Panelboards, like the one pictured below, can be found in virtually all buildings with electrical service. Switchgear and switchboards are much less familiar for the lay person, but they all serve the same purpose: taking an incoming power feed and splitting it up into smaller circuits protected by overcurrent devices. The panelboard below uses molded case circuit breakers for protection of branch circuits.
Panelboards, switchgear, and switchboards are all defined in NEC Article 100. The distinctions may not be immediately obvious. Small panels (in terms of both size and current ratings) that are accessible only from one side are usually called panelboards and large, standalone, metal-enclosed assemblies are called switchgear. Switchgear generally range in ratings from 800A to 6000A. Switchboards are somewhere between the two. In the 2020 edition of the National Electrical Code, Article 408 addresses panelboards, switchgear, and switchboards.
A Typical Electrical Panelboard for Residential Applications
High-Level Schematic of a Typical Panelboard with Circuit Breakers
Panelboards are required to be protected with an overcurrent device rated for no more than their nameplate rating. For example, a 300A or 400A fuse could be used to protect a panelboard, but not a 500A fuse. Switchboards and switchgear are not subject to this requirement, but it's still a good design practice.
Padmount Switchgear Installed in a Public Location
Neither Article 408 nor Article 215 (Feeders) requires that the feeders supplying the panelboard, switchgear, or switchboard have ampacity sufficient to carry the nameplate of the device. Instead, feeders are only required to carry the load anticipated to be fed through the panelboard (125% of continuous + 100% of noncontinuous). It may be a good choice to design for the nameplate of the panel if additional growth is expected for the installation. Additionally, remember that conductors must be protected in accordance with their ampacity. If a conductor is sized to carry a load less than the panel/switchgear rating, then overcurrent protection must be provided to limit the conductor from being used at a higher rating.
The National Electrical Code used to place a limit on the number of overcurrent protective devices that could be installed on panelboards, but that language has been modified in more recent editions of the Code. Now, the limit is by the manufacturer. Depending on the load served from the system, it may be reasonable to limit the sum of the downstream overcurrent protection trip ratings to be no more than the panel rating. However, this is often an unnecessarily conservative practice due to the limited number of standard overcurrent protective device ratings. It's common for very small loads in a home to all be supplied by 15A and 20A breakers, totaling to a downstream collective rating much larger than the sum of the panel. If the system is properly designed for coordination and overload protection, then there is no issue with having a higher total downstream trip rating than the main trip rating for the panel/switchboard/switchgear.
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