3-Wire AC Dual Run Capacitor Wiring Diagram: HVAC Terminals Explained
This is a free printable 3 wire ac dual capacitor wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A 3-wire AC dual run capacitor wiring diagram shows how the C (common), HERM (compressor), and FAN terminals connect to the compressor motor, fan motor, and AC supply in an HVAC system.
A dual run capacitor is a single cylindrical or oval capacitor housing that contains two capacitor sections sharing a common terminal. It is standard equipment in split-system air conditioners and heat pumps, where it provides run capacitance for both the compressor motor and the condenser fan motor simultaneously from one compact component.
The three terminals on a dual run capacitor are: C (Common), HERM (Hermetic compressor), and FAN. Understanding each terminal's role is essential for correct replacement and wiring.
The C terminal (Common) is the shared return terminal for both internal capacitor sections. In the HVAC circuit, C connects to the common terminal of the contactor — the line that delivers mains AC supply voltage to the capacitor. Both internal capacitor sections connect from their respective live terminals (HERM and FAN) back to C.
The HERM terminal connects to the start/run winding (sometimes called the auxiliary winding) of the compressor motor. The capacitor creates a phase-shifted current in this winding relative to the main (run) winding, maintaining a rotating magnetic field and keeping the motor running efficiently at full load. Without capacitance on HERM, the compressor motor runs overloaded, draws excessive current, and overheats.
The FAN terminal connects to the start winding of the condenser fan motor. Like the compressor, a permanent-split-capacitor (PSC) fan motor relies on the capacitor to create the phase difference needed to maintain rotation and efficiency.
When replacing a dual run capacitor, both the capacitance value (measured in microfarads, µF) and the voltage rating of each section must match or exceed the original specification. Dual capacitors are labelled with two capacitance values separated by a slash — for example, 45+5 µF — where the larger value is for HERM and the smaller for FAN. The voltage rating (typically 370 V AC or 440 V AC) must not be reduced.
This is a generic illustrative reference. Always isolate and discharge the capacitor before handling, and follow the HVAC equipment manufacturer's wiring diagram.
How to wire 3 wire ac dual capacitor wiring diagram
- Isolate the HVAC system Switch off the unit at the thermostat, then disconnect the power at the outdoor disconnect switch (the fused safety disconnect box near the outdoor unit). Open the disconnect and verify with a non-contact voltage tester that the terminal block inside the outdoor unit is dead.
- Discharge the dual run capacitor Even with power off, the capacitor can hold a charge. Using an insulated resistor of approximately 20 kΩ rated for the capacitor's voltage, connect it across each pair of capacitor terminals (C to HERM, then C to FAN) for approximately 5 seconds per connection to dissipate the stored charge. Verify with a multimeter on AC voltage mode — should read 0 V.
- Photograph the existing wiring before disconnecting Before removing any wires, photograph the existing capacitor with all wires attached. The photograph serves as a reference if terminal labels on the replacement are in a different orientation or order.
- Note the existing capacitor's ratings Read the capacitor label: note the dual capacitance values (e.g., 45+5 µF) and the AC voltage rating (370 V or 440 V). The replacement must match these values exactly — do not substitute with a lower capacitance or lower voltage rating.
- Remove and replace the capacitor Disconnect all wires from the old capacitor terminals — they may be push-on (spade) connectors or screw terminals. Remove the capacitor from its mounting strap or bracket. Install the new capacitor in the same orientation and secure the mounting strap.
- Reconnect wiring to correct terminals Using your photograph as reference: connect the wire from the AC supply contactor common to the C terminal; the wire from the compressor's run winding to the HERM terminal; and the wire from the fan motor's start winding to the FAN terminal. Ensure all push-on connectors are fully seated.
- Restore power and test operation Reinstate the fused disconnect. Set the thermostat to call for cooling. Verify the compressor starts and runs without tripping, and that the condenser fan runs at correct speed. Use a clamp meter to measure compressor running current and compare against the nameplate RLA (Rated Load Amps).
Specifications
| Capacitor terminal labels | C (Common), HERM (Hermetic compressor), FAN (condenser fan) |
|---|---|
| Typical HERM section capacitance range | 25–80 µF (varies by compressor; verify equipment nameplate) |
| Typical FAN section capacitance range | 3–10 µF (varies by fan motor; verify equipment nameplate) |
| Standard voltage ratings available | 370 V AC and 440 V AC (440 V provides greater safety margin at 240 V supply) |
| Acceptable capacitance tolerance for replacement | ±6 % of nameplate value (tighter is better; do not exceed rated value significantly) |
| Operating frequency | 50 Hz or 60 Hz (verify capacitor is rated for the system supply frequency) |
| Typical operating temperature range | −40 °C to +70 °C (verify manufacturer's specification for outdoor installation) |
| Discharge time before safe handling (with 20 kΩ resistor) | Minimum 5 seconds per terminal pair; verify 0 V with multimeter before touching |
Safety warnings
- Dual run capacitors store dangerous levels of electrical energy and can deliver a severe or fatal electric shock even after the power supply has been disconnected. Always discharge the capacitor using an insulated resistor and verify zero voltage with a multimeter before touching any terminals.
