Start and Run Capacitor Wiring Diagram
This is a free printable start run capacitor wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A start/run capacitor wiring diagram shows how electrolytic start capacitors and film-type run capacitors connect to single-phase AC motor terminals to provide the phase shift needed for starting torque and efficient continuous operation.
Single-phase AC motors cannot self-start from a stationary position because the main winding alone produces a pulsating magnetic field with no net rotation. Capacitors solve this by introducing a phase-shifted current in a secondary winding, creating a rotating magnetic field.
A start capacitor (typically 88–300 µF, electrolytic construction) is connected in series with the start winding and a centrifugal switch or potential relay. It is only in the circuit momentarily during run-up—usually less than 3 seconds—before it is switched out. Because it operates briefly, it is not rated for continuous duty. Leaving a start capacitor energised will cause it to overheat and fail.
A run capacitor (typically 2–50 µF, oil-filled or metallised-film construction) remains in circuit permanently. It improves power factor, reduces running current, and keeps the motor running smoothly. Run capacitors are rated for continuous AC duty, commonly 370 V AC or 440 V AC.
In HVAC systems, a dual-run capacitor (sometimes called a dual-round or dual-rated capacitor) serves both the compressor and the condenser fan motor from a single can. It has three terminals labelled C (common), HERM (hermetic compressor) and FAN. The C terminal connects to the common supply. HERM connects to the compressor run winding. FAN connects to the fan motor run winding. The two capacitances share the common terminal but are electrically isolated from each other inside the can.
Split-phase induction motors use a start capacitor only. Capacitor-start capacitor-run motors use both: a large electrolytic start capacitor switched out at speed, and a smaller film run capacitor that stays connected. Permanent split capacitor (PSC) motors use only a run capacitor with no switching.
Wiring follows the motor nameplate and terminal diagram. Terminals are typically labelled: MAIN (or T1/T2), START (or S), RUN (or R), and COMMON (or C). The capacitor is always wired between the start or run terminal and the supply or common line—never directly across line voltage unless rated for it.
How to wire start run capacitor wiring diagram
- Identify the motor type and required capacitor values Read the motor nameplate for capacitor ratings. Distinguish between PSC (run cap only), capacitor-start (start cap only), and CSR (both). Note the µF value and voltage rating for each capacitor required.
- Isolate and verify the circuit is de-energised Switch off the circuit breaker, apply a lockout/tagout device, and use a non-contact voltage tester or multimeter to confirm no voltage is present at the motor terminals before touching any wiring.
- Discharge existing capacitors Bridge a 10–20 kΩ, 5 W resistor across each capacitor terminal pair and hold for at least 10 seconds. Confirm zero DC voltage with a multimeter. Never assume a capacitor is discharged simply because power is off.
- Locate and label motor terminals Identify MAIN, START, RUN, and COMMON terminals on the motor terminal board or leads. For a dual-run capacitor, identify C, HERM, and FAN terminals. Photograph existing wiring before disconnecting anything.
- Connect the run capacitor Wire the run capacitor between the run (R) terminal on the motor and the common (C) supply line, or between C and HERM/FAN on a dual-run unit. Film run capacitors are non-polarised; either terminal can connect to either motor terminal.
- Connect the start capacitor if applicable Wire the start capacitor in series with the start winding and the start relay or centrifugal switch. The switch opens automatically near full speed to remove the start capacitor from the circuit. Ensure the switch mechanism is functional.
- Restore power and verify operation With covers replaced, restore power and observe startup. The motor should start promptly and run smoothly. Measure running current and compare to the nameplate full-load amperage. Investigate any humming, slow starting, or tripping.
Specifications
| Typical run capacitor range (single-phase motors) | 2 µF to 50 µF at 370 V AC or 440 V AC, continuous duty |
|---|---|
| Typical start capacitor range | 88 µF to 300 µF at 125 V AC or 250 V AC, intermittent duty (< 3 s on-time) |
| Run capacitor tolerance | ±6% of rated capacitance (IEC 60252-1 / ANSI/EIA 198) |
| Start capacitor tolerance | −10% / +50% of rated capacitance (typical; check datasheet) |
| HVAC dual-run capacitor common ratings | 35/5 µF, 40/5 µF, 45/5 µF, 50/5 µF at 370 V AC or 440 V AC |
| Motor start speed threshold for capacitor switch-out | Approximately 75% of synchronous speed |
| Operating temperature range (film run capacitor, typical) | −40 °C to +85 °C |
Safety warnings
- Fixed electrical work on mains-connected motor circuits must be performed by a licensed electrician in accordance with local regulations, including NEC/NFPA 70 (USA), BS 7671 (UK), AS/NZS 3000 (Australia/NZ), and IEC 60364 (international). Always check which standard applies in your jurisdiction.
- Capacitors store potentially lethal charge even after the power supply is disconnected. Always discharge capacitors using an appropriate resistor before handling, and verify zero voltage with a calibrated meter.
