This post will discuss how 3 phase panelboards are built and how they distribute power to loads.
How 3 Phase Panelboards are Built
Panelboards serve as a distribution medium for power to feed various loads. The Panelboards themselves must be fed from an upstream source with adequate capacity to feed all the loads it distributes power to. This is called the incoming feeders or the mains to the panel. A panelboard’s incoming feeders may come from the branch circuit of another panelboard or switchboard upstream, or may come directly from an upstream transformer. In either case for 3 phase 4 wire AC systems there are usually 4 incoming feeders plus ground.
The feeders are as follows:
Wire 1 – Phase A
Wire 2 – Phase B
Wire 3 – Phase C
Wire 4 – Neutral
The panel will have one bus for neutral, which is usually found at the top or bottom of the panel. There will be one bus for ground, which is usually off to the left or right side of the panel. This ground bus will also be connected to the metallic enclosure of the panel itself. There will be three larger copper busses that run down the length of the panel. These are the buses that connect to each of the phases A, B, and C of your incoming feeders. They are sometimes referred to as as the power mains or mains of the panel. The current rating of a panel (LINK) is dependant on the rating of these buses. Each of the loads being fed off the panel is connected to either one, two or three of these buses depending on whether its a single phase load, two phase load or a three phase load respectively. This is accomplished by smaller buses that run perpendicular to the power mains connecting to each of the phases in a standard order.These smaller buses are usually referred to as finger bars/buses in the industry. All manufacturers follow the standard ordering such that it is possible to determine which phase(s) a load is being fed off dependant on which branch breaker it is being fed from.
Branch Breakers, Main Breakers & Main Lugs
The incoming feeders enter the panel enclosure either at the top, bottom, or side depending on the constrains on site. Where the incoming feeders enter the panel is known as the “Entry”. Ideally you would prefer top entry. The entry of the incoming feeders plays a role in where the panel’s main breaker will be placed (if it has a main breaker). For a top entry panel the main breaker will ideally be mounted at the top of the panel, for a bottom entry panel the main breaker would be mounted at the bottom of the panel.
Branch breakers are the breakers that get connected to the loads. The branch breakers connect to the the main copper busses (via the finger bus in most panels) that run down the panel. Branch breakers are installed differently depending on the manufacturers and Interrupting Current (KAIC) rating one wishes to achieve. Breakers come in bolt-on type or snap-on type. Read (link) for more info on breakers.
It is also important to note that only the panel manufacturer’s