How to Wire a Photocell for Outdoor Lights: Installation, Sizing and Troubleshooting

A photocell, properly a photocontrol, is usually a three-wire switch that turns outdoor lighting on at dusk and off at dawn. Wiring one is simple. The trouble is usually in the details around it: the white lead on a 480 V circuit, an LED load that is inside the tungsten rating but outside the electronic ballast rating, a unit aimed at the parking lot light it controls, or a shorting cap left in a receptacle. This guide covers the installation side. If you are still choosing between a photocontrol and an interior daylight sensor, start with our daylight sensor vs photocell guide.

Before you start

Kill the circuit at the breaker or disconnect before you touch the photocontrol, the fixture or the receptacle. On two-hot and contactor circuits that can mean more than one: Intermatic's contractor box manual warns that "more than one circuit breaker or disconnect switch may be required to de-energize the equipment before servicing." Its EK4000 instructions add: "Do NOT use the OFF state of the photocontrol for equipment servicing." A photocontrol that is off in daylight is still wired to a live line.

This is line-voltage work, up to 480 V on commercial sites. Intermatic's contractor box manual requires installation "in accordance with National and Local Electrical Code requirements." Have a qualified electrician do it.

Wiring a button or stem-mount photocontrol

Start with the voltage rating, because the wiring depends on it. Photocontrols are sold by voltage range; Intermatic's EK4000 series, for example, comes in separate 120-277 V, 347 V and 480 V versions. A 347 V or 480 V circuit needs a control rated for 347 V or 480 V. Check the fixture's supply voltage, then order the control rated for it.

The Intermatic button and stem-mount photocontrols cited here have three leads. In Intermatic's EK4000 instructions the colors are:

• Black: Line 1, the hot from the breaker panel.
• White: neutral on a 120, 277 or 347 V circuit.
• Red: load, the switched hot that runs to the lights or to a contactor coil.

The white lead changes on two-hot circuits. For 208, 240 or 480 V, Intermatic's table puts the white lead on Line 2, the second hot, and the second load wire from the lights or contactor lands there too. Black still takes Line 1 and red still feeds the load. That connection is only safe with a control whose voltage rating covers the circuit: a 480 V circuit takes the 480 V model. Follow the wiring label on the device you are installing; lead colors and functions come from the manufacturer's instructions, not from this article.

For mounting, Intermatic's K4100 and K4400 stem-mount units fit standard outdoor fixtures and boxes with a 1/2"-14 NPSM thread or a 7/8" knockout, and come with a washer gasket and locking nut. The EK4000 instructions add a warning: "DO NOT MOUNT WITH THE WIRES FACING UP OR ON THE BOTTOM OF AN ENCLOSURE."

Twist-lock photocontrols and receptacles

On area and street lights, the photocontrol commonly plugs into a locking receptacle on top of the fixture instead of being hard-wired. On a fixture that ships with a receptacle, installing the control is a matter of seating it and twisting it to lock. That makes the receptacle, not the control, the part to check on the fixture spec sheet: whether the fixture ships with one, what voltage it is wired for, and how many contacts it has.

Dimming receptacles to ANSI C136.41, the standard for dimming control between a locking-type photocontrol and a dimmable driver, add signal contacts to the three power contacts. TE Connectivity's 2014 product sheet for its C136.41 receptacle describes a 5-position version (three power contacts plus two dimming contacts) and a 7-position version (three power plus four), supporting "either 0-10 VDC dimming methods or Digital Addressable Lighting Interface (DALI)." A three-pin photocontrol only switches the fixture on and off. The extra contacts are for networked or dimming controls, which is where the choice between 0-10V and DALI comes back into play outdoors.

Shorting caps and open caps

A receptacle with nothing in it is open to the weather, so it gets a cap. There are two kinds, and they do opposite things:

• Shorting cap: closes the circuit across the receptacle. TE Connectivity describes it as providing "a closed circuit when a photo control is not installed, keeping lights continually on for safety." Intermatic positions its shorting cap for fixtures that will be "controlled by a central contactor box or lighting control panel" instead of a photocontrol on each head.
• Open cap: opens the circuit. TE: "An open cap disconnects service when lighting is no longer needed." Intermatic's is for taking a fixture "out of service (turned OFF) while protecting the receptacle."

If a pole light burns all day and the control on top looks like a photocell, confirm it is not a shorting cap before you replace anything.

Read the LED rating, not the tungsten number

Load rating is where LED jobs go wrong. Many photocontrol labels list tungsten, ballast and electronic ballast (LED) ratings separately, and they do not move together from model to model. For the EK4000, every voltage family is rated 1000 W tungsten and 1800 VA ballast, but the electronic ballast rating drops from 6 A at 120-277 V to 2 A at 347 V and 0.35 A at 480 V. Intermatic notes the 480 V figure was "[t]ested with 150 W 480 V LED driver with 120 Apk /100µs In-rush current." RAB rates its R-3P-PCT twist-lock control at 1000 W tungsten, 1800 VA ballast, and 8 A at 120 V or 5 A at 208-277 V for electronic ballasts. NSi's Tork ZTL124, by contrast, carries the same 1000 W tungsten and 1800 VA ballast figures but an electronic ballast (LED) rating of 8 A or 2200 W at 277 V. The tungsten number tells you nothing about the LED rating. For an LED load, size against the electronic ballast or LED rating the manufacturer publishes, not the tungsten wattage. If the label gives no electronic ballast or LED rating, ask the manufacturer or switch the load through a contactor.

