5 Capacitor Basics

The term “35 5 capacitor” refers to a dual-run capacitor with two capacitance values: 35 microfarads (MFD or µF) and 5 microfarads. These aren’t two separate capacitors—it’s a single component housing two capacitor sections in one oval can. The 35 MFD section powers your compressor motor, while the 5 MFD section handles the fan motor.

I’ve worked with hundreds of these units over the years, and understanding the designation is crucial. When you see “35/5 MFD 440V” stamped on the capacitor, it tells you immediately what you’re dealing with: a dual-run capacitor with 35 µF for the compressor (HERM terminal), 5 µF for the fan (FAN terminal), and a voltage rating of 440 VAC.

Why Your AC System Needs a 35 5 Capacitor

Air conditioner motors require substantial starting torque to overcome inertia and begin rotating. The capacitor creates a phase shift in the motor winding, generating the rotating magnetic field necessary for starting and running efficiently. Without proper capacitance, your compressor and fan motors would struggle to start, draw excessive current, and eventually fail prematurely.

From an electrical engineering standpoint, the capacitor serves three critical functions:

Starting Assistance: Provides the phase angle difference needed for single-phase motors to develop starting torque Power Factor Correction: Improves the power factor of the motor circuit, reducing reactive power consumption Run Efficiency: Maintains optimal motor performance during continuous operation

Think of it like a battery for your AC motors, except instead of storing DC voltage, it stores and releases AC energy at specific phase angles. The larger 35 MFD section handles the heavy-duty compressor, while the smaller 5 MFD section is perfectly sized for the less demanding fan motor.

Technical Specifications of 35 5 Capacitors

Standard Electrical Ratings

Parameter	Specification	Notes
Primary Capacitance	35 µF ±6%	Compressor section (HERM)
Secondary Capacitance	5 µF ±6%	Fan motor section (FAN)
Voltage Rating	370V or 440V AC	440V provides better reliability
Frequency	50/60 Hz	Dual frequency compatible
Temperature Range	-40°C to +70°C	Operating environment limits
Tolerance	±6% standard	±5% for precision applications
Terminal Configuration	3-pin (C, HERM, FAN)	Common terminal mandatory

Physical Dimensions and Mounting

The physical packaging matters more than many technicians realize. I’ve seen countless installation issues stem from incorrect sizing assumptions. Here’s what you need to know:

Dimension Type	Typical Range	Critical Considerations
Height	3.5″ – 4.5″	Clearance for mounting bracket
Diameter	2.0″ – 2.5″	Oval shape varies by manufacturer
Terminal Spacing	0.5″ – 0.75″	Must match connector configuration
Mounting Bracket	Standard 2-hole	Verify existing bracket compatibility
Weight	8-12 oz	Consider vibration mounting

How to Identify if Your 35 5 Capacitor Has Failed

In my experience diagnosing hundreds of HVAC systems, capacitor failure presents distinct symptoms that any homeowner or technician can recognize. The capacitor doesn’t always fail completely—sometimes it degrades gradually, causing intermittent problems.

Common Failure Symptoms

Compressor Hums But Won’t Start: This is the classic sign. You hear the compressor trying to start with a humming sound, but it never kicks over. The capacitor has lost enough capacitance that it can’t provide adequate starting torque.

Fan Runs But Compressor Doesn’t: When the 35 MFD section fails but the 5 MFD section still works, your fan spins normally while the compressor sits silent. This split failure mode is actually common.

High Electric Bills: A degraded capacitor forces the motor to work harder, drawing more current and consuming more power. If your utility bills spike without explanation, check the capacitor.

Short Cycling: The system starts, runs briefly, then shuts off. This happens when the capacitor provides just enough boost for starting but fails under running load.

Bulging or Leaking Can: Physical deformation is a dead giveaway. If the metal can is swollen, venting, or leaking oil, replace it immediately—it’s a safety hazard.

