A zero reading between L1 and L2 would indicate the service is not fed from another source.

• A. L1 & L2
• B. L2 & S1
• C. L1 & S2
• D. L1 & S1

Answer: (A)

The key idea is how to tell if a second source could be feeding the service by comparing the two supply legs themselves. L1 and L2 are the two hot conductors that carry the service from the main source. When the system is energized from a single source, there’s a substantial voltage between these two lines (typically around 240 V). If you observe a zero voltage between L1 and L2, it means there is no potential difference between them, so there isn’t a second source backfeeding into the service. In other words, L1 and L2 being at the same potential indicates the service isn’t being fed from another source. Testing other pairs (involving S1 or S2) wouldn’t give a reliable indication of an alternate feed because those points are related to switches or other parts of the wiring and can be misleading about the presence of a second source.