How to Wire a Lighting Contactor

1. Pick mechanically held or electrically held

Electrically held: the coil stays energized as long as the control contact is closed; kill control power and the lights drop out. Cheaper, fine in a remote electrical room where nobody hears the coil hum. Mechanically held: a latch coil pulses it closed, an unlatch coil pulses it open, and a mechanical latch holds position with zero coil current — silent, no coil heat, rides through power blips. Near occupied space, mechanically held is the pick.

Mechanically held coils are momentary-rated, so you can't wire a maintained photocell contact straight to one. Use 3-wire momentary control or — far more common — the factory control module that converts a maintained 2-wire signal into latch and unlatch pulses, with coil-clearing contacts so the coils don't cook. Confirm the module ships with the contactor.

2. Size the contactor to the lighting load

Site lighting runs 3 hours or more, so it's a continuous load: conductors and overcurrent protection sized at 125% per NEC 210.19(A) and 210.20(A), and each pole rated at least equal to the circuit it switches. A 30A contactor switching 20A branch circuits is the bread-and-butter setup; 60A and 100A frames handle site lighting panel feeders.

Watch the load type. Contactors carry different ratings for tungsten, ballast, and general-use loads, and LED drivers pull inrush many times their running amps — size to the manufacturer's electronic-load rating where published. Each pole switches one ungrounded conductor: a 12-pole contactor runs twelve 277V circuits off one control signal. Neutrals don't get switched.

3. Select the coil voltage and control power source

Order the coil to match the control power you'll actually have — 120V and 277V are the common coils. Feed it by tapping the line side of one of the contactor's own poles (fused where required) or from a separate 120V control circuit. On 480V equipment, a control transformer stepping down to 120V keeps the photocell, clock, and HOA at the voltage most control devices want.

Under the 2023 NEC a 120V or 277V coil circuit is a non-power-limited remote-control circuit installed with ordinary Chapter 3 wiring methods (300.2 through 300.25) — the new Article 724 covers only power-limited Class 1 circuits (30V, 1000 VA), while under the 2020 and earlier editions this was a Class 1 circuit under Article 725. Either way, 14 AWG THHN is the standard field choice for the control wiring. If control power comes from a different panel than the load, label the enclosure as containing more than one source, and lock out both.

4. Mount the enclosure and land line and load poles

Match the enclosure to the environment — NEMA 1 indoors, 3R or better outdoors — with working clearance per NEC 110.26. Land supply conductors on the line lugs and lighting circuits on the load lugs, paired straight through: pole 1 line to pole 1 load. Torque every lug to the label value with a calibrated tool per NEC 110.14(D); a loose lug on a device loaded all night is the classic burn-up.

Outside the building you're in NEC Article 225 territory: minimum cover per Table 300.5 (generally 24 in. for direct-burial cable, 18 in. for PVC conduit), overhead clearances per 225.18, and 210.6(D) permits 480V area lighting circuits only on poles at least 22 ft high.

5. Wire the photocell and time clock for dusk-on, time-off

Wire the photocell contact and the time clock contact in series — both must be closed for the coil (or module input) to see power. At dusk the photocell closes and the lights fire, since the clock contact is already closed. At the off time the clock opens and kills them. Set the clock to re-close in the early afternoon, before the earliest winter sunset — or use an astronomic clock — so the circuit is armed for the next evening.

If part of the site needs dusk-to-dawn (entries, egress paths) and the rest needs time-off, use two contactors: one on the photocell alone, one on the series combination — one photocell can drive both. Mount the photocell facing north, seeing open sky but not the luminaires it controls; a photocell that sees its own lights cycles all night.

6. Add the Hand-Off-Auto switch

Wire a 3-position selector feeding the coil or module input: Hand connects control power directly, bypassing photocell and clock, so you can force the lights on at noon for service or aiming. Auto routes power through the series string. Off opens the control circuit.

One trap on mechanically held units: opening the control circuit doesn't drop a latched contactor — it stays wherever it was last pulsed. A proper control module treats loss of signal as an unlatch command and pulses it open; verify yours does, per the manufacturer. Leave the switch in Auto when you walk away.

7. Test, torque-check, and commission

Energize the control circuit first, load breakers off, and confirm coil voltage. Cycle Hand-Off-Auto and verify pull-in, dropout, and Auto response. Simulate dusk by covering the photocell — most have a built-in delay of 30 seconds to a couple minutes, per the manufacturer. Set the clock: time, day, daylight-saving handling, and events matching the sequence.

Energize the loads and confirm every pole switches by checking voltage at the load lugs in both states. Before closing up: re-verify torque, label the cover with panel and circuit numbers for every source inside, and write the control sequence on the door.

Should I use a mechanically held or electrically held lighting contactor?

Use mechanically held anywhere coil hum or heat matters, or where lights should hold their state through a control power blip — most site lighting jobs. Electrically held costs less but hums continuously and drops out on any control power loss.

How do I get lights on at dusk and off at a set time?

Wire the photocell and time clock contacts in series. The photocell closes at dusk and turns the lights on; the clock opens at the programmed time and turns them off. Set the clock to re-close in the early afternoon so it's armed before the next sunset.

What coil voltage should I order?

Match the coil to the control power available at the enclosure, which is 120V on most commercial jobs. On 480/277V systems, order a 277V coil or install a control transformer to 120V so standard photocells, clocks, and selector switches work.

Do LED retrofits change the contactor sizing?

They can. LED drivers draw inrush current many times their running amps, and some manufacturers publish a lower contactor rating for electronic loads. Size to that rating, and step up a frame size if the numbers are close.

Why doesn't the Off position turn off my mechanically held contactor?

A latched contactor stays in its last position when the control circuit simply opens, because no unlatch pulse was sent. The control module must treat loss of signal as an unlatch command and pulse the contactor open; verify that against the manufacturer's wiring diagram.