Testing and Measuring Your 35 5 Capacitor

As an engineer, I never trust assumptions. Always test before replacing. Here’s my systematic approach:

Using a Multimeter with Capacitance Function

Most modern multimeters include capacitance measurement capability. Here’s the proper procedure:

1. Safety First: Disconnect power at the breaker and discharge the capacitor using a 20kΩ 5W resistor across its terminals for 60 seconds
2. Terminal Identification: Note the HERM, FAN, and C (common) terminals
3. Measure HERM-C: Should read 35 µF ±6% (32.9-37.1 µF is acceptable)
4. Measure FAN-C: Should read 5 µF ±6% (4.7-5.3 µF is acceptable)
5. Record Readings: Document for future reference and trend analysis

Acceptable Tolerance Range Table

Rated Value	Lower Limit (-6%)	Upper Limit (+6%)	Action Required
35 µF	32.9 µF	37.1 µF	Replace if outside range
5 µF	4.7 µF	5.3 µF	Replace if outside range

If either section reads below -6%, the capacitor should be replaced. Some engineers use -10% as the replacement threshold, but I recommend -6% to maintain optimal efficiency and prevent motor damage.

Visual Inspection Checklist

Before breaking out the meter, a quick visual inspection often reveals the problem:

• Top Surface: Should be flat, not bulged or domed
• Bottom Surface: Check for rust or corrosion indicating moisture ingress
• Terminals: Look for burn marks, loose connections, or corrosion
• Insulation: The plastic sleeve should be intact without cracks
• Oil Stains: Any oil leakage indicates dielectric failure

Selecting the Right 35 5 Capacitor Replacement

Not all 35 5 capacitors are created equal, and this is where engineering knowledge pays dividends. I’ve tested capacitors from numerous manufacturers, and the quality variance is significant.

Voltage Rating: 370V vs 440V

This is probably the most misunderstood specification. Your AC unit likely uses a 370V compressor, so why do 440V capacitors exist?

The higher voltage rating provides better reliability and longer life. A 440V capacitor used in a 370V application runs cooler and handles voltage spikes better. The dielectric film is thicker, which means better insulation and reduced risk of breakdown. I always recommend 440V capacitors for residential installations—the minimal cost difference is worth the extended lifespan.

Engineering Rule: Never downgrade voltage rating. You can use a 440V capacitor in a 370V system, but never use a 370V capacitor if the original was 440V.

Capacitance Tolerance and Motor Matching

The ±6% tolerance might seem like wiggle room, but it’s actually critical for optimal motor performance. Motors are designed for specific capacitance values, and deviations affect:

• Starting torque: Lower capacitance reduces starting ability
• Running current: Incorrect values increase current draw
• Power factor: Non-optimal capacitance degrades efficiency
• Motor heating: Poor capacitor matching causes excessive heat generation

Brand Quality Comparison

Based on field testing and failure rate analysis, here’s my practical breakdown:

Brand Category	Expected Lifespan	Failure Rate	Best Application
Premium (GE, Carrier OEM)	7-10 years	<5%	New installations
Mid-tier (Mars, TurboMax)	5-7 years	5-10%	Standard replacements
Economy (Generic imports)	2-4 years	15-25%	Emergency temporary fix

I’ve dissected failed capacitors in the lab, and the quality differences are stark. Premium units use better metallization, higher-grade dielectric film, and superior sealing. The economy units often use thinner films and cheaper materials that degrade faster under thermal cycling.

Installation Guidelines for 35 5 Capacitors

Proper installation isn’t just about connecting three wires—it’s about ensuring long-term reliability and safety.

Wiring Configuration

The terminal markings aren’t arbitrary. Here’s the correct hookup:

• C (Common): Connects to the common side of both motors and the power supply neutral
• HERM (Hermetic/Compressor): Connects to the compressor motor start winding
• FAN: Connects to the condenser fan motor

A common mistake I see is reversing the HERM and FAN connections. While the system might run, the motors operate with incorrect phase relationships, reducing efficiency and lifespan.

Physical Mounting Best Practices

The mounting location affects thermal performance and longevity:

1. Vertical Orientation: Mount with terminals facing down to prevent moisture accumulation
2. Clearance: Maintain 2″ minimum clearance around the capacitor for air circulation
3. Vibration Isolation: Use rubber grommets if mounting directly to compressor housing
4. Sun Exposure: Avoid direct sunlight—UV radiation degrades the plastic housing
5. Moisture Protection: Ensure the electrical compartment has proper drainage

Electrical Safety Procedures

Never skip safety steps. Capacitors store charge even after power disconnection:

1. Lock Out Power: Turn off breaker and tag it
2. Verify Zero Voltage: Use a multimeter to confirm no voltage present
3. Discharge Capacitor: Short terminals through a 20kΩ resistor for 60 seconds
4. Insulated Tools: Use insulated screwdrivers and pliers
5. One Hand Rule: Keep one hand behind your back when working near live circuits

I’ve witnessed what happens when technicians skip the discharge step—it’s painful and dangerous. The stored energy in a 35 µF capacitor at 370V is substantial.

