Electrical Wiring for Residential Construction

A house has a very particular wiring plan. The code tells you which rooms need their own circuits, how far apart receptacles can sit, and how switches get their neutral. This guide covers that design and how each circuit is connected. It is about dwellings only. Commercial and industrial buildings work differently.

What You Will Be Able to Do

• Explain how a house gets 120 and 240 volts from one service.
• Read an NM cable label and match it to the right breaker.
• List the circuits a new home must have.
• Lay out receptacles using the 6-foot and 24-inch rules.
• Wire single-pole, three-way and four-way switch circuits, and a multiwire branch circuit.

The code sections below come from the National Electrical Code (NEC). The current edition is 2026, but many states still enforce 2023 or 2020, and states and cities add their own changes. Ask your supervisor or the building department which edition applies to your job. The adopted local rule wins.

120 and 240 Volts in a Home

A US house normally gets a single-phase service with three wires from the utility: two hot legs and a neutral. Between either hot leg and neutral you measure 120 volts. Between the two hot legs you measure 240 volts, because the two legs are opposite each other.

• 120-volt circuits use one hot, a neutral and a ground. These feed lighting, receptacles and small appliances.
• 240-volt circuits use one hot from each leg and a ground, and sometimes a neutral. These feed water heaters, ranges, dryers and air conditioners.

A two-pole breaker connects to both legs. If two hots from the same leg were connected to a 240-volt appliance, you would measure 0 volts between them and it would not run. That is why breaker placement in the panel matters.

Conductor Colors

A white conductor used as a hot in a cable, such as on a 240-volt circuit or a switch loop, must be re-identified, usually with black tape at each end.

The neutral and the EGC do different jobs. The neutral carries normal current back to the source. The EGC carries current only during a fault.

Cable and Circuit Sizes

Nonmetallic sheathed cable, Type NM-B, is the standard wiring method in wood-frame houses. People often call it Romex. The label gives the conductor size and count: 12/2 means two insulated 12 AWG conductors plus a bare ground. 14/3 means three insulated conductors plus a ground.

NM cable uses the 60 degree C ampacity column even though the conductors are rated 90 degrees C. In that column, 14 AWG is good for 15 amps, 12 AWG for 20 amps and 10 AWG for 30 amps. Never put a larger breaker on a smaller wire, because the wire would overheat before the breaker trips.

A 20-amp breaker feeds one branch circuit. That circuit may have several 15-amp receptacles on it. A 15-amp receptacle on a 20-amp circuit is allowed as long as there are two or more receptacle openings.

Other Residential Methods

• UF-B: gray cable rated for direct burial. The usual minimum cover in a yard is 24 inches. It can be 12 inches for a residential 120-volt circuit of 20 amps or less with GFCI protection. Check Table 300.5.
• Type MC, or EMT: required by some cities in place of NM, and used in places where cable needs protection, such as exposed garage walls.

The Circuits a New Home Needs

Bathrooms have two allowed layouts. A 20-amp circuit can serve receptacles in several bathrooms and nothing else. Or it can serve everything in one bathroom, including lights and fan, as long as nothing else is on it.

Small-appliance circuits must not feed lighting or outlets outside those kitchen and dining areas, with a few exceptions such as a clock or a gas range igniter.

Water heaters and other fixed loads follow their nameplates. A 4,500-watt, 240-volt tank heater is commonly on a 30-amp circuit with 10 AWG cable.

Receptacle Layout

The goal of NEC 210.52 is to keep people from running extension cords across rooms.

Wall spaces. In habitable rooms, no point along a wall can be more than 6 feet from a receptacle. In practice, that means one within 6 feet of each door or wall break and no more than 12 feet apart along a wall. Any wall space 2 feet wide or wider counts. Fixed glass panels count as wall space. In the 2026 NEC, fixed cabinets do not.

Countertops. No point along a counter can be more than 24 inches from a receptacle, so that means one every 4 feet at most. A counter space 12 inches wide or wider needs a receptacle. Rules for islands and peninsulas have changed in recent editions. Some editions require a receptacle, or provisions for adding one, based on the size of the counter. Read your adopted edition.

Other required outlets:

• Bathrooms: at least one within 3 feet of the outside edge of each sink.
• Outdoors: one at the front and one at the back of the home, reachable from grade.
• Hallways: one in a hall 10 feet or longer.
• Laundry, basement and garage: at least one in each.

Receptacle types:

• Tamper-resistant (TR): nonlocking 15- and 20-amp receptacles in a dwelling must be TR under NEC 406.12. Shutters block a single object pushed into one slot. If a homeowner asks for regular ones because plugs are stiff, the answer is no for new work. A few exceptions apply, such as a receptacle more than 5-1/2 feet above the floor or one behind an appliance that is not easily moved.
• Weather-resistant (WR): required in damp and wet locations, with an in-use cover where the location is wet.

