Electrical · Lesson 7 of 13Intermediate9 min read

Outlets, Switches and Lights

Outlets, switches and lights are the parts of the electrical system you touch every day. This lesson explains what the slots and screws on an outlet mean, how two switches at opposite ends of a hallway can both control one light, and why LED bulbs use so little power.

Receptacles

The everyday outlet is properly called a receptacle: a contact device at an outlet where you connect a plug. The most common kind in homes is a duplex (two-outlet) receptacle rated for 15 amps at 125 volts. The narrow slot connects to the hot wire, the wider slot to the neutral, and the third hole to the equipment ground. Inside the wall, the black hot wire goes to the brass-colored screw, the white neutral to the silver screw, and the bare or green ground to the green screw.

Ratings have to match the circuit. OSHA’s workplace rules allow only 15-amp receptacles on a 15-amp circuit; a 20-amp circuit can have 15- or 20-amp receptacles. Receptacles for different voltages, such as 240-volt dryer and range outlets, are shaped so the wrong plug won’t fit.

When a plug wobbles or falls out of an outlet easily, the contacts inside are worn. Loose-fitting plugs can overheat and cause fires, so CPSC says to have the outlet replaced.

Sources: University of Hawaiʻi Maui College, Building Maintenance & Construction, 5.6; OpenStax College Physics 2e, 23.8.

Single-pole switches

A single-pole switch is the simplest: two terminal screws and an ON/OFF lever. Flip it on and it closes a gap in the circuit so current can flow to the light; flip it off and it opens the gap. Many switches also have a green screw for the ground wire.

3-way and 4-way switches

A hallway or stairway light usually has a switch at each end. Either switch can turn the light on or off, no matter where the other one is. That takes a pair of 3-way switches; they always come in pairs.

A 3-way switch has no ON or OFF. It has one common terminal (usually a darker screw) and two traveler terminals, and its lever connects the common to one traveler or the other. At one switch the common is connected to the power source; at the other, the common goes to the light. Two traveler wires run between the switches, usually in a three-conductor cable with black, red and white wires plus a ground.

Current reaches the light only when both switches are set to the same traveler. Flip either one and the path breaks. Flip either one again and it’s complete. To control a light from three or more places, electricians add 4-way switches between the two 3-ways. A 4-way swaps the two travelers: in one position they pass straight through, in the other they cross over. Try it below.

Wire colors follow the cable described by Purdue Extension 4-H. Real installations vary, and in some the power feeds the light first. The current path, not the color, is what matters.

Dimmers

A dimmer lets you set the brightness of a light. The U.S. Department of Energy says most LEDs are dimmable, and dimmer-compatible LEDs are widely available. Not every LED works with every dimmer, though, so check the bulb’s package and the dimmer’s instructions to be sure they work together.

LED lighting

An old incandescent bulb makes light by heating a wire filament until it glows. About 90 percent of its energy comes out as heat and only about 10 percent as light. LEDs make light directly from electronics. DOE says LEDs use up to 90 percent less energy and last up to 25 times longer than incandescent bulbs, and stay cool to the touch.

Shop for lumens, a measure of brightness, not watts. DOE says that to replace a 75-watt incandescent bulb, look for an LED of about 1,100 lumens; a screw-in LED can do it with as little as 9 watts. The Lighting Facts label shows brightness, yearly energy cost, expected life and light color. Color is given in kelvins (K): lower numbers look warmer and more yellow, higher numbers cooler and bluer.

Every fixture has a maximum bulb wattage on its label. A bulb with more watts than the fixture allows can overheat the fixture and its wiring. LEDs make it easy to stay under the limit.

Source: U.S. Department of Energy, Consumer Guide to Energy-Efficient Lighting.

Parts and what they do
Part	What it does	Signs of trouble
Duplex receptacle (15 A, 125 V)	Two outlets for plug-in loads.	Loose plugs; warm, discolored or cracked faces.
GFCI receptacle	Outlet with shock protection; TEST and RESET buttons.	Doesn’t trip on TEST.
240-volt receptacle	For a dryer or range; shaped so 120-volt plugs won’t fit.	Scorching; a loose plug.
Single-pole switch	Opens or closes one circuit from one place.	Warm plate, crackling, a light that flickers when touched.
3-way switch	One of a pair that controls a light from two places.	Light works from only one switch.
4-way switch	Goes between two 3-ways for a third location.	Light works from some positions only.
Dimmer	Sets brightness.	Flicker with bulbs that aren’t made for dimmers.
Fixture and lampholder	Holds the bulb and connects it to the wiring.	Discoloration from bulbs over the wattage limit.
LED bulb	Makes light with far less power than incandescent.	Flickering, especially on a dimmer.

NoteReplacing a switch or outlet

Whether you may replace your own switch or receptacle depends on where you live: some places allow homeowners to do it, often with a permit, and others require a licensed electrician. Either way, these are the principles electricians follow:

Always de-energize the circuit first, at the breaker.
Prove it’s dead with a voltage tester rated for the job, after checking the tester on a known live source, and check it again afterward (Lesson 9).
Replace like with like: the same amp rating, and GFCI or tamper-resistant protection where it’s required.
Stop if you find aluminum wiring, scorched or brittle insulation, or anything you don’t understand, and call an electrician.

You canWhat you can safely do

Replace bulbs with LEDs that don’t exceed the fixture’s wattage limit.
Choose LEDs labeled as dimmable for lights on dimmers.
Notice loose-fitting plugs and warm switch plates, and have them checked.
Test GFCI receptacles monthly: Being an Adult 101.

Check yourself

Tap an answer to check it, or open “Show the answer.”

On a standard receptacle, which slot connects to the neutral?
Show the answer
The wider one. The wider slot is the neutral, the narrow one is hot, and the third hole is the ground.
Two 3-way switches control one light. When is the light on?
Show the answer
When both switches connect their commons to the same traveler. There’s a complete path only when both switches select the same traveler wire.
What do you add to control a light from a third place?
Show the answer
A 4-way switch between the two 3-ways. A 4-way switch either passes the travelers straight through or crosses them.
What should you look for to replace a 75-watt incandescent bulb?
Show the answer
About 1,100 lumens. Brightness is measured in lumens. DOE says about 1,100 lumens, which an LED can give with as little as 9 watts.
What can a 20-amp circuit have, under OSHA’s receptacle rules?
Show the answer
15- or 20-amp receptacles. A 15-amp circuit may have only 15-amp receptacles; a 20-amp circuit may have 15- or 20-amp ones.

Go deeper

University of Hawaiʻi Maui College, Building Maintenance & Construction (open textbook), 5.6 Receptacle and Switch Wiring An open textbook chapter on receptacle and switch wiring.
Purdue Extension 4-H Electric, Level 5: Wiring Diagrams for 3-Way and 4-Way Switch Combinations Purdue Extension 4-H diagrams for 3-way and 4-way switching.
U.S. Department of Energy: Consumer Guide to Energy-Efficient Lighting (DOE/EE-2465) DOE’s consumer guide to LED lighting.
Science Lab: Circuit Builder Build switch circuits of your own.
Being an Adult 101: Test and Reset a GFCI Outlet Test and reset a GFCI receptacle.

Sources

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A free course from Teddy Rook Book. It explains how home electrical systems work in general and follows U.S. sources; your local electrical code, your inspector and a licensed electrician are the final word for your home.