Panelboard

A panelboard is a factory-built assembly with a few standard parts:

• Enclosure (cabinet), with a trim and door over the dead front that keeps live parts out of reach
• Main lugs or a main breaker where the incoming feeder connects
• Bus bars that carry current to each breaker position
• Branch circuit breakers, or fusible switches, one for each circuit
• Neutral bar and equipment grounding bar
• Circuit directory (panel schedule) identifying what each breaker feeds

Power reaches a panelboard through a feeder from the main switchboard, another panel or a transformer. The bus bars distribute that power to the branch breakers, and each breaker feeds one circuit of lighting, receptacles or equipment. If a circuit draws too much current or has a short circuit, its breaker trips and disconnects that circuit without affecting the rest of the panel.

In commercial buildings, panelboards are usually located in electrical rooms or closets on each floor. A common pattern is a 277/480V panel for lighting and mechanical loads, a step-down transformer, and a 120/208V panel for receptacles and small equipment. A panel is described as main lug only (MLO) when it has no main breaker and is protected by a device upstream, or main breaker (MB) when it has its own main disconnect.

A panelboard is designed to be installed in a cabinet in or against a wall and accessed from the front. A switchboard is a larger, free-standing assembly that usually sits higher in the distribution system and handles the main service or large feeders. Both are covered by Article 408 of the National Electrical Code (NEC).

Load center is the term commonly used for a residential or light commercial panelboard, the breaker panel found in a house. People also say breaker box, electrical panel or distribution board, which is the common term outside the US.

Every panelboard should have an accurate circuit directory listing what each breaker serves. The NEC requires circuits to be clearly identified, and in practice an up-to-date panel schedule saves time in every outage, renovation and lockout. A full panel schedule also shows the panel name, voltage, phase, bus rating, main breaker or lug rating, feeder source and the load on each circuit.

Panelboards also need clear working space in front of them so they can be operated and serviced safely. NEC Section 110.26 sets the required depth, width and height of that space, and keeping it free of storage is a routine inspection item. Panels that may be worked on while energized also carry arc flash warning labels under NEC Section 110.16, with the label content defined by NFPA 70E.

Panelboard maintenance typically includes infrared scans under load to find hot connections, checking for signs of overheating or corrosion, making sure covers and filler plates are in place, torque checks on connections during planned shutdowns, and updating panel schedules after every change. NFPA 70B covers maintenance of electrical equipment, including panelboards.

Teams track each panel's name and location, its feeder source, the panels and equipment it feeds, spare breaker spaces and available capacity, arc flash label data, and the date of the last infrared scan. In a digital twin such as Syyclops, panelboards can be tracked as assets in the building model with their documents and work orders attached.