The fastest way to identify a starting capacitor versus a running capacitor is to check the microfarad (µF) rating printed on the case. Run capacitors fall in the 3–70 µF range with a single, specific value like 35 µF. Start capacitors are rated at 70 µF or higher and typically show a range, such as 88–108 µF. Beyond that rating, the two types differ in shape, construction, and how they behave in a circuit.
Check the Microfarad Rating First
Every motor capacitor has its capacitance printed on the housing, measured in microfarads (µF or MFD). This is the single most reliable way to tell the two apart. Run capacitors carry a fixed value between 3 and 70 µF. Start capacitors begin at 70 µF and go much higher, often into the hundreds. A label reading “35 µF at 370V” is a run capacitor. A label reading “88–108 µF at 250V” is a start capacitor.
Notice that start capacitors list a range rather than a single number. That range reflects the looser manufacturing tolerances acceptable for a component that only operates briefly. Run capacitors need a precise value because they stay in the circuit continuously and affect motor efficiency the entire time the motor runs.
Physical Appearance and Housing
Run capacitors are commonly housed in metal, oval, or round cans with a smooth metallic finish. In HVAC systems, you’ll often find them in two forms: a single oval capacitor or a dual round capacitor that combines two run capacitors in one housing (one for the compressor, one for the fan). A dual capacitor will have three terminals on top, usually labeled C (common), FAN, and HERM (hermetic compressor).
Start capacitors are almost always housed in a round black plastic or Bakelite casing. They tend to be physically larger for their voltage rating because they store more energy. If you open an outdoor HVAC unit or look at the back of a washing machine motor and see a black cylindrical plastic cap with two terminals, that’s very likely the start capacitor.
Internal Construction
The two types are built differently on the inside, and this matters for understanding why they can’t be swapped. Run capacitors use oil-impregnated metalized polypropylene film as their internal insulating layer. This construction handles continuous voltage cycling without overheating. Run capacitors are non-polar, meaning they have no positive or negative terminal and work in either direction with alternating current.
Start capacitors use an electrolytic construction, which packs much more capacitance into a smaller space. The tradeoff is that electrolytic designs generate more internal heat and can’t tolerate continuous operation. That’s why start capacitors are only in the circuit for a few seconds during startup. Leaving a start capacitor energized too long will cause it to overheat and fail.
How Each One Works in the Motor
A single-phase motor needs help getting its rotor spinning. The start capacitor connects to the motor’s starting winding and delivers a large burst of energy that rapidly brings the motor to about 75% of full speed. Once the motor reaches that threshold, a centrifugal switch or relay disconnects the start capacitor from the circuit entirely. Its job is done in a matter of seconds.
The run capacitor connects between the main winding and the start winding and stays energized the entire time the motor operates. It maintains a phase shift between the two windings that keeps the motor running smoothly and efficiently. Without it, the motor draws more current, runs hotter, and loses torque. In HVAC systems, the run capacitor typically serves both the compressor motor and the condenser fan motor through a single dual capacitor.
Voltage Ratings
Both types will have a voltage rating printed alongside the capacitance. Common run capacitor voltages are 370V and 440V. Start capacitors are typically rated at 125V, 165V, or 250V. The voltage rating indicates the maximum continuous voltage the capacitor can safely handle, not the voltage it produces. You should always replace a capacitor with one that matches or exceeds the original voltage rating. Using a lower voltage rating risks failure. Using a higher voltage rating is safe but physically larger and more expensive.
Testing With a Multimeter
If the label is worn off or you want to confirm a capacitor still works, a multimeter can help. The method is the same for both types.
Before touching any capacitor, discharge it first. Even after the power is off, capacitors hold a charge that can shock you. The safest approach is to place a resistor (anywhere from 1,000 to 100,000 ohms, rated for 2–5 watts) across both terminals. Hold it there for several seconds. The stored energy dissipates as heat through the resistor. You can also buy a dedicated capacitor discharge pen with a built-in LED that turns off when the charge is gone.
- Capacitance mode: If your multimeter has a capacitance setting (marked with a capacitor symbol), select it and touch the probes to the terminals. A healthy capacitor reads within 10–20% of its labeled value. A reading far above or below that range means the capacitor has failed.
- Resistance mode: Set your meter to the ohm range around 1kΩ. Touch the probes to the terminals. On a digital meter, you should see the reading start low and climb steadily toward infinity as the capacitor charges from the meter’s internal battery. On an analog meter, the needle sweeps from low resistance toward high. If the reading jumps straight to infinity, the capacitor is open. If it stays near zero, it’s shorted.
- Continuity mode: Select continuity (the sound wave symbol). A working capacitor produces a brief beep that fades as it charges. If the beep is continuous, the capacitor is shorted internally.
For a start capacitor, the labeled range gives you more room. A reading of 95 µF on an 88–108 µF capacitor is perfectly fine. For a run capacitor labeled at exactly 35 µF, a reading of 30 µF or below means it’s weak and should be replaced.
Symptoms of a Failed Capacitor
Knowing which capacitor failed helps you buy the right replacement. The symptoms differ because the two capacitors do different jobs.
A bad start capacitor prevents the motor from starting at all. You’ll typically hear a loud humming sound as the motor tries to spin but can’t overcome its initial inertia. In an HVAC system, you might hear repeated clicking every few seconds as the relay tries again and again to engage the compressor. The unit draws heavy current during these failed attempts, which can eventually trip a breaker or damage the motor windings.
A bad run capacitor produces subtler problems. The motor may start but run sluggishly, overheat, or shut off on its thermal overload. In a heat pump or air conditioner, a common sign is the compressor running while the condenser fan sits motionless, or the fan spinning weakly. The system might cool or heat poorly without an obvious explanation. Over time, a weak run capacitor forces the motor to work harder, shortening its lifespan.
Physically, both types can show visible signs of failure. A bulging or swollen top, cracked housing, oily residue leaking from the base, or burn marks all indicate a capacitor that needs replacing. Start capacitors are more prone to dramatic failure (bulging, splitting) because of their electrolytic construction. Run capacitors tend to degrade gradually, losing capacitance over months or years before symptoms become noticeable.
Quick Reference Comparison
- Capacitance range: Run capacitors: 3–70 µF (single value). Start capacitors: 70 µF and above (listed as a range).
- Typical voltage: Run capacitors: 370V or 440V. Start capacitors: 125V, 165V, or 250V.
- Housing: Run capacitors: metal oval or round can. Start capacitors: black plastic cylinder.
- Construction: Run capacitors: oil-filled polypropylene film. Start capacitors: electrolytic.
- Duty cycle: Run capacitors: continuous, the entire time the motor runs. Start capacitors: momentary, only during the first few seconds of startup.
- Terminals: Run capacitors (dual type): three terminals labeled C, FAN, HERM. Start capacitors: two terminals.