Capacitor Wiring Diagram: Start, Run, and Motor Capacitors

Capacitors in motor circuits serve a specific purpose: they phase-shift the current in the start or auxiliary winding to create the rotating magnetic field that a single-phase AC motor cannot produce on its own. Wire the wrong capacitor type, install the wrong value, or connect the terminals incorrectly, and the motor either fails to start, overheats, or trips its thermal overload. This guide covers start and run capacitor wiring, the dual-run capacitor found in HVAC systems, and how to discharge and safely handle capacitors before working on the circuit.

Start Capacitor vs. Run Capacitor

These two component types look similar but are fundamentally different:

Start Capacitor

Start capacitors provide a large phase shift for high starting torque. They are always used with a centrifugal switch or potential relay that disconnects them once the motor reaches 65--80% of synchronous speed.

Run Capacitor

Run capacitors improve power factor and running efficiency. They stay in the circuit permanently. Never substitute an electrolytic start capacitor for a run capacitor -- the electrolytic will fail within minutes in continuous service.

CBB60 and CBB61 Film Capacitors

Two IEC/Chinese designations you will encounter in residential equipment:

Both are film capacitors rated for continuous AC duty -- they are run capacitors.

Wiring a Start Capacitor

Start capacitors wire in series with the start winding and a switching device (centrifugal switch or potential relay):

L1 ──────────────────── Main winding ─── L2
L1 ── [Centrifugal switch] ── [Start cap] ── Start winding ─── L2

The two terminals on a start capacitor are not polarized -- either lug can connect to either side of the circuit. Polarity does not matter for AC-rated capacitors.

Step-by-step:

  1. Disconnect and lock out power.
  2. Identify the start winding terminals (typically T3/T4 or S1/S2 on the motor terminal board, or a separate lead pair from the motor).
  3. Connect one terminal of the start cap to the junction of the centrifugal switch output and T3.
  4. Connect the other terminal of the start cap to the remaining start winding terminal T4, or to L2 (depending on motor design -- check the nameplate diagram).
  5. Verify the centrifugal switch is in series (not bypassed).

Wiring a Run Capacitor (2-Terminal)

A 2-terminal run cap wires in series with the auxiliary winding -- with no centrifugal switch, because it stays in the circuit permanently:

L1 ──── Main winding ─── L2
L1 ──── [Run cap] ─── Aux winding ─── L2

One terminal of the cap goes to L1 (line); the other connects to one end of the auxiliary winding. The other end of the auxiliary winding goes to L2.

On many PSC (permanent split-capacitor) motors -- ceiling fans, condenser fans, furnace blowers -- the run capacitor is already connected internally. If you are replacing the motor, match the µF rating and voltage exactly. A run cap that is off by more than 10% in capacitance will cause the motor to run hot, have reduced torque, or draw excess current.

Dual-Run Capacitor (HVAC)

HVAC systems -- specifically split-system air conditioners and heat pumps -- use a single can that contains two independent capacitors. This is the dual-run capacitor (also called a dual-rated or combo capacitor).

Terminals

A dual-run capacitor has three terminals labeled:

The value is printed as two numbers separated by a slash: for example, 45/5 µF, 440 V AC. The larger value (45 µF) serves the HERM terminal; the smaller (5 µF) serves the FAN terminal. C is the common return.

Wiring

Compressor: HERM terminal ─── Compressor start/run winding terminal
Fan motor:  FAN terminal ─── Fan motor run winding terminal
Common:     C terminal ─── L1 (line, through contactor)

Terminal labels (by wire color on most residential HVAC systems):

Always follow the original wiring diagram on the unit's access panel -- color conventions vary by manufacturer.

Replacing a Dual-Run Capacitor

  1. Photograph the existing wiring before disconnecting anything.
  2. Discharge the capacitor (see safety section below).
  3. Note the µF ratings (e.g., 45/5 µF) and voltage rating (370 V AC or 440 V AC). A 440 V AC cap can replace a 370 V AC rating; do not substitute a lower voltage rating.
  4. The capacitance must match within ±5% on the HERM value and ±10% on the FAN value. Do not substitute a 35/5 µF for a 45/5 µF -- the compressor will overheat.
  5. Connect HERM, FAN, and C as documented.

Safety: Discharging Capacitors

This is not optional. Motor run and start capacitors store charge at line voltage (120--240 V AC peak) and can deliver a painful or dangerous shock even after power is removed. The dual-run capacitor in an HVAC unit can hold its charge for several minutes to hours.

Discharge procedure:

  1. Disconnect and lock out the power supply.
  2. Obtain a discharge resistor: 20 kΩ, 5 W (or two 10 kΩ, 5 W resistors in series). Do not use a screwdriver to short the terminals -- the energy dump can damage the capacitor and produce a dangerous arc.
  3. With insulated pliers or holding only the insulated resistor body, touch one end of the resistor to each terminal of the capacitor and hold for 5 seconds.
  4. For a dual-run cap: discharge HERM to C, then FAN to C.
  5. Verify with a multimeter (DC volts setting) across each terminal pair that the voltage has dropped to near zero before handling.

Common Wiring Faults

Start cap left in circuit permanently: Someone bypassed the centrifugal switch or the switch failed open while the cap was wired directly in. The electrolytic overheats and fails within minutes.

Wrong µF value: A start cap value that is too low reduces starting torque (motor may fail to start under load). Too high causes overcurrent and heating in the start winding.

Wrong voltage rating (too low): A 250 V AC run cap in a 240 V system sees 339 V peak. Running below rated voltage causes premature dielectric breakdown.

Mixing cap types: Using a run cap as a start cap. Film caps have too low a capacitance for good starting torque. Using a start cap as a run cap -- far more dangerous -- causes rapid failure.

Disconnected C terminal on dual-run cap: The fan or compressor motor runs on main winding only, heating the winding rapidly.

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Key Takeaways