Inrush current is part of the reason. NSi says its Tork ZTL series uses zero-cross switching "to minimize high inrush currents, which are commonly found in LED luminaires," and RAB lists "Zero-Cross Technology Inside" for the R-3P-PCT.

When the load is bigger than the photocontrol's contacts, put a contactor in between. Intermatic's PCCB32A2P contractor box, for example, combines a twist-lock photocontrol receptacle with a 32 A, 2-pole contactor and a 40 A, 2-pole breaker. The photocontrol then switches the contactor coil and the contactor carries the lighting circuit.

Fail-on or fail-off

Photocontrols also differ in what happens when they fail. NSi's specification sheet for its Tork 5200 series explained the fail-on choice this way: "in the event of failure, light should stay ON for safety reasons." Its current ZTL-series brochure lists fail-on models alongside a fail-off version, the ZTL124F, and RAB lists "Fail-On / Fail-Off Modes Available" for its R-3P-PCT.

Fail-on keeps a lot, walkway or entrance lit if the control dies, and the failure shows up as a light burning in daylight. Fail-off saves the energy but leaves the area dark until someone notices. Pick the mode deliberately for each site and confirm it on the spec sheet for the exact part number.

Placement: keep the fixture out of its own photocell

A photocontrol reads the light that hits its window. Point it at a bright source and it will decide it is daytime. Intermatic's instruction is to "[o]rient the lens of the photocontrol so it faces NORTH and away from artificial light sources that may cause the load to turn off at night," and it includes a metal light shield (stem-mount models) or a plastic light dome (button models), plus self-adhesive labels, for installs that cannot avoid stray light.

Photocontrols also switch on and off at different light levels, and the Intermatic thermal and NSi models below add a delay before switching off. Intermatic's thermal stem-mount units are rated to activate on at 1 to 5 foot-candles and off at 3 to 15 foot-candles, with a "delay feature [that] prevents unwanted OFF switching." NSi's Tork 5237-UL and ZTL series are rated to turn on at 1.5 foot-candles (plus or minus 0.2) and off at 2.25 foot-candles (plus or minus 0.3), with instant on and a 3 to 5 second off delay. Check the published levels for your exact model.

Troubleshooting a photocell that misbehaves

• Lights cycle on and off at night. The unit is seeing the lights it controls, or another bright source. Re-aim it, move it, or add the manufacturer's light shield.
• Lights stay on all day. Check for a shorting cap in the receptacle, a failed fail-on unit, or a load wired around the photocontrol.
• Lights never come on. Confirm the voltage family matches the circuit, the white lead is on neutral (120/277/347 V) or Line 2 (208/240/480 V) as the instructions specify, and the load is within the control's rating for that load type.
• Test it. Intermatic's procedure for the EK4000 is to cover the lens with a leather glove or similar; the control "should react to a change within 30 seconds." Other models have different delays, so use the time in your control's instructions.

Photocells and the energy code

A photocontrol handles dusk-to-dawn switching and nothing else. If the job is in California, the outdoor lighting control requirements it plays a part in, including scheduling and motion sensing, are covered in our Title 24 outdoor lighting controls guide.

Shop photocontrols at Rock

Rock Lighting & Electric stocks photocells and photocontrols from brands including Westgate and RAB, among them 120-277 V and 480 V twist-lock controls and a twist-lock shorting cap, plus the fixtures they install on. Our RAB area and pole light guide and RAB wall pack guide cover the heads. Create a contractor account for project pricing, request a quote, or contact us with the fixture schedule and supply voltage and we will match the controls.

Frequently asked questions

How do you wire a photocell? Start with a photocell rated for the circuit voltage: a 347 V circuit needs a 347 V model and a 480 V circuit a 480 V model. A hard-wired button or stem-mount photocell typically has three leads. In Intermatic's instructions, black goes to Line 1 (the hot), white goes to neutral on 120, 277 or 347 V circuits, and red is the switched load to the lights or a contactor. On 208, 240 or 480 V circuits, the white lead goes to Line 2 instead of neutral. Turn off the power first and follow the wiring label on your specific device.

What does the red wire on a photocell do? The red lead is the load: the switched hot that feeds the lights, or the coil of a contactor when the lighting load is larger than the photocell can carry. In Intermatic's EK4000 instructions, red connects to the lights or contactor and black connects to the incoming line.

What is a photocell shorting cap? A shorting cap plugs into a twist-lock photocontrol receptacle in place of a photocell and closes the circuit, so the fixture stays on whenever it has power. It is used when the fixture will be switched by a central contactor or lighting control panel instead of a photocell on each head. An open cap does the opposite and keeps the fixture off.

What is the difference between fail-on and fail-off photocells? A fail-on photocell leaves the lights on if it fails; NSi's 5200 series sheet explained that "light should stay ON for safety reasons." A fail-off unit leaves them off. Fail-on protects the site but wastes energy until the failure is found; fail-off saves energy but leaves the area dark. Some models, such as RAB's R-3P-PCT, are available in either mode.

Why does my photocell turn the lights off at night? Usually the photocell is seeing light from the fixture it controls or another bright source, decides it is daytime, and switches off. Intermatic recommends facing the lens north and away from artificial light, and includes a light shield or dome plus labels for installs that cannot avoid stray light.

Can a photocell handle an LED load? Yes, if it is rated for it. Many photocontrol labels list tungsten, ballast and electronic ballast ratings separately, and the electronic ballast figure can be far lower. Intermatic's EK4000 is rated 1000 W tungsten at every voltage but 0.35 A electronic ballast at 480 V. Size LED loads against the electronic ballast or LED rating; if the label has none, ask the manufacturer or use a contactor.

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