The transformers supplying the plant loads can be connected on the load (secondary) side either in delta or in wye configuration. Either of these configurations can serve balanced three-phase three-wire loads equally well but they differ greatly in the way they accommodate single-phase loads. The other difference is the way the system can be grounded.
Standardized system voltages are based on integer multiples of 120 Volts and are historically related to the use of delta systems. That is why voltages that have the 1.732 as a multiplier (208 V; 4,160 V; 12.47 kV) are wye connections. A wye-connected secondary requires a primary winding connected in delta or supplied from a four-wire system with a firm neutral.
The most common three-phase wye system is the four-wire system shown here. The fourth wire is connected to the common point where all three transformer phase windings are connected together. NEC Section 250-5 requires that the system be grounded for AC systems of 50 Volts to 1,000 volts where the neutral is carried through as the fourth wire as in a 480Y/277-Volt system.
This wye system provides the three lines to serve three-phase loads at the system line voltage. It provides three other line-to-neutral circuits to serve the single-phase loads at 277 Volts, a voltage which is 57.7% of the line voltage. Common arrangements are 480Y/277Volts and 208Y/120-Volts. Single-phase loads can be connected line-to-line as well, if adequately insulated at both ends and protected by two-pole, common-trip circuit breakers.
A single-phase load connected in a line-to-neutral configuration only loads the transformer phase that it is connected across. A single-phase load connected line-to-line loads two transformers with a derating penalty of about 14%. (If these loads are balanced line-to-line across all three phases, this penalty cancels out.) Single-phase loads are almost always served line-to-neutral instead of line-to-line.
The common neutral connection provides a convenient place to apply ground-fault-current detection or protection. The grounding conductor at the neutral carries current only if current returns outside the normal four wires.
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