MSS550 v2 is designed to be used with electric boxes with load wires (also called live wires). The following points can help you find the correct electric box to install.
1. Choose the electrical box far away from the fuse box.
2. There is no voltage in the load line, please use an electrical tester to verify.
3. If you are still unsure, just select any electrical box to start using. The Meross app will help you detect the correct electrical box in the future.
For MSS550 v3, you can install it in the fuse side or load side electrical box.