Wiring diagrams

Single-pole switch, power at the switch

One switch controls one light, and the power reaches the switch box first. The most common layout.

Steps

1. Turn off the breaker and verify the circuit is dead (live-dead-live).
2. Switch box: land the incoming black (hot) on one switch screw and the black going to the light on the other. A single-pole switch’s two screws are interchangeable.
3. Switch box: splice the two whites together. The neutral passes through; it never goes on the switch.
4. Light box: black to the fixture’s black (or brass screw), white to the fixture’s white (or silver screw).
5. Grounds: splice the grounds in each box, pigtail one to each device’s green screw, and bond metal boxes.

Good to know

• Switch the hot, never the neutral.
• 14 AWG on a 15 A breaker, 12 AWG on a 20 A breaker.

NEC: 404.2(B), 240.4(D), 250.148

Single-pole switch, power at the light

The power reaches the light box first and a cable drops down to the switch. Run 3-wire cable so the switch box has a neutral.

Steps

1. Turn off the breaker and verify dead.
2. Light box: splice the incoming black to the black going down to the switch. That black is always hot.
3. Light box: connect the red coming back from the switch (the switched leg) to the fixture’s black.
4. Light box: splice the incoming white, the fixture’s white and the white going to the switch.
5. Switch box: black on one screw, red on the other. Cap the white; it’s there for a smart switch or dimmer that needs a neutral.
6. Splice and pigtail the grounds in both boxes.

Good to know

• Older homes often have a 2-wire switch loop: the white is the always-hot to the switch and must be re-marked black or red at both ends.
• New work generally needs a neutral at switch locations. Running 3-wire to the switch covers it.

NEC: 404.2(C), 200.7(C)(1)

One switch, two lights

A single switch turns on two or more lights, with power at the switch.

Steps

1. Turn off the breaker and verify dead.
2. Switch box: hot black on one screw, the black to Light 1 on the other; splice the whites through.
3. Light 1 box: splice the switched black from the switch, Light 1’s black and the black going on to Light 2.
4. Light 1 box: splice the incoming white, Light 1’s white and the white going on to Light 2.
5. Light 2 box: black to black, white to white. Splice and pigtail the grounds in every box.

Good to know

• Add up the fixtures’ load. Many LED fixtures on one switch are fine; heavy loads may need their own circuit.

NEC: 404.2(B), 314.16

Half-switched receptacle

A wall switch controls the bottom half of a receptacle for a lamp; the top half stays on.

Steps

1. Turn off the breaker and verify dead.
2. Receptacle: break off the brass tab between the two hot screws. Leave the silver (neutral) tab in place.
3. Switch box: splice the incoming black with a pigtail to the switch and the black going to the receptacle (always hot).
4. Switch box: the red from the switch’s other screw goes to the receptacle. Splice the whites through.
5. Receptacle: black to the top brass screw, red to the bottom brass screw, white to a silver screw.
6. Splice and pigtail the grounds.

Good to know

• In most habitable rooms a switched receptacle can stand in for the required lighting outlet. Kitchens and bathrooms need a switched light.
• If the two halves are on two different breakers (a multiwire circuit), the breakers need a common handle tie or a 2-pole breaker.

NEC: 210.70(A)(1) Ex. 1, 210.7, 210.4(B)

Three-way switches

Two switches control one light, like at the top and bottom of stairs. Power at the first switch.

Steps

1. Turn off the breaker and verify dead.
2. Find each switch’s common screw: it’s the dark (black) one. The two lighter screws are travelers.
3. 3-way #1: the incoming hot black goes on the common.
4. Run 3-wire between the switches. The red and the black are the travelers: land them on the traveler screws of both switches. Either traveler can go on either traveler screw.
5. 3-way #2: the common goes to the black running to the light.
6. Splice the whites through every box to the light. Splice and pigtail the grounds.

Good to know

• If the light works only from one switch, or only in some positions, the hot or the light leg is on a traveler screw. Find the commons again.

NEC: 404.2(A), 404.2(C)

Four-way switch (three locations)

Three or more switches control one light. Any number of 4-ways go between the two 3-ways.

Steps

1. Turn off the breaker and verify dead.
2. Wire the two 3-ways like a normal three-way: hot to the first common, the second common to the light.
3. The 4-way’s screws come in pairs (matched by color or marked IN/OUT). The two travelers from 3-way #1 go on one pair; the two travelers to 3-way #2 go on the other pair.
4. Splice the whites through every box to the light. Splice and pigtail the grounds.

