If you are asking what does a capacitor do in an ac unit, the direct answer is that it acts as a temporary energy reservoir to shift the alternating current's phase. Specifically, it provides the initial rotational torque to start the compressor and fan motors (start capacitors) and maintains a continuous phase shift to keep them spinning efficiently under load (run capacitors). Without it, a single-phase 240V motor lacks a rotating magnetic field; it will simply hum, draw locked-rotor amps, and trip your breaker.
The Physics: How AC Capacitors Create Motor Torque
Single-phase AC power, which feeds almost all residential HVAC systems in North America, pulses but does not naturally rotate. To make a cylindrical motor rotor spin, the stator needs a rotating magnetic field. The capacitor solves this by connecting in series with the motor's start or auxiliary winding.
Because current through a capacitor leads the voltage by 90 degrees electrically, the capacitor delays the current in the auxiliary winding relative to the main run winding. This artificial phase split mimics a two-phase power supply, creating the magnetic push required to turn the rotor. Once the motor reaches about 75% of its rated RPM, a centrifugal switch or potential relay disconnects the start capacitor, while the run capacitor stays in the circuit to improve the power factor and running efficiency.
Start vs. Run vs. Dual: Which Type for Which Job?
Selecting the wrong capacitor type will result in immediate motor failure or a vented dielectric fluid mess. Here is the selection criteria based on internal construction and thermal limits.
| Capacitor Type | Internal Construction | Tolerance | Tempco / Operating Temp | Typical HVAC Use |
|---|---|---|---|---|
| Start Capacitor | Non-polarized Electrolytic (AC) | ±20% | Max 65°C (Short duty cycle) | High-torque compressor starts; disconnected by relay after 1-3 seconds. |
| Run Capacitor | Metalized Polypropylene Film (CBB65) | ±5% to ±6% | 70°C to 85°C (Continuous) | PSC (Permanent Split Capacitor) fan motors; stays energized continuously. |
| Dual Run Capacitor | Two CBB65 windings in one cylindrical can | ±5% to ±10% | 70°C to 85°C (Continuous) | Combined compressor (HERM) and condenser fan (FAN) circuits in outdoor units. |
The Golden Rule: Never use a start capacitor as a run capacitor. The electrolytic construction of a start cap cannot dissipate the heat generated by continuous AC ripple current; it will overheat, swell, and explode within minutes if left in a run circuit.
Decoding the Label: How to Read Capacitor Markings
HVAC capacitors use standardized nomenclature. Let us break down a typical dual run capacitor label, such as the ubiquitous Genteq or AmRad 45/5 µF 440VAC CBB65.
| Marking | Meaning | Selection Criteria |
|---|---|---|
| 45/5 µF | Microfarad rating. 45µF for the compressor (HERM), 5µF for the fan. | Must match the motor's design spec within ±5%. Incorrect µF alters the phase angle, causing overheating. |
| 440 VAC | Maximum continuous alternating voltage rating. | Can be substituted with a higher voltage (e.g., 440V replacing 370V), but never lower. |
| 50/60 Hz | Frequency rating. Capacitive reactance (Xc) changes with frequency. | Standard for North American (60Hz) and European (50Hz) grids. |
| CBB65 | IEC/Chinese standard code for metalized polypropylene film, oil-filled, metal can. | Ensures you are buying a continuous-duty run capacitor, not an electrolytic start cap. |
| HERM / FAN / C | Terminal designations: Hermetic Compressor, Fan Motor, Common. | C is the shared return path. Wiring HERM to FAN will instantly destroy the fan motor winding. |
Visual and Electrical Failure Modes
Capacitors degrade over time due to thermal stress and voltage spikes. According to U.S. Department of Energy HVAC maintenance guidelines, failing capacitors are a leading cause of summer compressor burnouts. Recognize these failure modes before they destroy your $2,500 compressor.
Visual Symptoms
- Domed or Swollen Top: The internal dielectric fluid breaks down, generating gas. The pressure switch pops the top dome upward to prevent a catastrophic casing rupture. If the top is not perfectly flat, replace it immediately.
