What a panelboard is
A panelboard is an assembly that receives power from a feeder and divides it into several branch circuits, each protected by its own breaker or fuse. It is usually mounted in a wall-mounted or floor-standing metal cabinet, with a hinged cover that exposes the breaker handles but not the live parts behind them. In many facilities it is the last stage of distribution before the wiring that runs to lights, receptacles, small motors and equipment.
Terminology varies by country and by standard. Some regions say “distribution board” for a similar device. The ideas below are general; the code that applies to a site decides the details.
Main parts
- Enclosure. The metal box, often called a cabinet or can, plus the front trim and door. Its rating describes protection against dust, water and contact, and it should suit the environment, whether a dry electrical room or a wet, dusty production area.
- Bus. Metal bars, usually copper or aluminum, that carry current from the incoming connection to the breakers. The bus has a current rating, and a short-circuit rating describing the fault current it can withstand.
- Main device. A main breaker or main disconnect that can switch off the entire panelboard. Some panelboards are “main lugs only” and rely on a protective device upstream.
- Branch breakers. Devices that open the circuit on overload or short circuit. They can also serve as the switch for the circuit. The differences between breakers and fuses are explained under circuit breakers.
- Neutral and ground bars. Terminal bars where neutral and protective ground conductors connect. How they are connected, and whether neutral and ground are bonded in this panel, follows the code and the system design, which relates to grounding and bonding.
- Interior barriers and labeling. Covers and a circuit directory that identifies each circuit.
How a panelboard differs from switchgear
The two overlap in function, but they differ in scale and construction.
Panelboards are generally lighter-duty. They hold molded-case breakers in a compact, standardized layout, and they are fed from a larger source upstream. Switchgear is a heavier assembly with larger, more robust breakers in compartments, designed to handle higher currents and fault levels and often built so that individual breakers can be withdrawn for service. It tends to sit at the head of the distribution system, as in switchgear and medium-voltage basics.
Typically, then, switchgear protects main feeders and large loads, while panelboards serve groups of smaller loads. A third category, the motor control center, groups motor starters, and is dealt with separately in articles on motor control centers.
Choosing and specifying
Matters that an electrical designer typically settles include:
- Voltage and number of phases.
- Bus current rating, with some spare capacity for growth.
- Short-circuit rating that exceeds the available fault current at that point.
- Number of circuit spaces, including spares.
- Type of mains and any ground-fault or arc-fault protection the code requires.
- Enclosure type for the environment, and whether the panel is surface or flush mounted.
- Mounting height and clear working space in front.
The rating of each breaker needs to match the conductor it protects, not simply the load, which is one reason that changes should not be improvised.
Care and operation
Panelboards are among the most commonly handled parts of an electrical system, and that makes tidiness important. Practices that typically help are keeping the area in front free of storage, keeping covers in place and closing unused openings, and keeping the circuit directory accurate. Regular inspection of panels by qualified staff, covered in panel maintenance routines and in the inspection and testing of panels, can find loose connections, overheating, dust and damage before they cause a failure.
Safety
Inside a panelboard are live parts that can cause electric shock, burns and arc flash. Removing covers, working on terminals and replacing breakers should be done only by trained, qualified personnel, using lockout and tagout style procedures, appropriate protective equipment and the manufacturer’s instructions, in line with local regulations. Operators who only switch breakers should do so only as their site procedures allow. This article is for general information, not as engineering advice.