Inside your main electrical panel, you’ll see labels or lists indicating which breaker controls which circuit. These labels are a reasonably good guide, but they’re not completely reliable, especially in older homes that have been through remodeling projects.
For example, you might find that there’s one outlet in a room that’s not on the same circuit as all the others in that room, yet it’s not listed elsewhere on the panel. That orphan outlet could be connected to almost any other circuit in your home. In some cases, you might even find wires from different circuits in the same junction box.
The bottom line: Always use a voltage detector to make sure the power is off before you do any electrical work.