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Three-phase transformer with four-wire output for 208Y/120 volt service: one wire for neutral, others for A, B and C phases. Three-phase electric power (abbreviated 3ϕ [1]) is a common type of alternating current (AC) used in electricity generation, transmission, and distribution. [2]
Even when unmarked, it is generally easy to identify this type of system, because the B phase (circuits #3 and #4) and every third circuit afterwards will be either a three-pole breaker or a blank. Current practice is to give separate services for single-phase and three-phase loads, e.g., 120 V split-phase (lighting etc.) and 240 V to 600 V ...
Clock diagram for the regular (>50 V) connector series, viewed looking in to the socket end. The extra-low voltage connector also supports keying, although in this case the angle is the position of the minor key, assuming the major key is at the 6h position (180°).
Single-pole circuit breakers feed 120 V circuits from one of the 120 V buses within the panel, or two-pole circuit breakers feed 240-volt circuits from both buses. 120 V circuits are the most common, and used to power NEMA 1 and NEMA 5 outlets, and most residential and light commercial direct-wired lighting circuits.
A distribution board (also known as panelboard, circuit breaker panel, breaker panel, electric panel, fuse box or DB box) is a component of an electricity supply system that divides an electrical power feed into subsidiary circuits while providing a protective fuse or circuit breaker for each circuit in a common enclosure.
Rural services normally try to minimize the number of poles and wires. It uses higher voltages (than urban distribution), which in turn permits use of galvanized steel wire. The strong steel wire allows for less expensive wide pole spacing. In rural areas a pole-mount transformer may serve only one customer.
Two-pole common-trip breakers are common on 120/240-volt systems where 240 volt loads (including major appliances or further distribution boards) span the two live wires. Three-pole common-trip breakers are typically used to supply three-phase power to powerful motors or further distribution boards.
Each row of breakers in the distribution board is fed from a different phase (L1, L2, and L3), to allow 3-pole common-trip breakers to have one pole on each phase. The distributor's service head will contain a main fuse for each phase. Nominal voltage is usually 230 V AC between phase and neutral, and 400 V AC between phases.