Gas Fitter Practice Test

Where is a 120-volt power supply sourced from in a residential electrical circuit?

1. Line side of one branch circuit breaker and a ground lug

2. Load side of one main circuit breaker and the line side of a second breaker

3. Load side of one main breaker and the load side of the second main breaker

4. Load side of one branch circuit breaker and a neutral lug

The correct answer is: Load side of one branch circuit breaker and a neutral lug

A 120-volt power supply in a residential electrical circuit is sourced from the load side of one branch circuit breaker and a neutral lug. In typical residential wiring, the 120-volt circuit is derived by using one of the hot wires (usually black or red) that feeds through a branch circuit breaker. The other necessary component to complete the circuit is the neutral wire, which connects to the neutral bus bar or neutral lug in the electrical panel. This configuration works because the breaker allows for the control of electrical flow, providing protection against overloads, while the neutral lug serves as a return path for the current. The voltage between the hot wire and the neutral wire produces the 120 volts required for standard household appliances and fixtures. Other options do not adequately provide a proper 120-volt supply. For instance, sourcing from the line side of a circuit breaker generally refers to incoming voltage, not the usable voltage needed for devices. Additionally, using load sides of multiple main breakers or other configurations would typically yield 240 volts or result in improper voltage sources, which are not suitable for a standard 120-volt application.