In general, we recommend keeping a single line through a single sensor. One situation where you can add multiple lines through a single sensor would be if both lines are on the same phase of power. If using two lines from separate phases of power through a single sensor, then their measurements would be cancelled out against each other. In a situation where you do have the same phase wires running through a sensor, it'll provide the total usage from both lines (or more) as a single measurement in the app. Standard amperage/usage limitations still apply, which would be 50A for the sensor on a Gen 1, and 75A total for the Gen 2 side sensor.
Here is a simple illustration of the Phase A(L1) and Phase B(L2) of a single split phase panel found in most North American Homes. The sensors should not be placed around a wire from phase 'A' and 'B' at the same time for the reason explained above.
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This knowledge base is pretty new for the Emporia team. Our goal is to provide all of the information we can to help you manage your energy in better ways. If this article wasn't helpful, or we could be more clarifying on any points please reach out to the Customer Support team and we'll certainly work to improve these guides.