Troubleshooting Common 35 5 Capacitor Issues

Premature Failure Analysis

When capacitors fail before their expected lifespan, there’s usually an underlying cause:

Voltage Spikes: Lightning strikes or utility voltage fluctuations stress the dielectric Thermal Cycling: Extreme temperature swings in unconditioned spaces accelerate aging Overvoltage: Operating above rated voltage degrades insulation rapidly Harmonics: VFDs or other nonlinear loads inject harmonic currents Poor Connections: Loose terminals create resistance and heat Wrong Capacitance: Oversized capacitors stress motor windings

Diagnostic Decision Tree

Symptom	Test Result	Probable Cause	Solution
Won’t start, hums	Capacitance <32.9µF	Capacitor degraded	Replace capacitor
Starts then stops	Normal capacitance	Thermal overload	Check refrigerant, clean coils
High current draw	Capacitance >37.1µF	Wrong capacitor	Install correct rating
Intermittent operation	Visual bulging	Dielectric breakdown	Replace immediately

Alternative Configurations and Universal Replacements

Sometimes you need a creative solution when the exact replacement isn’t available.

Using Two Single-Run Capacitors

In a pinch, you can replace a dual-run 35 5 capacitor with two single-run capacitors:

• Install a 35 µF single-run capacitor for the compressor
• Install a 5 µF single-run capacitor for the fan
• Connect both common terminals together
• Ensure both have adequate voltage rating

This works electrically but requires more mounting space and additional wiring. I’ve used this approach for temporary repairs, but it’s not ideal for permanent installations.

Round vs Oval Capacitors

The shape is largely about packaging efficiency. Round capacitors contain single capacitance values, while oval cans typically house dual capacitance sections. The electrical performance is identical if the specifications match.

Resources and Technical References

As an engineer, I believe in providing access to solid technical information. Here are resources I regularly reference:

Manufacturer Datasheets

• Genteq (Mars) Motor/Capacitor Database: Technical specifications and cross-references for most residential HVAC capacitors
• Carrier OEM Parts Catalog: Original equipment specifications for Carrier, Bryant, and Payne systems
• Emerson Climate Technologies: Motor and capacitor selection guides with electrical calculations

Testing Equipment Recommendations

• Fluke 117 Multimeter: Reliable capacitance measurement with CAT III 600V rating
• Klein MM700 Multimeter: Budget-friendly option with adequate capacitance range
• Supco CR87 Capacitor Checker: Dedicated capacitor analyzer for field testing

Online Calculation Tools

• Motor Capacitor Calculator: Determines required capacitance based on motor specifications
• Power Factor Correction Calculators: For engineers designing custom HVAC installations
• Voltage Drop Calculators: Essential for proper wire sizing in capacitor circuits

Industry Standards

• NEMA MG-1: Motors and generators standard covering capacitor applications
• UL 810: Standard for capacitors used in HVAC equipment
• ANSI/AHRI 1060: Performance rating of air conditioning and heating equipment

Long-Term Reliability and Maintenance

Capacitors are consumable components with finite lifespans. Here’s what affects longevity:

Environmental Factors

Temperature: Every 10°C increase in operating temperature roughly halves capacitor life. Installing equipment in shaded locations extends capacitor lifespan significantly.

Humidity: Moisture ingress degrades the dielectric and causes internal corrosion. Ensure proper enclosure sealing in humid climates.

Vibration: Constant mechanical stress fatigues internal connections. Use rubber mounts on compressor-mounted capacitors.

Preventive Maintenance Schedule

Interval	Action	Purpose
Annual	Visual inspection	Detect physical degradation early
Biennial	Capacitance measurement	Track degradation trend
5 Years	Proactive replacement	Prevent unexpected failure
Post-storm	Surge damage check	Identify lightning-related stress

I replace capacitors proactively at 5-7 years in critical applications, even if they test within specification. The cost of replacement is minimal compared to compressor damage from capacitor failure.

Cost Analysis and Value Engineering

Understanding the economics helps make informed decisions.

