120-V Branch Circuits: Wiring and Safety Essentials

The article discusses the wiring of typical 120-V branch circuits, focusing on receptacle outlets, switch outlets, and light outlets. It covers essential safety features, grounding requirements, and the identification of conductors in residential electrical systems.

Typical 120-V branch circuits provide power to outlet boxes for receptacles, lights, and switches. Every branch circuit is protected by a circuit breaker or a fuse at the main panel or at a subpanel.

Receptacle Outlet in Branch Circuit

Figure 1 illustrates how a duplex receptacle is wired in a 20-A branch circuit using a type of NM cable and a metal outlet box. Although not shown in this figure, cable clamps are provided to secure the cable to the box. The cable sheathing (again not shown) extends through the clamp to prevent the clamp from damaging the conductor insulation. Notice in Figure 1 that jumper bars between the two neutral connections and between the two hot connections provide the duplex receptacle with current paths between the two receptacles. The wire connector on the neutral ensures that the neutral will have a continuous path even when the receptacle is removed.

A complete electrical path from the neutral bus in the main distribution panel to every load (and outlet) is an absolute requirement for a residential wiring system. The equipment-grounding conductor must also have a continuous path; however, it is not electrically connected to the load. It connects only to the frame, the enclosure, and/or other metallic parts of the electrical device that provides the load.

Receptacle outlet in a 120-V, 20-A branch circuit 

Figure 1. Receptacle outlet in a 120-V, 20-A branch circuit 

Notice several other features of the duplex outlet. First, the neutral side of a receptacle is identified for the user as the longer of the two slots that receive the plug. The narrow slot connects to the hot side. For the electrician, the neutral side of the receptacle is identified by the light-colored screws, whereas brass-colored screws are used on the hot side of the receptacle. Also, there is only one ground screw, and it is the green screw. Third, the grounding wire from the receptacle goes to a wire connector unless the outlet box is nonmetallic and there is no outgoing cable. Then the equipment grounding conductor terminates at the receptacle.

Often the grounding conductors are connected together with an uninsulated crimped connector. Also, the grounding conductor that grounds the outlet box is often a bare conductor.

Switch Outlet

The cable out of the box in Figure 1 could go to a switch outlet box like the one in Figure 2. The ordinary on-off wall switch is often referred to as a snap switch. Electrically, it is a single-pole, single-throw (SPST) switch. Again, notice that both the neutral and the ground have continuous paths to the next outlet box and to the lighting outlet box. Only hot conductors are switched or protected by circuit breakers or fuses.

Plastic outlet box for a wall switch

Figure 2. Plastic outlet box for a wall switch

If a switch fails and the hot part of the switch makes contact with the switch frame, then the metal cover plate (or the metal screws attaching a plastic cover plate) also becomes hot. That is, the cover plate or screws would be 120 V with respect to any grounded object or surface. This would be a real shock hazard—especially in kitchens, where many exposed surfaces are electrically grounded. Grounding the switch frame as shown in Figure 2 (and required by the NEC) removes this shock hazard by keeping the switch frame at the same potential as all other grounded surfaces.

With the switch frame grounded, the switch failure described above would trip the circuit breaker in the branch circuit. Homes wired before the NEC required grounding switch frames (straps) to have this shock potential if plastic outlet boxes were used. If metal outlet boxes were used and grounded, then the switch frame (mounting strap) would be grounded by the screws that attach the strap to the box.

Light Outlet

The continuation of the lighting circuit started by the switch in Figure 2 is shown in Figure 3. A simple light fixture with a porcelain base (like those often used in garages, shops, and unfinished basements) may not have a ground connection or flexible leads. In this case, the cable’s grounding conductor connects directly to the metal outlet box. The porcelain-base fixture has a light-colored and a dark-colored screw. The ­neutral conductor connects to the light-colored screw. This is an important safety feature because the light-colored screw connects to the screw shell (the part the light bulb screws into) of the fixture. When a light bulb is changed, it is relatively easy to make contact with this part of the fixture and/or light bulb. You would not want the screw shell to be hot if you accidentally touched it when your body was also touching a grounded surface!

