A voltage check with a zero reading between ______ and the neutral will confirm that the service is not fed from another source.

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

Answer: (D)

The key idea here is checking for backfeed from another source by comparing the two sides of the main disconnect. L1 is a line (incoming hot) and S1 is the switch terminal on the load side that feeds the panel.

If another source were energizing the service, the load side could be energized independently of the incoming line, and you would see a voltage between the line side and the load-side switch terminal. A zero reading means there’s no potential difference between those two points, indicating the service isn’t being backfed by another source and is being supplied from a single source.

So, testing between the line side conductor and the corresponding switch terminal on the load side is the correct way to confirm there isn’t another source feeding the service.