2-Terminal Capacitor Wiring Diagram
This is a free printable 2 terminal capacitor wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A clear reference for wiring single run or start capacitors with two terminals in HVAC, fan, and single-phase motor applications.
A 2-terminal capacitor is the simplest capacitor form used in AC motor circuits. Unlike a 3-terminal dual-run capacitor that serves both the compressor and the fan motor, a 2-terminal (single) capacitor has exactly two leads and serves a single purpose: either a run capacitor or a start capacitor.
Run capacitors are permanently in the circuit while the motor operates. They maintain a phase shift between the main and auxiliary windings, improving efficiency and torque. Typical values range from 2 µF to 70 µF at 370 VAC or 440 VAC rating. HVAC fan motors most commonly use 5 µF or 7.5 µF run capacitors rated at 370 VAC or 440 VAC.
Start capacitors are switched out of circuit once the motor reaches approximately 75–80% of synchronous speed, typically via a centrifugal switch. Start capacitors carry much higher capacitance values (typically 88–400 µF) at lower voltage ratings (125–330 VAC) because they are only energised for a fraction of a second.
In a standard single-phase motor wiring diagram with a 2-terminal run capacitor: one terminal connects to the auxiliary (start) winding lead, and the other connects to the line side of the supply. The main winding connects directly across the supply. The capacitor shifts the current phase in the auxiliary winding by approximately 90 degrees, creating a rotating magnetic field.
Voltage rating must always meet or exceed the circuit voltage. Never substitute a start capacitor in place of a run capacitor — they are not thermally rated for continuous duty and will fail or rupture. The capacitor must be discharged before handling, as 2-terminal run capacitors can retain dangerous charge after de-energisation.
This diagram serves as a general illustrative reference only. All installation and replacement work must be carried out by a qualified electrician in compliance with applicable codes including NEC/NFPA 70, BS 7671, AS/NZS 3000, or IEC 60364.
How to wire 2 terminal capacitor wiring diagram
- Isolate and verify the circuit is dead Switch off the motor's disconnect or circuit breaker. Lock out / tag out per your local safety procedure. Use a calibrated AC voltmeter to confirm zero voltage at both supply terminals before touching any wiring.
- Discharge the existing capacitor Bridge a 10 kΩ, 5 W (or higher) resistor across both capacitor terminals and hold for at least 5 seconds. Verify discharge with a DC voltmeter set to 600 V DC before handling.
- Record and photograph existing wiring Note which terminal connects to the auxiliary (start) winding and which connects to the supply line. Photograph before disconnecting any wires. Capacitor lead labelling varies by manufacturer.
- Match replacement capacitor specifications Match the capacitance value exactly (e.g. 7.5 µF ±6%). Select a voltage rating equal to or greater than the original. Confirm the capacitor type (run or start) is correct for the application.
- Connect the replacement capacitor One terminal connects to the auxiliary winding lead (often marked HERM or AUX on the motor terminal block). The other terminal connects to the supply-side line conductor. Tighten terminals securely to prevent arcing under load.
- Restore power and test motor operation Re-energise the circuit. Measure running current with a clamp meter and compare against the motor nameplate full-load current. Elevated current can indicate a capacitor value mismatch or a failing winding.
Specifications
| Typical run capacitor range | 2 µF – 70 µF |
|---|---|
| Run capacitor voltage ratings (standard) | 370 VAC, 440 VAC |
| Typical start capacitor range | 88 µF – 400 µF |
| Start capacitor voltage ratings (standard) | 125 VAC, 165 VAC, 250 VAC, 330 VAC |
| Capacitance tolerance (run capacitors) | ±5% to ±6% (per ANSI/EIA-464) |
| Operating temperature range (typical) | -40 °C to +70 °C (run); -40 °C to +65 °C (start) |
Safety warnings
- Always isolate, lock out, and tag out the power supply before working on any capacitor or motor circuit. Verify the circuit is dead with a calibrated voltmeter.
- Capacitors can retain a lethal charge after de-energisation. Always discharge using an appropriate resistor and verify discharge with a DC voltmeter before touching terminals.
- Never replace a run capacitor with a start capacitor. Start capacitors are not rated for continuous duty and will overheat, rupture, or cause fire.
- Installation and replacement must be performed by a licensed electrician in compliance with NEC/NFPA 70 (US), BS 7671 (UK), AS/NZS 3000 (Australia/NZ), or IEC 60364 (international).
- Never exceed the motor manufacturer's specified capacitance value. Incorrect sizing causes overheating and premature motor winding failure.
Tools needed
- Calibrated AC/DC digital multimeter (CAT III 600 V minimum)
- Capacitance meter or multimeter with capacitance function
- Insulated screwdrivers (flat and Phillips)
- Discharge resistor (10 kΩ, 5 W)
- Clamp meter for running current measurement
- Lockout/tagout kit
- Wire strippers and insulated crimp terminals
Common mistakes
- Fitting a start capacitor in place of a run capacitor — start caps are not thermally rated for continuous duty and will fail rapidly.
- Ignoring voltage rating — installing a 370 VAC capacitor on a 440 VAC circuit reduces service life and risks dielectric breakdown.
- Oversizing capacitance — more is not better; excessive µF causes the auxiliary winding to draw excessive current and overheats the motor.
- Failing to discharge the old capacitor before disconnecting it, risking electric shock.
- Not photographing the original wiring before removal, leading to incorrect reconnection of auxiliary versus main winding leads.
Troubleshooting
- Motor hums but does not start
- Cause: Open or failed capacitor (run or start); capacitor not delivering phase shift required to develop starting torque Fix: Test capacitor capacitance with a capacitance meter. A reading significantly below rated value (more than 10% low) indicates failure. Replace with correct specification capacitor.
- Motor runs but draws excessive current
- Cause: Capacitance value too high, or incorrect capacitor type installed Fix: Measure running current and compare to nameplate FLA. Verify installed capacitor value matches motor nameplate specification exactly.
- Capacitor housing is bulged, cracked, or shows burn marks
- Cause: Capacitor has failed due to voltage spikes, overheat, or end-of-service-life Fix: Replace immediately. Do not attempt to test or re-use a visibly damaged capacitor. Discharge carefully even if damaged.
Frequently asked questions
What is the difference between a run capacitor and a start capacitor?
A run capacitor remains in circuit continuously during motor operation and is rated for continuous AC duty (typically 370 VAC or 440 VAC). A start capacitor is switched out once the motor reaches speed and uses higher capacitance at lower voltage ratings. They are not interchangeable.
Can I use a higher capacitance run capacitor than specified?
No. Oversizing a run capacitor increases current draw in the auxiliary winding, overheats the motor, and can cause premature winding failure. Always match the manufacturer's specified capacitance value within the ±5–10% tolerance stated on the motor nameplate.
Why does a 2-terminal capacitor have no polarity markings?
Motor run and start capacitors are non-polarised AC capacitors (typically metalized film or electrolytic AC type). AC voltage alternates in direction each half cycle, so either terminal can connect to either winding or line lead.
How do I discharge a 2-terminal capacitor safely?
After de-energising the circuit and verifying it is dead with a multimeter, use a resistor (typically 5–20 kΩ, 5 W or higher) bridged across both terminals for at least 5 seconds. Never short the terminals directly — the arc can destroy the capacitor and cause injury.
What voltage rating should I choose for a replacement run capacitor?
Select a capacitor with a voltage rating equal to or higher than the circuit's AC voltage. A 370 VAC-rated capacitor can be replaced with a 440 VAC-rated unit of the same capacitance; the higher rating increases service life. Never use a lower-rated replacement.
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