4 terminal capacitor wiring diagram

4 Terminal Capacitor Wiring Diagram — circuit diagram showing component connections230V SupplyC1 (Start)C2 (Common)4 Terminal Capacitor Wiring Diagram
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A 4-terminal capacitor wiring diagram shows the internal construction and terminal connections for dual-capacitor units commonly used in HVAC systems controlling both the compressor motor and condenser fan motor through a single capacitor assembly. The diagram identifies: terminals labeled H (herm/hermetic compressor), FAN, COM (common), and GND (ground). The hermetic terminal connects to the compressor motor run circuit, the fan terminal connects to the condenser fan motor, and the common terminal connects to the AC line power. The ground terminal connects to the equipment chassis providing electrical safety. Internally, the 4-terminal capacitor contains two separate capacitors: the hermetic capacitor (50-100µF typical) providing the phase shift for compressor motor starting and running, and the fan capacitor (3-5µF typical) supporting the fan motor. Dual-capacitor design eliminates the need for separate capacitors, reducing cost and simplifying field replacement. The diagram shows proper voltage rating selection (370V or 440V) based on the supply voltage, and the microfarad (µF) capacity matching motor specifications. Understanding 4-terminal capacitor wiring enables proper replacement in HVAC systems, safe handling of high-voltage energy stored in capacitors, and troubleshooting of motor starting failures.

How to wire 4 terminal capacitor wiring diagram

  1. Isolate power and discharge the capacitor before touching terminals Switch off the unit disconnect and wait 5 minutes. Then discharge each capacitor terminal pair through a 20kΩ 5W resistor held across the terminals. Never short capacitor terminals directly — the arc can burn you and damage terminals.
  2. Photograph all capacitor wiring connections before removal Take a clear photo of every wire connected to C, HERM, and FAN terminals. Capacitors are easy to rewire incorrectly, and transposing HERM and FAN wires causes the wrong motor to receive the incorrect capacitance, leading to immediate failure.
  3. Test old capacitor MFD values With wires removed, test with a capacitor meter between HERM and C, then between FAN and C. Record values and compare to nameplate rating. If either is more than 10% low, proceed with replacement.
  4. Source correct replacement capacitor Match MFD values exactly — HERM and FAN ratings are separate. Voltage rating must equal or exceed original. Round dual-run capacitors interchange between brands; oval capacitors are interchangeable only if bracket fits.
  5. Mount new capacitor and reconnect wires Secure new capacitor in bracket. Reconnect wires per your photograph: compressor run winding to HERM, fan motor run winding to FAN, and both motor run-circuit returns to C. Insulate any exposed terminals with push-on terminal covers.
  6. Restore power and measure running current Energise unit and measure compressor and fan motor current with a clamp meter. Compare to nameplate RLA values. High current after capacitor replacement indicates a winding fault in the motor itself.

Frequently asked questions

What does each terminal on a 4-terminal dual-run capacitor do?

A dual-run capacitor has three electrical terminals: C (common) connects to both motors' run circuit returns, HERM (hermetic) connects to the compressor motor run winding, and FAN connects to the condenser fan motor run winding. The fourth terminal is the earth/chassis bonding stud — not part of the motor circuit.

Can I replace a dual-run capacitor with two separate single-run capacitors?

Yes. Use one single-run capacitor at the HERM MFD rating for the compressor and a separate single-run capacitor at the FAN MFD rating for the fan motor. Both capacitor commons connect together at the C terminal wire. This is a valid field replacement when an exact dual-run replacement is unavailable.

How do I test a dual-run capacitor before replacement?

Discharge the capacitor first by shorting HERM to C and FAN to C through a 20kΩ resistor. Then measure MFD between HERM and C, and between FAN and C, with a capacitor meter. If either reading falls more than 10% below the nameplate MFD value, replace the capacitor.

Why does a weak capacitor cause the compressor to trip on overload?

The run capacitor provides the phase-shift current that keeps the compressor motor running efficiently. A low-MFD capacitor forces the motor to draw higher running current on its main winding to compensate, pushing it over the overload trip threshold — especially in hot weather when the motor is already thermally stressed.

Is the capacitor voltage rating critical?

Yes. Always replace with the same MFD values but you can fit a higher voltage rating capacitor if the exact voltage is unavailable — for example, a 440VAC capacitor can replace a 370VAC unit in the same circuit. Never fit a lower voltage rating than the original, as it will fail rapidly or rupture.

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