3-Terminal Capacitor Wiring Diagram
This is a free printable 3 terminal capacitor wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A 3-terminal capacitor is an HVAC dual run capacitor with C (common), HERM (compressor), and FAN terminals, providing run capacitance for both the compressor and condenser fan motor from a single unit.
A three-terminal capacitor — also called a dual run capacitor or dual capacitor — is a single cylindrical component containing two separate capacitors in one housing, sharing a common terminal. It is the standard component used in residential and light commercial HVAC systems to provide run capacitance for both the compressor motor and the condenser fan motor simultaneously.
The three terminals are universally labelled in North American HVAC practice as: - C (Common): the shared return connection for both capacitors - HERM (Hermetic): the terminal for the compressor motor run capacitor section - FAN: the terminal for the condenser fan motor run capacitor section
The capacitance between HERM and C is typically larger (25 µF to 50 µF for most residential compressors) because compressor motors are larger and require more capacitance for efficient phase-shift and torque production. The capacitance between FAN and C is typically smaller (5 µF to 15 µF for condenser fan motors).
A run capacitor serves a different function from a start capacitor. A start capacitor provides a large capacitance boost for a brief moment during start-up to produce the required starting torque, then disconnects via a start relay. A run capacitor remains permanently in circuit and shifts the phase of the current in the auxiliary winding of a single-phase motor by approximately 90 degrees, creating the rotating magnetic field needed for efficient continuous operation. Running a single-phase motor without its run capacitor causes excessive current draw, overheating, reduced efficiency, and noise.
On the wiring diagram, the C terminal connects to the neutral return of the control circuit. The HERM terminal connects to the auxiliary (start) winding terminal of the compressor. The FAN terminal connects to the auxiliary winding terminal of the condenser fan motor. Both the compressor and fan motor run winding terminals (main winding) connect directly to the line supply.
Dual run capacitors fail through internal dielectric breakdown, which can manifest as a short circuit, open circuit, or partial capacitance loss. A capacitance meter or a capacitor analyser is required to verify the actual capacitance — a multimeter's basic resistance or continuity test is not sufficient to confirm a capacitor is within specification.
How to wire 3 terminal capacitor wiring diagram
- Isolate and lock off the HVAC unit Switch off the unit using the system disconnect and, for outdoor condensing units, lock off the outdoor electrical disconnect box. Verify no voltage is present with a non-contact voltage tester on all terminals before proceeding. Capacitors store charge after de-energisation — discharge them before handling.
- Discharge the capacitor safely Using an insulated screwdriver or a purpose-made capacitor discharge tool (a resistor of approximately 10 kΩ to 20 kΩ in series with a pair of insulated probes), short across C-to-HERM and C-to-FAN independently to discharge stored charge. Never short a capacitor directly with a screwdriver blade — the arc can cause injury and damage the terminals.
- Photograph the original wiring before removal Take a clear photograph of all wires connected to the capacitor before removing anything. Note the wire colour, terminal label, and origin of each wire. This is the most reliable reference for correct reconnection and catches any previous incorrect wiring.
- Measure the capacitance of the original capacitor Using a capacitor analyser or multimeter with capacitance function, measure HERM-to-C and FAN-to-C. Compare against the nameplate values. Record results for the service record. This confirms whether the capacitor has failed before condemning it.
- Select a replacement with matching ratings The replacement must match both the capacitance (µF) values for HERM-to-C and FAN-to-C exactly (within ±6 % acceptable range), and the voltage rating must equal or exceed the original (commonly 370 V AC or 440 V AC for residential HVAC). Never fit a lower-voltage-rated capacitor — it will fail prematurely.
- Connect the replacement using the photograph as reference Connect each wire to the correct terminal: C wire to C, compressor auxiliary winding wire to HERM, and fan motor auxiliary winding wire to FAN. Ensure all connections are fully seated on the terminals. Use push-on spade connectors — do not twist or tape bare wires.
- Energise and verify operation Remove tools and restore power. Observe the unit starting: the compressor should start and run smoothly, and the condenser fan should spin at normal speed with no humming or struggling. Measure compressor and fan motor running currents and compare against nameplate rated load amps (RLA) to confirm correct capacitor sizing.
Specifications
| Terminal designations (North American HVAC standard) | C (Common), HERM (Hermetic compressor), FAN (condenser fan motor) |
|---|---|
| Typical HERM-to-C capacitance range (residential) | 15 µF to 55 µF |
| Typical FAN-to-C capacitance range (residential) | 3 µF to 15 µF |
| Standard voltage ratings (North American HVAC) | 370 V AC and 440 V AC (60 Hz); 250 V AC and 450 V AC (international) |
| Acceptable capacitance tolerance in service | Within ±6 % of rated value (industry practice) |
| Capacitor dielectric type (modern HVAC) | Metallised polypropylene film (dry type, self-healing) |
| Operating temperature range (typical) | –40 °C to +70 °C |
| Standard spade terminal size (HVAC) | 6.35 mm (1/4 inch) quick-connect spade |
Safety warnings
- Capacitors store high-voltage charge that can persist for hours after power is removed. Always discharge the capacitor using a purpose-made discharge tool or a resistor in series with insulated probes before handling. Direct shorting of capacitor terminals creates a dangerous arc.
