An air conditioner capacitor stores and releases electrical energy that helps the compressor and fan motors start and run efficiently. When your AC hums but the fan never spins, or the compressor clicks and quits, a spent capacitor is often the reason. This small, inexpensive component sits inside the outdoor condensing unit, feeding motors the electrical push they need.
For the full breakdown, see our best Air Conditioner Capacitor guide.
What Does An Air Conditioner Capacitor Do?
A capacitor energizes a motor’s startup and smooths its operation, like a short-term battery. Residential systems hide these parts inside the outdoor condensing unit, serving the compressor motor, the condenser-fan motor, or both.
- Start capacitor: Delivers a short torque burst during startup, then a relay removes it. Values run roughly 70–400+ µF at 110–330 V.
- Run capacitor: Stays connected the entire time the motor runs, providing the phase shift needed to spin efficiently. Typical HVAC ratings fall between about 2.5 and 80 µF at 370 or 440 VAC.
Without a healthy cap, a motor either won’t start or strains to keep going — driving up heat and energy draw.
How To Read The Ratings On A Capacitor Label
Capacitance is measured in microfarads — written µF, MFD, or mfd — and residential HVAC values stretch from 3 to 1,200 µF, though most land between 5 and 80 µF. A single run capacitor carries one value for one motor. A dual run capacitor packs two sections into one housing — a label like 45/5 MFD means 45 microfarads for the compressor (terminal HERM) and 5 for the condenser fan (terminal FAN), with a shared C terminal.
Common run voltage ratings are 370 VAC and 440 VAC. A 440-VAC replacement can stand in for a 370-VAC original when capacitance and fit also match, but never the reverse.
| Capacitor Type | Typical Ratings | Job In The Unit |
|---|---|---|
| Start capacitor | ~70–400+ µF, 110–330 V | Startup torque burst only |
| Single run capacitor | 2.5–80 µF, 370/440 VAC | One motor, continuous operation |
| Dual run capacitor | Two values, e.g. 45/5 MFD | Compressor plus condenser fan |
| Common capacitance range | 3–1,200 µF overall | Most units sit 5–80 µF |
| Common voltage ratings | 370 VAC or 440 VAC | Equal or higher is required |
| Typical tolerances | ±5% or ±6% | Sets the acceptable measured range |
| Frequency marking | 50/60 Hz | Must match equipment spec |
There is no universal capacitor size based on air-conditioner tonnage alone. Confirm the correct value from the equipment data plate, the original capacitor label, or the manufacturer’s documentation. When comparing replacement options, this breakdown of top-rated air conditioner capacitors pairs specs with real-world picks.
Signs Of A Failing Capacitor
Symptoms include a compressor or fan that won’t start, a motor that hums without turning, difficult startup, reduced cooling, unusual noise or vibration, overheating, and higher energy consumption. A bulging, leaking, or ruptured can is a clear red flag — but appearance alone can’t confirm electrical health. A dual capacitor can fail in just one section, so test the compressor and fan sides separately. If either section reads outside its printed tolerance, replace the dual capacitor as one assembly.
How Do You Replace One Safely?
Follow a fixed, safety-first order from the Home Depot repair guide. A capacitor can hold a dangerous charge even with the breaker off.
- Turn the thermostat off, then shut off the outdoor unit’s circuit breaker or disconnect its main power.
- Remove the outdoor-unit access panel.
- Treat the capacitor as energized until safely discharged — use an appropriate resistor-equipped jumper method. Switching off power does not remove stored energy.
- Photograph or note every wire position before touching anything.
- Disconnect the wires and test capacitance between C–FAN and C–HERM on a dual capacitor.
- Remove the retaining strap, install a replacement with matching capacitance and equal-or-higher voltage, and reconnect wires to the same terminals.
- Restore the panel and power, then confirm the fan and compressor run normally.
Never test or swap the component without insulated tools, eye protection, appropriate gloves, electrical training, and a verified de-energization procedure. Common field errors: choosing by tonnage, fitting a wrong µF value, dropping to a lower voltage rating, and mixing up the HERM, FAN, and C terminals.
Common Questions
Can a capacitor fail without any visible damage?
Yes. A capacitor can drift out of its tolerance range while looking perfectly normal. Testing capacitance with a multimeter against the label’s stated tolerance is the only reliable way to confirm condition.
Will a higher voltage rating hurt my system?
No. A 440-VAC capacitor can safely replace a 370-VAC one when capacitance and physical fit also match. The reverse is not acceptable — never install a lower voltage rating than the original.
Why does my AC hum but not cool?
A humming motor that won’t turn often points to a weak or failed capacitor that can’t deliver enough startup torque. A capacitance test is the fastest way to rule the capacitor in or out before calling a technician.
Sources
- Ferguson. “AC Capacitor Guide.” Supports capacitor types, capacitance and voltage ratings, and labeling details.
- The Home Depot. “AC Capacitor Replacement.” Supports the replacement sequence, testing steps, and failure symptoms.