It is easier to install a porcelain fixture if you connect stranded, flexible leads to the fixture rather than connecting the cable’s solid conductors directly to the fixture. Also, notice that some yellow-colored wire connectors are used in Figure 3. Different sizes of wire connectors (sometimes referred to as wire nuts) to accommodate different gages and numbers of conductors have different colored insulating covers.

Plastic outlet box for a wall switch

Plastic insulated wire connectors.

Lighting outlet box diagram

Figure 3. Lighting outlet box diagram

Because of the physical locations of a light fixture and a wall switch, it is sometimes advantageous to have the light fixture in an outlet box with feed-through cables and the switch in the “dead-end” box. This arrangement is shown in Figure 4, where plastic outlet boxes are used. If metal outlet boxes were used, the light fixture’s grounding wire could be connected to the outlet box. This would be done by connecting the grounding wire to the fixture’s mounting strap, which is screwed to the metal box.

Figure 4 illustrates another case when a white conductor has to be used as a hot conductor. It is important that the white conductor is used in the continuously hot leg of the switching circuit instead of the switched (return) leg. If a white wire were used in the switched leg, the two white wires would be connected to the light fixture. Then it could be difficult to determine which is a neutral conductor and which is a switched hot conductor. Remember, if properly wired, a white conductor connected to either a switch or a black (or red) conductor is a hot conductor and must be reidentified with black tape. As shown in Figure 4 (and Figure 2), the switching device has a ground connection as required by the NEC.

Snap switches used in residential wiring before the late 1990s were made without a ground screw on the mounting strap. In this case, the cable’s equipment grounding conductor in a switching circuit like the one shown in Figure 4 was connected directly to the box if the box was metal. If the box was plastic, the grounding conductor was not connected to anything.

Figure 4. Lighting circuit in plastic outlet boxes. A white wire is a hot conductor in this circuit.

The circuits in Figures 1 through 4 may look a little confusing because of grounding wires and wire connectors in the boxes. The essence of all these circuits connected in a single branch circuit is shown in schematic form in Figure 5. This figure shows that a branch circuit is just a parallel circuit with some of the branch loads controlled by switches.

A branch circuit in schematic form

Figure 5. A branch circuit in schematic form. Most branch circuits have more branches than shown here.

Again, notice in Figure 5 that the neutral (white or light gray) conductor is never grounded at any point except at the main distribution panel. Neutral conductors are insulated and meant to carry load current. In a 120-V circuit, they carry the same current as the hot (black) conductor does. In contrast, the ground conductor (bare or green) should never carry any current unless a ground fault occurs in the system.

Review Question:

  1. In a typical 120-V branch circuit, an ­outlet box contains a(n) ______ or a(n) ______, or a(n) ______.
  2. The shorter of the two slots on a ­receptacle is connected to the ­conductor.
  3. The neutral side of a receptacle can be identified by the ______ -colored screw.
  4. The ground screw on a receptacle is a(n) ______ -colored screw.
  5. True or false. A jumper bar connects the neutral screw pads together on a duplex receptacle.
  6. When can an equipment-grounding conductor terminate on a receptacle grounding screw?
  7. The screw shell of a lamp fixture should be connected to the ______ conductor.
  8. True or false. A white conductor connected to a snap switch should be a continuously hot conductor.


  1. switch, receptacle, light fixture
  2. hot or black
  3. light
  4. green
  5. T
  6. Termination on the grounding screw is allowed when a plastic outlet box is used at the end of a branch circuit.
  7. Neutral
  8. T

120 V Branch Circuit Key Takeaways

Understanding the wiring and safety essentials of 120-V branch circuits is crucial for ensuring reliable and safe electrical installations in residential and commercial settings. Properly wired outlets, switches, and light fixtures not only prevent electrical hazards but also ensure that the electrical system operates efficiently and meets regulatory standards. This knowledge is essential for electricians and homeowners alike to maintain the integrity and safety of electrical systems, reducing the risk of electrical fires, shocks, and system failures.