Before installing a socket meter, what is the voltage measured between the top two energized source lugs on a single-phase, 3-wire service?

In a single-phase, 3-wire service, there are typically two hot wires (often referred to as phase conductors) and one neutral conductor. The measurement between the top two energized source lugs, which are the two hot wires, represents the total voltage of the system available for the circuit.

For a standard residential setup, the voltage between these two hot wires is 240 volts. This is derived from the typical configuration in which each leg of the service provides 120 volts with respect to the neutral. When measuring the voltage between the two hot wires, the voltages effectively add up, resulting in a total of 240 volts.

This information is fundamental in understanding electrical systems, as it helps ensure correct installation practices and safety. The other voltage options represent different service configurations or types, which are not applicable to the standard single-phase setup described in the question. For example, 120 volts would be the measurement across a single hot wire and the neutral, while 208 volts may pertain to certain three-phase systems, and 480 volts is generally associated with high-voltage industrial applications, none of which apply in this case.