Lighting Contactor Wiring Diagram
This is a free printable lighting contactor wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A lighting contactor wiring diagram shows how an electrically or mechanically held contactor switches large lighting loads from a low-current control circuit, covering coil, main contacts, and auxiliary wiring.
A lighting contactor is a heavy-duty relay designed specifically for switching AC lighting loads — fluorescent, HID (high-intensity discharge), LED drivers, and incandescent — which can generate high inrush currents at the moment of energisation. Standard motor contactors are not always rated for lighting duty (AC5a/AC5b in IEC terminology), so specifying the correct contact rating is important.
The two fundamental architectures of lighting contactors differ critically in behaviour during power interruptions:
An electrically held contactor requires continuous coil energisation to keep its contacts closed. When control power is removed — whether intentionally or due to a power failure — the contactor opens and the lighting load drops out. This is the default behaviour of most standard IEC contactors and is generally preferred where fail-safe de-energisation on power loss is desirable (emergency luminaire circuits excluded).
A mechanically held (latching) contactor uses two coils: a latch coil and an unlatch coil. A brief pulse energises the latch coil, mechanically latching the main contacts closed. The contacts remain closed with no further coil current. A second pulse on the unlatch coil releases the mechanism. Because no continuous coil power is required, mechanically held contactors generate no coil heat during the held state, draw no holding current, and remain in their last state through a power failure. They are used in large installations where hundreds of contactors must be held closed continuously and the energy saving on coil power is significant, or where lighting must remain on after a momentary control signal.
Control circuits for lighting contactors commonly use push-button stations, key switches, building management system (BMS) relay outputs, time-clock relays, or photocell (dusk-to-dawn) controllers wired to the coil circuit. Auxiliary contacts on the contactor provide status feedback signals to BMS panels or indicator lights.
Illumination of high-bay HID luminaires (metal halide, high-pressure sodium) presents a specific challenge: if the contactor drops out and re-closes immediately while the lamp is hot, the arc cannot restrike until the lamp cools — a period of 5–15 minutes. Wiring must account for this re-strike delay in any automatic control sequence.
How to wire lighting contactor wiring diagram
- Determine contactor type and rating Select an electrically held or mechanically held contactor based on the control strategy. Confirm the main contact current rating is appropriate for the total lighting load, including inrush, using the IEC AC5a or AC5b duty rating rather than the AC3 motor rating.
- Wire the main (power) contacts Connect the three-phase or single-phase supply conductors to the line-side main terminals (typically L1, L2, L3 or A1 supply side). Run load conductors from the load-side main terminals (T1, T2, T3 or equivalent) to the lighting distribution board or luminaire circuits.
- Connect the coil supply Run the control circuit live from the appropriate control voltage source to the A1 coil terminal. The control circuit neutral (or DC negative) connects to the A2 coil terminal. Ensure the control circuit is protected by a separate fuse or MCB rated for the coil VA, not the main load.
- Wire the control devices Connect control switches (push-buttons, key switch, BMS relay output, timer, or photocell) in series with the coil circuit between the control live and A1. For a mechanically held contactor, wire the latch coil and unlatch coil separately with momentary-contact push-buttons or BMS pulse outputs.
- Wire auxiliary contacts for feedback Connect normally open (NO) auxiliary contacts from the contactor to BMS digital inputs or indicator lamps to provide on-state confirmation. Connect normally closed (NC) auxiliary contacts for off-state or fault monitoring if required.
- Verify wiring and test Before energising, check all terminal connections are secure and correctly labelled. Restore control power only and verify the coil operates (contactor closes and opens correctly with control signals). Then restore main power and confirm load switching is correct.
Specifications
| Main contact duty rating (discharge lamps) | AC5a per IEC 60947-4-1 |
|---|---|
| Main contact duty rating (incandescent / resistive) | AC5b per IEC 60947-4-1 |
| Typical coil control voltages | 24 V AC/DC, 120 V AC, 230 V AC (model-dependent) |
| Contactor type options | Electrically held (standard) or mechanically held (latching / impulse) |
| HID lamp hot re-strike delay | 5–15 minutes (lamp type dependent) |
| Auxiliary contact rating | Typically 10 A at control circuit voltage (confirm datasheet) |
| Applicable standards | IEC 60947-4-1, NEC/NFPA 70, BS 7671, AS/NZS 3000 |
Safety warnings
- Isolate and lock out / tag out both the main supply and control circuit supply before working inside a lighting contactor enclosure. Verify all conductors are de-energised. Contactor enclosures can contain live bus bars adjacent to the switching terminals.
- Never assume a mechanically held contactor is de-energised because control power has been removed. The main contacts remain closed after a latch pulse until an unlatch pulse is received. Always verify the main contacts are open with a voltage tester before touching load-side conductors.
