Rule 7 of 40 · Chapter I — Before You Touch Anything
Map and label your panel
Why this rule exists
In an emergency you do not want to be guessing which breaker kills a circuit. Mislabeled or blank panels waste the seconds that matter and lead people to work circuits they wrongly believe are dead. The NEC requires circuit directories to be legible and specific, not lights or misc. A clear map also reveals multiwire branch circuits, shared neutrals, and double-tapped breakers, the arrangements most likely to surprise you. Knowing your system before you work it is the map you navigate by when things go wrong.
In practice
Walk the panel and identify each circuit by turning breakers off and confirming what dies, ideally with a helper and a receptacle tester. Write specific labels: room, device, and any shared-neutral partner. Note the panel's main disconnect and any subpanels it feeds. Flag anything unusual, a breaker feeding two rooms, a circuit that will not fully de-energize, tandem breakers. Update the directory whenever the wiring changes. Keep a flashlight and the panel location known to everyone in the building.
When it doesn't apply
Circuit mapping by switching breakers can crash computers, disrupt medical or life-safety equipment, and reset controls, so coordinate before you kill power. For circuits you cannot safely interrupt, map from documentation and non-intrusive tracing. If the panel is a recalled or damaged type, or shows scorching, stop and have it evaluated rather than probing it.