Replacement Cost Breakdown

Cost Component	DIY	Professional	Notes
Capacitor	$15-$35	$15-$35	Quality varies significantly
Labor	$0	$150-$250	Typical service call charge
Diagnostic Fee	$0	$75-$125	Often waived if repair made
Total	$15-$35	$240-$410	Professional warranty included

The DIY approach saves substantial money if you’re comfortable working with electrical systems. However, HVAC technicians provide warranty coverage and can identify related issues.

Long-Term Value Optimization

Investing in premium components makes financial sense:

• Premium capacitor: $35, lasts 8 years = $4.38/year
• Economy capacitor: $15, lasts 3 years = $5.00/year

Plus, the economy capacitor requires two additional service calls over 8 years. The premium unit is actually cheaper in the long run.

Frequently Asked Questions

Can I Replace a 35 5 Capacitor with a 40 5?

Technically yes, but I don’t recommend it for long-term use. The compressor motor is designed for 35 µF, and the 40 µF capacitor will increase starting torque beyond design specifications. In the short term, the system will start more aggressively. Long term, the increased starting current can stress motor windings and reduce compressor lifespan. If your system struggles to start with the correct 35 5 capacitor, investigate the root cause—low voltage, weak compressor, or dirty coils—rather than compensating with oversized capacitance.

Why Does My 35 5 Capacitor Keep Failing?

Repeated capacitor failure indicates an underlying problem, not just bad luck with components. From my diagnostic experience, the most common causes are: voltage issues (check utility supply for spikes or sustained overvoltage), compressor problems (a hard-starting compressor stresses the capacitor), poor ventilation (inadequate airflow causes thermal stress), incorrect wire sizing (undersized conductors create voltage drop), and lightning-induced surges (install a surge protector on the outdoor unit). Document how long each capacitor lasts and what symptoms occur. Pattern recognition reveals the root cause.

What’s the Difference Between MFD and µF on Capacitors?

Absolutely nothing. MFD stands for microfarads, and µF is the proper electrical engineering symbol for microfarads. Capacitor manufacturers use MFD because it’s easier to print and read on component labels. Some newer capacitors use UF as another variant. All three terms (MFD, µF, UF) mean exactly the same thing: one millionth of a farad. Don’t let the terminology confuse you—a 35 MFD capacitor is identical to a 35 µF capacitor. The electrical engineering community prefers µF as standard notation, but HVAC industry uses MFD predominantly.

Can a Bad 35 5 Capacitor Damage My Compressor?

Yes, absolutely, and I’ve seen it happen many times. When the capacitor fails, your compressor attempts to start without adequate phase shift. This creates excessive starting current—sometimes 4-6 times normal running current—that flows directly through the motor windings. The increased current generates heat through I²R losses in the copper windings. If the compressor repeatedly attempts to start and fails (known as locked rotor condition), the winding temperature skyrockets. Most compressors have thermal overload protection that trips, but repeated thermal cycling degrades the winding insulation. Eventually, the insulation breaks down, causing short circuits or open windings. A $25 capacitor failure can easily lead to a $1,500 compressor replacement.

Should I Replace My 35 5 Capacitor Myself or Hire a Professional?

This depends on your comfort level with electrical work and local codes. From a technical standpoint, capacitor replacement is straightforward: disconnect power, discharge the old capacitor, note the wire connections, install the new capacitor matching the terminal configuration. However, working with 240V AC systems poses real dangers. If you’re not experienced with electrical systems or uncomfortable working in outdoor electrical panels, hire a licensed technician. The professional service includes proper diagnostics to ensure the capacitor is actually the problem, warranty coverage if the replacement fails, and expertise to identify related issues. If you choose DIY, invest in a good multimeter with capacitance function, follow proper discharge procedures, and verify your local code allows homeowner electrical work.

Conclusion: Engineering Perspective on 35 5 Capacitors

After years working with these components, I’ve learned that the 35 5 capacitor represents a critical intersection of power electronics and practical HVAC engineering. It’s a simple device—just two capacitor sections in a can—but its role in motor starting and running efficiency makes it essential for reliable air conditioning operation.

The key insights from an engineering perspective: always test before replacing, invest in quality components with 440V rating, follow proper safety procedures, address underlying issues causing premature failure, and maintain a proactive replacement schedule. Understanding these fundamentals transforms capacitor service from random parts swapping into systematic troubleshooting and reliability engineering.

Whether you’re a homeowner trying to save on HVAC costs, a technician building your skills, or a fellow engineer appreciating the elegant simplicity of passive power correction, the 35 5 capacitor deserves respect as a workhorse component keeping millions of homes comfortable.