Typical heights are set by the plans, not by the NEC:

The NEC does not set a maximum height for most receptacles, but one more than 5-1/2 feet above the floor does not count toward the required spacing. Accessible designs follow their own limits.

Connecting Devices

• Hot goes to the brass screw, neutral to the silver screw, ground to the green screw.
• Wrap the conductor clockwise around the screw, so tightening draws it in.
• Pigtail connections use a short length of wire from a splice to the device. If one device fails or is removed, the rest of the circuit keeps working.
• Push-in "backstab" holes accept 14 AWG only on most devices, and they are a well-known cause of later failures. Most electricians use the screws or a back-wire clamp.
• On a multiwire circuit, neutrals must be pigtailed, so removing a device cannot break the neutral for the rest of the circuit.

Switch Circuits

Switches always go in the hot, never in the neutral.

The neutral at the switch. Smart switches and occupancy sensors need a neutral. Since the 2011 NEC, a neutral must be present at most switch boxes, even if the switch does not use it. The 2023 edition has this rule at 404.2(C). The 2026 edition moved rules for device-type switches into Article 406.

Single-Pole

1. Power comes into the switch box.
2. Hot connects to one switch terminal.
3. The switched hot leaves on the other terminal and goes to the light.
4. Neutrals are spliced straight through.
5. Grounds connect at every box and to the switch's green screw.

Three-Way

Three-way switches control a light from two places. Each has one common terminal, usually a darker screw, and two traveler terminals.

1. At the first switch, the common receives power from the line.
2. Two traveler wires run from the first switch to the second, usually in a /3 cable, using red and black.
3. At the second switch, the common sends switched power to the light.

The two travelers are the only connection between the switches. Whichever traveler is live depends on the position of the first switch. The light is on when the second switch connects that traveler to the fixture. Mixing up a common and a traveler is the classic mistake. The light may work only in some switch positions.

Four-Way

A four-way switch goes between two three-way switches, to add a third or fourth control point. It has four traveler terminals and no common. Each added location needs one more four-way switch.

Multiwire Branch Circuits

A multiwire branch circuit (MWBC) uses two hots from opposite legs and one shared neutral, often in a /3 cable. It saves wire, and the shared neutral carries only the difference between the two hots.

The rules exist for a reason:

• The hots must be on opposite legs. If both are on the same leg, the neutral carries the sum of the two currents instead of the difference, and it can overheat with no breaker to protect it.
• Both hots must disconnect together. NEC 210.4(B) requires a two-pole breaker or two single-pole breakers with an identified handle tie, so that a person working on one side cannot be shocked by the other.
• The neutral must stay unbroken, so it is pigtailed at devices.

An MWBC also needs a two-pole GFCI or AFCI device if that protection is required.

Smoke and Carbon Monoxide Alarms

Smoke alarm rules come from the residential building code, not the NEC. Under the International Residential Code, new homes need hardwired alarms with battery backup in each bedroom, outside each sleeping area and on every level, interconnected so one sets off all. Carbon monoxide alarms are needed where a home has fuel-burning appliances or an attached garage. Local adoption varies, so follow the plans.

Old Wiring You May Find

Knob-and-tube wiring uses separate conductors on ceramic supports and has no ground. If you find it during a remodel, stop. Do not add loads to it, and do not bury it in insulation. Talk to your supervisor about rewiring the affected circuits.

Aluminum branch wiring from the 1960s and 1970s needs devices or connectors listed for it. A standard copper-only device on aluminum wire can overheat.

Common Mistakes

Check Your Understanding

1. A long wall in a living room has no doors or breaks. How far apart can the receptacles be?

Up to 12 feet, so that no point on the wall is more than 6 feet from one. The first receptacle must be within 6 feet of the end of the wall space.

2. An 8-foot kitchen counter has receptacles at each end. Does it meet the 24-inch rule?

No. The middle of the counter is 4 feet from each one, and no point can be more than 24 inches from a receptacle. It needs at least one more.

3. A switch works the light only when the other switch is in one position. What is the likely fault?

A traveler and the common have been swapped at one of the three-way switches.

4. You open a junction box and find two hots from two separate single-pole breakers sharing one neutral. Why is that a concern?

This is a multiwire circuit. Confirm the breakers are on opposite legs, because on the same leg the neutral can overload. Either way, they need an identified handle tie or a two-pole breaker.

5. What makes a 240-volt appliance circuit different from a 120-volt circuit?

It uses one hot from each leg through a two-pole breaker, and it may or may not use a neutral.

Key Takeaways

• A house gets 120 volts from hot to neutral, and 240 volts between the two hot legs.
• 14 AWG goes with 15 amps, 12 AWG with 20 amps and 10 AWG with 30 amps.
• Kitchens need two 20-amp small-appliance circuits. Baths, laundry and garages each need a 20-amp circuit.
• Wall receptacles follow the 6-foot rule, and countertop receptacles follow the 24-inch rule.
• Three-way switches use one common and two travelers.
• A multiwire circuit needs opposite legs, disconnects that work together and a continuous neutral.