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.

Create Your Own Capacitor Wiring Diagram

CircuitDiagramMaker handles motor capacitor circuits clearly:

Create your own capacitor wiring diagram -- free

Testing a Capacitor with a Multimeter

A failed capacitor is one of the most common reasons a single-phase motor hums but does not start, or runs weak. Testing confirms whether the capacitor is within spec before you replace it.

  1. Disconnect and lock out power, then discharge the capacitor per the safety procedure above.
  2. Disconnect at least one terminal so the capacitor is isolated from the rest of the circuit -- testing it in-circuit gives an inaccurate reading.
  3. Set the multimeter to capacitance mode (µF, often marked with a capacitor symbol). Not all multimeters have this function.
  4. Touch the meter leads to the two terminals. For a dual-run capacitor, test each section separately: HERM to C, then FAN to C.
  5. Compare the reading to the nameplate value. A capacitor within ±10% of its rated µF is good. A reading far below rating (or near zero) means the capacitor has failed open. A reading that keeps climbing and never settles, or an unstable/very high reading, suggests a shorted or leaking capacitor.
  6. If the meter has no capacitance function, use the resistance (Ω) setting on a high range instead. Touch the leads to the terminals: a good capacitor shows a brief low-resistance "kick" that climbs toward OL (infinite resistance) as the meter's internal battery charges it. A capacitor that stays near zero ohms is shorted; one that reads OL immediately with no kick is open.

Capacitive Reactance and Capacitor Sizing

A run capacitor works by introducing capacitive reactance into the auxiliary winding circuit, which shifts the current's phase relative to the main winding and creates the rotating field a single-phase motor needs to develop torque.

Capacitive reactance is calculated as:

Xc = 1 / (2 × π × f × C)

Where Xc is reactance in ohms, f is line frequency in Hz (60 Hz in the US), and C is capacitance in farads.

Example: a 35 µF run capacitor on a 60 Hz line has a reactance of approximately 1 / (2 × π × 60 × 0.000035) ≈ 75.8 Ω.

Lower reactance (a larger µF value) allows more current through the auxiliary winding, which increases running torque but also increases current draw and heat in that winding. This is why the nameplate µF rating is not arbitrary -- it is matched to the auxiliary winding's turns and wire gauge to produce a specific phase angle, and substituting a significantly higher µF value causes overcurrent and winding heat even though the motor may seem to run fine at first.

Motor Types and Capacitor Configurations

Not every single-phase motor uses the same capacitor arrangement. The motor type determines whether it needs a start capacitor, a run capacitor, both, or neither:

Motor Type Abbreviation Start Capacitor Run Capacitor Typical Use
Capacitor Start, Induction Run CSIR Yes (electrolytic) No Small pumps, low-torque fans
Capacitor Start, Capacitor Run CSCR (or CSR) Yes (electrolytic) Yes (film) Compressors, air handlers -- high starting torque
Permanent Split Capacitor PSC No Yes (film) Condenser fans, blower motors, ceiling fans
Shaded Pole -- No No Small low-torque fans (uses a shading coil instead)

CSCR motors use both capacitor types together: the start capacitor is in the circuit briefly through the centrifugal switch for high starting torque, and the run capacitor stays connected continuously once the switch opens. PSC motors skip the start capacitor entirely because the load does not need extra starting torque -- the single run capacitor handles both starting and running duty, which is why PSC motors have noticeably lower starting torque than CSIR or CSCR designs.

Key Takeaways

Frequently asked questions

What happens if a capacitor is wired backwards?

Motor start and run capacitors used in AC circuits are non-polarized, so there is no "backwards" to worry about -- either terminal can connect to either side of the circuit. Polarity only matters for polarized electrolytic capacitors used in DC circuits, which are not the type used in motor start/run applications.

Can I replace a start capacitor with a higher microfarad rating?

Going slightly higher in µF is generally more tolerant than going lower, but staying within the manufacturer's stated range (often printed as a range like 88-108 µF) is safest. A capacitor well above the rated range increases current through the start winding and can overheat it or shorten the life of the centrifugal switch.

How long does it take for a capacitor to discharge on its own?

Most motor run and start capacitors have no built-in bleed resistor, so they can hold a dangerous charge for minutes to hours after power is removed, especially dual-run HVAC capacitors. Never assume a capacitor has discharged on its own -- always discharge it manually through a resistor before handling the terminals.

What size wire do I use for capacitor connections?

There is no single standard gauge -- match the wire size already used on the motor's start or auxiliary winding leads, since capacitor circuit current is limited by the winding and the capacitor's reactance rather than by a separate rating. Check the motor's nameplate or original factory wiring diagram rather than guessing.

Is it safe to touch a capacitor with a screwdriver to discharge it?

No. Shorting a charged capacitor directly with a metal screwdriver releases the stored energy almost instantly, which can cause a bright arc, pit the terminals, damage the capacitor, and startle or injure whoever is holding the tool. Use an insulated discharge resistor (around 20 kOhm, 5W) instead, which bleeds the charge safely over a few seconds.

Can a bad capacitor damage the motor it's connected to?

Yes. A failed start capacitor that stays in the circuit (because a centrifugal switch failed closed) will overheat the start winding within minutes. A weak run capacitor causes the motor to draw excess current and run hot continuously, which shortens motor life and can trip the thermal overload repeatedly.

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