Electrical controls in oil heating systems shall be connected in which circuit?

The correct choice is to connect electrical controls in oil heating systems to the line circuit. This is because the line circuit carries the voltage necessary to power the controls, ensuring they function correctly. By connecting the controls to the line, you provide the necessary electrical supply for the operation of components like the burner, pumps, and other essential systems within an oil heating unit.

Connecting controls to the line circuit facilitates reliable performance and allows the controls to energize when required, such as during the heating cycle or when initiating system diagnostics. This connection method aligns with standard electrical practices, ensuring safety and efficiency within the system.

In contrast, connecting controls to a neutral circuit would not provide the necessary voltage and may result in the controls not functioning at all. A ground connection is meant for safety and protection against electrical faults, and while service circuits are used for maintenance purposes, they do not supply power in the same way as the line circuit. Therefore, the line circuit is the appropriate choice for connecting electrical controls in oil heating systems.