- Oil Leakage: A rusted seam or blown pressure vent leaking a clear, oily substance. Modern CBB65 caps use non-PCB mineral oil or soybean-based dielectric fluid. Clean the contactor box if you see this.
- Burnt or Melted Terminals: Indicates a loose spade connector causing high resistance arcing, not necessarily an internal capacitor failure. Always use a crimping tool, never rely on friction-fit pliers.
Electrical Testing
A multimeter reading capacitance (µF) is the only definitive test. If the nameplate says 45µF ±5%, the acceptable range is 42.75µF to 47.25µF. A reading of 38µF means the polypropylene film has degraded, reducing the phase shift and forcing the compressor to draw excess amperage.
Safe Substitution: What to Do When the Exact Part is Missing
It is 95°F on a Saturday, the supply house is closed, and you need to get the AC running. Follow these strict substitution rules outlined in standard electrical safety practices (such as those referenced by the NFPA 70 National Electrical Code regarding equipment ratings):
- Voltage (VAC) Can Go Up: You can safely use a 440VAC capacitor to replace a 370VAC capacitor. The higher voltage rating simply means a thicker dielectric film, which will actually run cooler and last longer. Never use a 370VAC cap in a 440VAC circuit; it will arc internally and fail in hours.
- Microfarads (µF) Must Match: Do not deviate more than ±10% from the original µF rating. Installing a 55µF cap to replace a 45µF cap to "give it more starting power" is a myth that will overheat the start winding and melt the motor insulation.
- Combining Single Caps into a Dual: If your dual 40/5µF cap fails and you only have single caps, you can wire a single 40µF and a single 5µF capacitor in parallel to the Common (C) terminal, provided they physically fit in the equipment compartment and are securely strapped to prevent vibration shorts.
- Physical Form Factor: Oval and round capacitors are electrically identical. Choose based on the mounting bracket constraints inside the condenser's electrical panel.
Frequently Asked Questions
Can I run my AC unit with a bad capacitor?
No. If the run capacitor is weak or open, the compressor will attempt to start but will fail to reach full RPM, entering a "locked rotor" state. In this state, it draws 5 to 8 times its normal running amps (often exceeding 80A on a 3-ton unit). This massive current will quickly overheat the contactor points, melt the wiring insulation, and trigger the compressor's internal thermal overload. Repeated attempts will permanently burn out the compressor windings, turning a $25 capacitor fix into a $3,000 system replacement.
Why does my AC capacitor keep blowing every summer?
Heat is the primary enemy of the polypropylene dielectric film. Every 10°C increase in ambient operating temperature cuts a capacitor's lifespan in half. If your capacitor fails annually, check two things: First, is the condenser coil clogged with dirt and cottonwood, trapping 110°F+ air inside the electrical compartment? Second, check the capacitor's temperature rating. If the original OEM part was rated for 70°C, upgrade to an 85°C rated "severe duty" capacitor (like the AmRad Tough Turbo or Genteq 97F series) to survive the peak summer thermal load.
Will a hard start kit fix my failing AC compressor?
A hard start kit (which consists of a high-microfarad start capacitor paired with a potential relay) provides an extra burst of torque to help aging compressors overcome mechanical wear and high equalized refrigerant pressures. If your compressor is "hard starting" due to age or a long refrigerant line set, a hard start kit (like the 5-2-1 or Supco SPP6) can buy you a few more years of life. However, if the compressor is mechanically seized (pistons locked) or has internally shorted windings, no amount of external capacitance will fix it. A hard start kit is a band-aid for wear, not a cure for mechanical failure.
How much does a replacement AC capacitor cost in 2026?
If you are sourcing the part yourself from an HVAC parts distributor or online retailer, a standard 35/5µF or 45/5µF dual run capacitor costs between $18 and $35 in 2026. Premium, American-made, severe-duty 85°C rated models cost closer to $40 to $55. If you hire a licensed HVAC technician for an emergency weekend service call, expect to pay between $180 and $350 total, which includes the diagnostic trip fee, the marked-up part, and the labor to safely discharge, swap, and amp-test the system.
