NATE Air Conditioning and Heat Pumps Practice Exam

Black and white leads on a step-down control transformer indicate a ____.

1. 24-V ac secondary

2. 24-V dc secondary

3. 120-V ac primary

4. 208-V ac primary

The correct answer is: 120-V ac primary

In the context of a step-down control transformer, black and white leads typically represent a 120-V AC primary. This is important in understanding transformer wiring since the primary side of a step-down transformer connects to the input voltage, which is commonly found in residential or commercial electrical systems. The primary winding of a step-down transformer is designed to operate at a higher voltage, such as 120 volts, converting it to a lower voltage on the secondary side, like 24 volts for control circuits. The identification of black and white conductors often correlates with the standard color-coding in electrical systems, where black typically signifies a live (hot) wire and white signifies a neutral wire in an AC circuit. On the other hand, a 24-V AC or DC secondary would be associated with the output of the transformer, not the primary. Therefore, understanding the function and connections of the transformer assists in properly identifying its components based on standard practices.