What would the wire size be for a 3 phase 4 wire - 480/277V - 400A panelboard. Would it be (3) 600mcm w/ (1) 1/0 ground?
Well not quite: 1. The panelboard is 4 wire therefore the (3) in the description would be (4). The 4th wire is for the neutral and required if you are using the 277. Otherwise the panelboard is a 3 wire board and no 277 2. 600 kcmil copper with 75°C insulation is able to carry 420 A as long as the ambient temperature < 86°F (30°C) and are installed by themselves (not in a raceway/cable tray with other power conductors). If the conductors are aluminum, the temperature is hot, the installation has multiple conductors in the same raceway/cable tray, or the distance is of sufficient length resulting in unacceptable voltage drop, the cable must be derated -- read you must increase the size to 750 kcmil or increase the insulation rating to 90°C The ground conductor should be of sufficient size
Three phase 3 wire is often called a 'delta' connection since the load(s) are distributed between each pair of phases. The 4 wire systems are also called 'Y' systems since the loads are distributed between each phase and a 4'th 'common' phase. The 4 wire system is intrinsically a bit safer since the common phase can also be made electrically neutral (ground). Doug
