Lighting circuits and controls
What you’ll learn
- How single-pole, 3-way and 4-way switching fit together, and how many of each you need.
- What dimmers, occupancy/vacancy sensors and photocells actually switch.
- Why a lighting contactor lets a small control switch a big lighting load.
- How to work out a lighting circuit’s current from the fixture wattage.
Every lighting control, from a plain toggle to a building-wide lighting panel, does the same job: it opens and closes the hot conductor feeding the light. What changes is what decides when — a person, a second switch across the room, a motion sensor, the daylight, a time clock — and how big a load it has to carry. Get that straight and the devices stop looking different from each other.
Switching from one, two and three places
A single-pole switch opens and closes one hot conductor: one light, one place. To control a light from two places you use two 3-way switches joined by two travellers; each one connects its common to one traveller or the other, and the light is on when both commons land on the same traveller. For every extra location, a 4-way switch goes in the middle of the travellers and crosses them over or passes them straight. The site’s interactive 3-way and 4-way diagrams let you flip them and watch the path.
- 1 place1 single-pole
- 2 places2 × 3-way
- 3 places2 × 3-way + 1 × 4-way
- n places2 × 3-way + (n − 2) × 4-way
Dimmers, sensors and photocells
A dimmer cuts part of every AC half-cycle so the lamp gets less power. Incandescent lamps do not care how; LED drivers do, which is why compatibility lists matter. A fan speed control is a different device — never use a lamp dimmer on a motor unless it is rated for it.
An occupancy sensor (infrared, ultrasonic or both) switches on when it sees someone and off after a set time with nobody. A vacancy sensor only switches off — someone has to turn the light on. A photocell switches on at dusk and off at dawn. A time clock switches on a schedule. Many buildings combine them: a photocell and a time clock, so the parking lot lights come on at dark and go off at midnight.
Lighting contactors: small switch, big load
A contactor is a heavy-duty relay: a coil and a set of power contacts. Put the photocell, time clock or a building-automation relay in series with the coil, and the contacts switch circuits far bigger than the control could handle on its own. The same idea runs through the motor control course — a lighting contactor is a motor starter without the overload.
Low-voltage lighting control takes it one step further: the switches on the wall run at a safe low voltage and only signal relays in a panel, which switch the line-voltage circuits. The wall wiring is then control cable, not branch-circuit wiring.
Worked examples
1. The current of a lighting zone
- Twelve LED fixtures, 40 W each: total 12 × 40 = 480 W.
- On 120 V: I = P ÷ V = 480 ÷ 120 = 4 A (taking the drivers’ power factor as close to 1; check the fixture data for the input VA).
- The same fixtures on 277 V: 480 ÷ 277 = 1.73 A. On 347 V (common in Canadian commercial work): 480 ÷ 347 = 1.38 A.
- How much of the breaker you may use is a code-book question; the arithmetic of what the fixtures draw is this.
2. What the photocell actually carries
- A contactor coil is labelled 120 V, 12 VA sealed.
- Coil current = VA ÷ V = 12 ÷ 120 = 0.1 A while held in. That is all the photocell switches.
- The contactor’s contacts, meanwhile, might each carry a full lighting circuit. The coil takes a brief higher “inrush” VA as it pulls in; the label gives that too, and the control device must handle it.
Try one: a hallway switched from four places
Two 3-way switches at the ends and 4 − 2 = two 4-way switches between them.
On the job
Tips from the job
- Before you pull a switch out, find which conductor is the hot and which is a switch leg or traveller — test it, do not trust colours alone.
- Many electronic controls (sensors, smart switches, timers) need a neutral in the box. Pull one to every switch location unless you know you will never need it.
- Write down which breaker, which contactor and which photocell or time clock feeds each lighting zone. The next person will thank you, and that is often you.
- Label contactor coils and their control transformers in the panel. A dead lighting zone is often a blown control fuse, not a dead circuit.
Common mistakes
- Mixing up the common and the travellers on a 3-way. The common is the odd-coloured screw; the travellers go switch to switch.
- Switching the neutral instead of the hot.
- Putting a standard incandescent dimmer on a fan or on LED drivers it is not listed for.
- Loading a photocell or a sensor’s own contact beyond its rating instead of using a contactor or relay.
- Forgetting that a contactor’s coil voltage and its contact voltage can be different — read both on the label.
Check yourself
7 questions. Tap an answer — it is marked straight away and the reason opens under it. Get 80% or better and the lesson is ticked off on the year page.
Not started yet.
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To control one light from three places you need…
Answer and why
B. A 3-way at each end and a 4-way for every location in between. Three places = two 3-ways + one 4-way; four places = two 3-ways + two 4-ways.
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Twelve 40 W LED fixtures run on 120 V. What current do they draw together (treat the power factor as 1)?
Answer and why
B. 12 × 40 = 480 W. I = P ÷ V = 480 ÷ 120 = 4 A.
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The same twelve fixtures on 277 V draw about…
Answer and why
A. 480 ÷ 277 = 1.73 A. The same power at a higher voltage needs less current — one reason commercial lighting often runs at 277 V (on a 480Y/277 V system) or 347 V in Canada (600Y/347 V).
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Why use a lighting contactor with a photocell instead of running all the lights through the photocell?
Answer and why
B. A photocell is rated for a modest load. Let it switch a coil drawing a fraction of an amp, and the contactor’s power contacts can switch many circuits of lighting at once.
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In a switch loop, the switch is placed in…
Answer and why
C. Switch the hot, so the lamp and its socket are dead when the switch is off. Switching the neutral leaves the fixture live with the light out — a shock waiting for whoever changes the lamp.
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An occupancy sensor in “vacancy” mode (manual-on) will…
Answer and why
B. Occupancy mode is auto-on/auto-off. Vacancy mode is manual-on/auto-off — it saves more, because the lights do not come on when daylight is enough.
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An LED lamp flickers on a dimmer that worked fine with incandescent lamps. The most likely cause is…
Answer and why
B. Dimmers built for incandescent loads often misbehave with LED drivers, especially at light loads. Use a dimmer and lamps the makers list as compatible, and check the dimmer’s minimum and LED ratings.
Questions people ask
What is the difference between a 3-way and a 4-way switch?
A 3-way switch has a common and two traveller terminals and connects the common to one traveller or the other. A 4-way sits in the middle of the travellers and either passes them straight through or crosses them over. You always need two 3-ways; each extra switching location adds one 4-way.
Do occupancy sensors need a neutral?
Many do, because the sensor’s electronics need a small, steady supply of their own. Some older two-wire models leak a tiny current through the load instead, which can make LED lamps glow or flicker. Check the sensor’s instructions — and pulling a neutral to the switch box keeps your options open.
Related
- Single-pole switch wiring
- 3-way switch wiring diagram
- 4-way switch wiring diagram
- Photocell wiring
- Occupancy sensor wiring
- Contactor wiring diagram
- Low-voltage and data basics
- All second year lessons
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