- HVAC equipment operates at mains voltage (typically 208–240 V AC in North America, or 220–240 V AC in other regions). All work must be performed by a qualified HVAC technician or licensed electrician in accordance with NEC/NFPA 70, BS 7671, AS/NZS 3000, or the applicable national standard. Regulations in many jurisdictions prohibit DIY work on permanently wired HVAC equipment.
- Never replace a dual run capacitor with one rated at a lower voltage. Under-rated capacitors may fail explosively when energised at the system operating voltage.
- Do not operate the compressor for extended periods with a failed or missing run capacitor — the compressor motor will overheat and sustain winding damage. If the capacitor has failed, replace it before attempting to run the system.
- Verify that the outdoor unit's fused disconnect is locked out before opening any panels or touching any internal wiring. Use a non-contact voltage tester to confirm all conductors are de-energised.
Tools needed
- Non-contact voltage tester
- Digital multimeter with capacitance (µF) measurement function
- Insulated discharge resistor (20 kΩ, 5 W)
- Clamp meter (for measuring running current after replacement)
- Insulated screwdrivers (flat and Phillips)
- Needle-nose pliers (for removing push-on connectors)
- Camera or smartphone (to photograph existing wiring before disconnection)
- HVAC equipment manufacturer's wiring diagram
Common mistakes
- Wiring the compressor run winding to the FAN terminal and the fan motor to the HERM terminal — this crosses the capacitor sections and can prevent both motors from starting correctly or damage the windings.
- Replacing a 440 V-rated capacitor with a 370 V-rated equivalent when the system operates at 240 V AC — while 370 V may be technically sufficient in some cases, the lower voltage rating reduces the safety margin and can lead to premature failure.
- Not discharging the capacitor before handling and receiving a painful or dangerous shock from the stored charge.
- Selecting a replacement based on physical size rather than the rated capacitance and voltage values on the label, installing an incorrectly rated unit.
- Leaving existing wires disconnected from the capacitor when installing the new unit because they 'look the same length' — always trace and reconnect each wire to its correct terminal using the wiring diagram and original photograph.
Troubleshooting
- Compressor hums and trips thermal overload but condenser fan runs normally
- Cause: Failed HERM section of the dual capacitor; compressor motor lacks run capacitance Fix: Isolate and discharge the capacitor. Measure capacitance on the HERM section — a reading significantly below rated value confirms failure. Replace the dual capacitor with a correctly rated unit.
- Condenser fan does not spin or runs at reduced speed; compressor operates normally
- Cause: Failed FAN section of the dual capacitor; fan motor lacks run capacitance Fix: Measure capacitance on the FAN section. A low or zero reading indicates failure. Replace the dual capacitor. Note that a completely dead fan motor (windings open-circuit or shorted) will also fail this way — verify motor winding resistance if a new capacitor does not resolve the issue.
- Both compressor and fan fail to start
- Cause: Failed C terminal connection, failed contactor, blown fuse in disconnect, or total capacitor failure Fix: Check for mains voltage at the contactor output with the thermostat calling for cooling. If voltage is present at the contactor output, verify the C terminal wire connection on the capacitor. Test capacitance on both sections. Replace the capacitor if both sections are out of specification.
Frequently asked questions
What do the terminals C, HERM, and FAN mean on a dual run capacitor?
C (Common) is the shared return terminal connecting both internal capacitor sections back to the AC supply. HERM connects to the compressor motor's run/auxiliary winding. FAN connects to the condenser fan motor's start winding. Both HERM and FAN are internally connected back to C through their respective capacitor sections.
Can I replace a dual run capacitor with two single capacitors?
Yes. Two separate single-run capacitors — one rated for the compressor (HERM) and one for the fan — can substitute for a dual capacitor when an exact dual replacement is unavailable. Wire each capacitor's terminals to match the original dual capacitor connections. Ensure each single capacitor's ratings match the corresponding section of the original dual unit.
What happens if the HERM capacitor section fails?
Without run capacitance on the HERM circuit, the compressor motor loses its phase-split run winding support. It will draw high current, run hot, and the internal thermal overload protector will trip, shutting the compressor off. Repeated thermal trips without fixing the capacitor will eventually burn out the compressor winding.
How do I read dual capacitor ratings?
A dual run capacitor is labelled with two capacitance values separated by a slash and a voltage rating — for example, '45+5 µF, 440 V AC'. The first value (45 µF in this example) is the HERM section; the second (5 µF) is the FAN section. The voltage rating applies to both sections and must not be replaced with a lower-rated unit.
Are dual run capacitors dangerous after being disconnected from power?
Yes. Run capacitors store electrical energy and can retain a dangerous charge for several minutes after the power supply is disconnected. Always use an insulated resistor (approximately 20 kΩ, 5 W rated for the capacitor's voltage) to discharge the capacitor before handling any terminals. Verify discharge with a multimeter before touching.
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