- HID luminaire circuits (metal halide, high-pressure sodium) may re-energise the contactor while lamps are still in the hot re-strike delay period. Ensure control sequences account for the lamp re-strike delay of 5–15 minutes to avoid wasted switching cycles.
- Lighting contactor installation must comply with NEC/NFPA 70 Article 410, BS 7671, AS/NZS 3000, or IEC 60364 as applicable. Large commercial or industrial installations require permits and inspection. Only qualified electricians should perform this work.
- This diagram is for illustrative and reference purposes only. Actual wiring must follow the specific contactor manufacturer's instructions and the applicable electrical installation code.
Tools needed
- Calibrated voltmeter or non-contact voltage tester (CAT III minimum)
- Clamp-type ammeter for load current verification
- Insulated screwdrivers
- Wire ferrule crimping tool
- Cable labels and marker
- Lockout/tagout devices
Common mistakes
- Wiring the contactor coil at the wrong control voltage — connecting a 24 V DC coil to a 230 V AC control circuit will immediately destroy the coil.
- Using an AC3-rated motor contactor for a heavy fluorescent or HID lighting load without checking the AC5a/AC5b derating — contact erosion will occur rapidly.
- Omitting a separate control circuit fuse, leaving the coil wiring protected only by the main circuit breaker which is far too large to protect the small control conductors.
- For mechanically held contactors, wiring both latch and unlatch coils to the same continuous signal instead of using momentary pulses — this either burns out one coil or prevents correct latching operation.
- Installing the contactor without auxiliary contact feedback to the BMS, making it impossible to remotely verify whether the lighting load is actually on or off.
Troubleshooting
- Contactor fails to pull in when control signal is applied
- Cause: Control circuit voltage absent at coil terminals, blown control fuse, coil failed open-circuit, or control device contacts not making Fix: Measure voltage at the A1 and A2 coil terminals with the control signal applied. If voltage is present and correct but the contactor does not close, the coil has failed — replace the contactor or coil. If voltage is absent, trace the control circuit for a blown fuse, open contact, or wiring fault.
- Contactor chatters or buzzes continuously
- Cause: Control circuit voltage is too low (below coil pull-in threshold), the shading ring on an AC coil is broken, or severe voltage fluctuation on the control supply Fix: Measure control circuit voltage under load. If low, investigate cable voltage drop or inadequate control transformer sizing. Inspect the coil face for a broken or missing shading ring (copper ring embedded in the pole face) — a broken shading ring is a common cause of buzzing and the contactor must be replaced.
- Mechanically held contactor does not release on unlatch signal
- Cause: Unlatch coil circuit is open, unlatch signal is not reaching the coil, or the mechanical latch mechanism is worn or jammed Fix: Verify voltage at the unlatch coil terminals when the unlatch signal is applied. Check continuity of the unlatch coil. If voltage is present and correct but the contactor does not release, the mechanical latch has failed — the contactor must be replaced. Do not attempt field repair of the latch mechanism.
Frequently asked questions
What is the difference between an electrically held and a mechanically held lighting contactor?
An electrically held contactor keeps its contacts closed only while its coil is continuously energised — remove control power and the contactor opens. A mechanically held contactor latches closed after a brief coil pulse and stays closed with no ongoing coil current, remaining in its last state through power interruptions.
Why can't a standard motor contactor always be used for lighting loads?
Fluorescent and HID lamps generate inrush currents many times their steady-state operating current, which stresses contactor contacts differently from motor inrush. IEC contact duty ratings AC5a (discharge lamps) and AC5b (incandescent lamps) are specifically designed for these high-inrush switching cycles. A motor-rated contactor may suffer premature contact erosion if used for heavy lighting duty.
What control voltage options are typical for lighting contactor coils?
Lighting contactor coils are available in a range of control voltages including 24 V AC or DC (common in BMS applications), 120 V AC (North America), and 230 V AC (Europe and many other regions). Select the coil voltage to match the available control circuit supply. Using the wrong coil voltage will cause the contactor to fail to operate or to burn out.
How does a photocell (dusk-to-dawn sensor) connect to a lighting contactor?
The photocell's switched output is wired in series with the contactor coil circuit. When ambient light falls below the photocell threshold (dusk), its contact closes, energising the coil and switching on the lighting load. At dawn the photocell contact opens, de-energising the coil and switching the load off. The photocell rating must exceed the coil's VA demand.
Can auxiliary contacts on a lighting contactor provide a status signal to a BMS?
Yes. Most lighting contactors include one or more auxiliary contact blocks (normally open and normally closed) that change state with the main contacts. These are wired to BMS digital inputs to provide real-time on/off status feedback for energy monitoring, fault detection, and automated scheduling.
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