Types of HVAC Capacitors
HVAC systems use two types of capacitors, and they serve completely different functions. Using the wrong type or wrong specifications can damage motors or prevent them from starting.
Run Capacitor
The run capacitor stays in the circuit continuously while the motor operates. It creates the phase shift needed to keep a single-phase AC motor spinning efficiently. Without it, a PSC (Permanent Split Capacitor) motor cannot run. Found on compressors, condenser fan motors, and indoor blower motors.
Start Capacitor
The start capacitor provides a large burst of additional torque during the first fraction of a second of motor startup. It is switched out of the circuit once the motor reaches 75-80% of operating speed (via a potential relay or start relay). Start capacitors are polarized electrolytics rated for intermittent duty only - they will fail quickly if left in circuit continuously.
Dual Run Capacitor
A single physical capacitor with three terminals (C, HERM, FAN) that replaces two separate run capacitors. The HERM side runs the compressor; the FAN side runs the condenser fan motor. Very common in residential condensing units - saves space and cost vs two separate caps.
Reading a Capacitor Label
A capacitor label provides everything you need to select a replacement:
- MFD (or °F): Microfarad rating - the capacitance value. The most critical spec.
- VAC: Voltage rating - the maximum AC voltage the capacitor is rated for. Always meet or exceed the original.
- Hz: Frequency rating - always 60 Hz in North America.
- Dual ratings example: 45+5 MFD 370/440 VAC - 45 MFD on HERM, 5 MFD on FAN, rated for either 370V or 440V systems.
Run Capacitor MFD Tolerance
The replacement MFD must match the original within °6% per AHRI standards. Do not go outside this range - too low and the motor runs hot and inefficiently; too high and the motor can overheat or draw excess amperage.
| Original MFD Rating | Acceptable Replacement Range (°6%) |
|---|---|
| 3 MFD | 2.82 - 3.18 MFD |
| 5 MFD | 4.70 - 5.30 MFD |
| 7.5 MFD | 7.05 - 7.95 MFD |
| 10 MFD | 9.40 - 10.60 MFD |
| 15 MFD | 14.10 - 15.90 MFD |
| 20 MFD | 18.80 - 21.20 MFD |
| 25 MFD | 23.50 - 26.50 MFD |
| 30 MFD | 28.20 - 31.80 MFD |
| 35 MFD | 32.90 - 37.10 MFD |
| 40 MFD | 37.60 - 42.40 MFD |
| 45 MFD | 42.30 - 47.70 MFD |
| 50 MFD | 47.00 - 53.00 MFD |
| 55 MFD | 51.70 - 58.30 MFD |
| 60 MFD | 56.40 - 63.60 MFD |
| 70 MFD | 65.80 - 74.20 MFD |
| 80 MFD | 75.20 - 84.80 MFD |
Voltage Rating Selection
The replacement voltage rating must equal or exceed the original. Using a higher-rated capacitor is acceptable and often preferred for longer life. Never use a lower-rated capacitor.
| Original Voltage Rating | Acceptable Replacements |
|---|---|
| 370 VAC | 370 VAC or 440 VAC ? |
| 440 VAC | 440 VAC only (not 370 VAC) |
| 370/440 VAC (dual rated) | Works on either 208-230V or 277V systems |
Most residential condensing units run on 208-230VAC. The 370V vs 440V distinction matters most in commercial applications with 277VAC circuits. For residential: a 440VAC capacitor always works. A 370VAC capacitor is only suitable if the system voltage is definitively 230V or below.
Common Run Capacitor Specifications by Equipment Type
| Equipment | Typical MFD Range | Voltage | Type |
|---|---|---|---|
| Condenser fan motor | 3-10 MFD | 370 or 440 VAC | Single run |
| Compressor (1-2 ton) | 25-35 MFD | 370 or 440 VAC | Single or dual run |
| Compressor (3 ton) | 35-45 MFD | 370 or 440 VAC | Single or dual run |
| Compressor (4 ton) | 45-55 MFD | 370 or 440 VAC | Single or dual run |
| Compressor (5 ton) | 55-70 MFD | 370 or 440 VAC | Single or dual run |
| Dual run (compressor+fan) | 35+5, 40+5, 45+5, 50+5 MFD | 370 or 440 VAC | Dual run (oval or round) |
| Indoor blower motor (PSC) | 3-10 MFD | 370 VAC | Single run |
Start Capacitor Sizing
Start capacitors are sized in MFD with a tolerance of °20% and rated in voltage (typically 165V, 250V, or 330V). Match the voltage rating and stay within the MFD range printed on the original. Common residential hard start kit capacitors: 88-108 MFD / 250V, 108-130 MFD / 250V.
Physical Sizing Considerations
- Round vs oval: Round capacitors are standard; oval capacitors fit in tighter spaces. Both work identically - choose based on fit in the electrical compartment.
- Terminal type: Spade terminals (° inch) are standard. Some motors use screw terminals or push-on pins.
- Height and diameter: Must fit inside the condensing unit electrical compartment. Measure if the original is in an unusually tight location.
Warranty Impact
Replacing a run capacitor with an OEM-equivalent or matching aftermarket unit does not void equipment warranty. Using a drastically wrong MFD (more than °6%) or wrong voltage can cause motor failure that may not be covered under warranty. Always document the original capacitor specifications and the replacement installed.