- Never substitute a run capacitor for a start capacitor or vice versa. A run capacitor installed in a start position will fail quickly and can cause a fire. An electrolytic start capacitor left in a continuous-duty circuit will overheat and may rupture.
- Always isolate the supply and apply lockout/tagout before opening any motor or HVAC enclosure. Use a non-contact voltage tester to verify the circuit is dead before touching any conductors.
- Observe capacitor polarity where marked. Although run capacitors are non-polarised, some start capacitors and polarised types must be connected with correct polarity to avoid failure or explosion.
Tools needed
- Multimeter (AC/DC voltage and capacitance measurement)
- Non-contact voltage tester
- Insulated screwdrivers (flat and Phillips)
- Discharge resistor (10–20 kΩ, 5 W)
- Lockout/tagout kit
- Insulated pliers and wire strippers
- Camera or phone (to photograph existing wiring before disconnection)
Common mistakes
- Installing a run capacitor where a start capacitor is required (or vice versa): the electrolytic start capacitor will overheat in continuous service, and an undersized run capacitor will not provide adequate starting torque.
- Failing to discharge the capacitor before handling: residual charge can deliver a severe shock even minutes after power is removed.
- Replacing a dual-run capacitor with a single-section unit and misidentifying the HERM and FAN terminals, causing the wrong motor to receive incorrect capacitance.
- Exceeding the voltage rating: replacing a 440 V AC capacitor with a 370 V AC unit in a 440 V AC circuit causes premature failure.
- Leaving a start capacitor in circuit because the centrifugal switch or potential relay was not repaired or replaced, causing the start capacitor to burn out rapidly.
- Ignoring tolerance: run capacitors must be within ±6% of rated value; excessive deviation causes motor heating and reduced efficiency.
Troubleshooting
- Motor hums but will not start
- Cause: Failed start capacitor or open start relay/centrifugal switch, preventing phase-shifted current in the start winding Fix: Discharge and test the start capacitor with a capacitance meter. Check the relay or centrifugal switch for continuity in the de-energised state. Replace any failed component with a correctly rated equivalent.
- Motor starts slowly and draws high current
- Cause: Start capacitor is below rated capacitance, reducing starting torque, or the switch opens prematurely Fix: Measure capacitance with a meter; replace if more than 20% below the rated value. Check centrifugal switch operation and adjust or replace if it opens before 75% speed.
- Motor runs hot and draws higher than nameplate current
- Cause: Run capacitor has failed open or its capacitance has drifted significantly low, causing power factor degradation and increased current in the main winding Fix: Measure run capacitor capacitance and compare to nameplate rating (must be within ±6%). Replace if outside tolerance with an equivalent continuous-duty film capacitor at the correct voltage rating.
- Dual-run capacitor bulging or leaking oil
- Cause: The capacitor has reached end of life, possibly due to over-voltage, thermal stress, or simply age Fix: Replace the dual-run capacitor with a unit matching the capacitance values (µF) and voltage rating printed on the old unit's label. Do not operate the equipment further with a visibly failed capacitor.
- HVAC compressor trips overload but fan runs
- Cause: The HERM section of the dual-run capacitor has failed, leaving the compressor motor without its run capacitor Fix: Measure capacitance between C and HERM terminals (compare to label). If open or significantly low, replace the dual-run capacitor. Also check compressor windings for ground faults or winding resistance imbalance.
Frequently asked questions
What is the difference between a start capacitor and a run capacitor?
A start capacitor is an electrolytic type rated for brief use only (seconds), providing high capacitance to boost starting torque. A run capacitor is a film or oil-filled type rated for continuous operation, providing lower capacitance to improve running efficiency and power factor. Using the wrong type will cause premature failure.
What do the terminals C, HERM, and FAN mean on a dual-run capacitor?
C is the common terminal shared by both capacitor sections and connects to the supply common. HERM connects to the compressor motor run winding. FAN connects to the condenser fan motor run winding. The two capacitor sections share the C terminal but are internally isolated, so each has its own rated capacitance and voltage rating.
Can I replace a dual-run capacitor with two separate run capacitors?
Yes. Match the rated capacitance and voltage for each section independently. Wire the compressor section between the compressor run terminal and common, and the fan section between the fan run terminal and common. Use capacitors rated for continuous AC duty at the correct voltage (typically 370 V AC or 440 V AC).
How do I know if a capacitor has failed?
Signs include a motor that hums but will not start, a motor that starts slowly or with a spin assist, a bulged or leaking capacitor casing, and a capacitance reading significantly outside the rated tolerance (typically ±6% for run, ±20% for start). Measure with a capacitance meter after safely discharging the capacitor.
How should I discharge a capacitor before handling it?
After de-energising the circuit and verifying it is dead, use a resistor of approximately 10–20 kΩ (rated 5 W or higher) bridged across the capacitor terminals for at least 10 seconds. Never short the terminals directly: the resulting arc can damage the capacitor and cause injury. Verify discharge with a multimeter set to DC voltage.
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