NCCER Residential Wiring Practice Exam

In newly constructed homes, smoke detectors are typically required to be powered by a 120-volt AC source. This is because the majority of modern smoke detectors are designed to operate directly from the standard household voltage, allowing them to benefit from a reliable power source without the need for battery replacements.

Using 120 volts AC ensures that smoke detectors are always operational as long as the power is on, which is essential for safety in residential settings. Many codes and regulations specify this voltage to ensure compatibility with existing wiring standards in homes, simplifying installation and increasing the overall reliability of smoke detection systems.

While other voltages such as 24 volts AC, 240 volts AC, and 12 volts DC are used in certain applications, they are not standard for primary smoke detector power in residential construction, leading to their exclusion as the correct choice for this question.