Service equipment, panels, and clearances

The service is where utility power becomes the building's responsibility. Residential service is typically 120/240 V single-phase three-wire: two hot legs 180° apart plus a neutral. Line-to-neutral gives 120 V for lighting and receptacles; line-to-line gives 240 V for ranges, dryers, and HVAC. The service disconnect must be able to kill everything with no more than six operations of the hand.

Working space is a life-safety rule with hard numbers: 30 inches wide (or the equipment width, whichever is greater), 36 inches deep in front for typical residential voltages, and 6½ feet of headroom. That space must stay clear — no storage, no shelving. Inspectors fail panels for boxes on the floor more often than for anything inside the enclosure.

Panelboards have their own limits: a lighting and appliance panelboard is limited to 42 overcurrent devices, and each ungrounded conductor needs its own device. Breakers must be listed for the panel they sit in — mixing brands voids the listing even when the bus physically accepts the breaker.

Exam focus: 120/240 V three-wire arrangement, the six-operations disconnect rule, and the 30/36/78-inch working-space trio. Working-space questions appear on virtually every residential exam.

Quiz — 3 questions