Good to know

• If the light acts strangely from the 4-way, a traveler from one side is on the wrong pair. Put each side’s two travelers on the same pair.

NEC: 404.2(A)

Smart switch or dimmer that needs a neutral

Smart switches, timers, occupancy sensors and many LED dimmers need a neutral to power their electronics.

Steps

1. Turn off the breaker and verify dead.
2. Connect the incoming hot to the device’s LINE lead or screw.
3. Connect the black going to the light to LOAD.
4. Splice the whites with a pigtail to the device’s neutral.
5. Ground the device. Follow the maker’s instructions: some have extra leads for 3-way use.

Good to know

• No neutral in the box? Look for a 2-wire switch loop. Some devices work without a neutral; most smart switches don’t.
• Match the dimmer to the lamps: use LED-rated dimmers for LED lamps, and check the minimum load.

NEC: 404.2(C), 404.14(E)

Ceiling fan and light on two switches

One switch runs the fan motor and another runs the light kit, with 3-wire from the switches to the fan.

Steps

1. Turn off the breaker and verify dead.
2. Switch box: splice the incoming hot with pigtails to both switches.
3. Switch box: the fan switch’s other screw goes to the black of the 3-wire; the light switch’s other screw goes to the red. Splice the whites through.
4. Fan box: black to the fan’s black (motor), red to the fan’s blue (light kit), white to white.
5. Ground the fan bracket and the box.

Good to know

• The box has to be listed for fan support (marked “for fan support”), and the fan must be hung from it as listed.

NEC: 314.27(C), 422.18

GFCI protecting the receptacles after it

One GFCI receptacle protects itself and every receptacle wired from its LOAD terminals.

Steps

1. Turn off the breaker and verify dead. Find which cable brings the power: that one goes on LINE.
2. GFCI: the incoming black and white go on the LINE hot (brass) and LINE neutral (silver) terminals.
3. GFCI: the cable to the next receptacles goes on LOAD, black to hot and white to neutral.
4. Wire the next receptacles normally. They’re now GFCI protected; mark them “GFCI Protected.”
5. Turn the power on, press TEST then RESET at the GFCI, and check the downstream receptacles go dead on TEST.

Good to know

• Line and load swapped? Most GFCIs won’t reset, or won’t protect what’s after them.
• If you don’t want the next receptacles protected, splice through with pigtails and use only LINE.

NEC: 210.8, 406.4(D)

Photocell controlling an outdoor light

A dusk-to-dawn light: the photocell turns the light on when it gets dark.

Steps

1. Turn off the breaker and verify dead.
2. Photocell black (line) to the incoming hot.
3. Photocell red (load) to the black going to the light.
4. Photocell white to the neutral splice with the incoming white and the light’s white.
5. Mount the eye facing away from the light it controls (north if you can), so the light doesn’t switch itself off.

Good to know

• Use a weatherproof box and cover; outdoor receptacles need in-use covers.

NEC: 314.15, 406.9

Motor start-stop (3-wire control)

A magnetic starter with momentary START and STOP buttons. The holding contact keeps the coil on after you let go of START.

How it works

1. Press START: current flows through STOP (closed), START and the overload contact to the coil. The starter pulls in.
2. The starter's auxiliary contact (M) closes in parallel with START, so the coil stays on when START springs back open.
3. Press STOP, or an overload trips: the circuit opens, the coil drops out and the holding contact opens. The motor won't restart by itself when power comes back (low-voltage protection).

Wiring it at the starter

1. Lock out the motor circuit and verify dead, including the control source.
2. Run three wires to the push-button station: the control hot, the wire between STOP and START, and the wire from START back to the coil.
3. At the starter, the wire between STOP and START lands on one side of the holding contact, and the START-to-coil wire on the other side.
4. Keep the overload contact in series with the coil on the X2 side, so a trip always drops the coil.

Good to know

• Two-wire control (a toggle, float or pressure switch in place of the buttons, no holding contact) restarts the motor by itself after a power outage. Use it only where that's safe.
• Stop buttons in series, start buttons in parallel: that's how you add more stations.
• Ground one side of the control transformer secondary where the design calls for it, and protect the control conductors.

NEC: 430.72 (control circuit protection), 430.74 (disconnecting the control circuit), 430.32 (overloads)