- HVAC condensing units contain voltages up to 240 V AC (single-phase) or 208 V AC to 460 V AC (three-phase). Always isolate and verify dead at the outdoor disconnect before working on any electrical components. Do not rely solely on the thermostat 'off' setting as an isolation means.
- A swollen, leaking, or ruptured capacitor casing indicates internal failure and possible risk of rupture under power. Remove and replace it without energising the unit — never attempt to reform or reuse a physically damaged capacitor.
- Run capacitors should only be replaced by technicians with appropriate training and certification. In many jurisdictions, HVAC electrical work requires a licensed HVAC technician or licensed electrician. All work must comply with applicable codes including NEC Article 440 (USA), BS 7671, IEC 60364, or local equivalent.
- Never replace a run capacitor with one having a lower voltage rating than the original specification. A 370 V AC capacitor installed in a 440 V AC rated position will overstress the dielectric and fail prematurely, potentially with force.
Tools needed
- Non-contact voltage tester
- Capacitor discharge tool or 10–20 kΩ power resistor with insulated leads
- Multimeter with capacitance measurement function or dedicated capacitor analyser
- Clamp meter (for measuring running current after replacement)
- Insulated screwdrivers
- Pliers (for spade terminal removal)
- Camera or smartphone (to photograph original wiring)
Common mistakes
- Connecting the compressor auxiliary winding wire to the FAN terminal and the fan motor wire to the HERM terminal — this provides incorrect capacitance to each motor, causing both to run incorrectly or fail.
- Selecting a replacement with the correct µF values but a lower voltage rating than the original — the capacitor operates closer to its dielectric limit and fails prematurely, often within one season.
- Attempting to test a run capacitor with only the resistance or continuity function of a multimeter — a capacitor can appear open on resistance test but have severely reduced capacitance that only a capacitance measurement will reveal.
- Forgetting to discharge the capacitor before handling and touching the terminals — even a unit that has been off for some time can retain a charge sufficient to cause a painful and potentially dangerous electric shock.
- Replacing only the dual run capacitor without checking the start capacitor and start relay (if fitted) — a compressor that struggles to start may have multiple capacitor-related faults.
Troubleshooting
- Compressor hums but does not start; trips thermal overload
- Cause: HERM-to-C section of dual run capacitor has failed (open circuit or severely reduced capacitance); compressor auxiliary winding receives no phase-shift current Fix: Isolate power, discharge capacitor, and measure HERM-to-C capacitance. A reading significantly below rated value or an open circuit confirms failure. Replace the dual run capacitor with one matching the original specifications. Verify compressor winding resistance before restart.
- Condenser fan does not start or runs slowly
- Cause: FAN-to-C section of dual run capacitor has failed; fan motor auxiliary winding not receiving phase-shift current Fix: Measure FAN-to-C capacitance after isolating and discharging. If below rated value, replace the dual run capacitor. Also check fan motor winding resistance — a motor that has been stalled due to capacitor failure may have thermal damage and require replacement as well.
- Capacitor body is hot to the touch during operation
- Cause: Capacitor is overloaded due to incorrect capacitance value (too low causes excess current through capacitor), or ambient temperature exceeds capacitor rating Fix: Measure actual capacitance and verify it matches the system specification. Check condenser unit airflow is not obstructed — adequate airflow is required to keep capacitor and motor temperatures within limits. A capacitor that runs hot has a shortened service life and should be replaced.
Frequently asked questions
What does a dual run capacitor do in an HVAC system?
A dual run capacitor provides the phase-shift needed to create a rotating magnetic field in both the single-phase compressor motor and the single-phase condenser fan motor simultaneously. It stays permanently in circuit during operation, improving efficiency and maintaining the correct phase angle between the main and auxiliary motor windings. Without it, single-phase motors cannot develop adequate torque and will overheat.
How do I identify which terminal is C, HERM, and FAN?
Modern dual run capacitors have C, HERM, and FAN stamped or printed on the terminal housing. If labels are missing or faded: C is the shared common terminal, HERM connects to the larger capacitance section (compressor winding), and FAN connects to the smaller capacitance section (fan motor winding). The capacitance values are stated on the capacitor label as two figures separated by a plus sign (e.g. '35+5 µF').
Can I replace a dual run capacitor with two separate run capacitors?
Yes. A dual run capacitor can be replaced by two separate single capacitors: one matching the HERM-to-C capacitance (for the compressor) and one matching the FAN-to-C capacitance (for the fan motor). Connect the C terminals of both capacitors together and to the original C wire. Both wiring configurations are electrically identical — dual capacitors are simply more compact.
What is the maximum acceptable capacitance deviation for a run capacitor?
Industry practice in HVAC maintenance accepts a run capacitor within ±6 % of its rated capacitance as serviceable. A reading more than 6 % below rated value indicates the capacitor is degraded and approaching failure. A reading significantly above rated value or a shorted capacitor (zero impedance) requires immediate replacement.
Why does the compressor hum but not start when the capacitor fails?
A failed run capacitor means the compressor's auxiliary winding receives no phase-shifted current. Without the rotating field, the motor produces no starting torque and stalls at locked rotor. The high locked-rotor current rapidly overheats the motor and will trip the thermal overload protector within seconds to minutes. This is the classic symptom of a failed HERM-to-C capacitor section.
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