Wye Systems 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 threephase threewire loads equally well but they differ greatly in the way they accommodate singlephase 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 wyeconnected secondary requires a primary winding connected in delta or supplied from a fourwire system with a firm neutral. The most common threephase wye system is the fourwire system shown here. The fourth wire is connected to the common point where all three transformer phase windings are connected together. NEC Section 2505 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/277Volt system. This wye system provides the three lines to serve threephase loads at the system line voltage. It provides three other linetoneutral circuits to serve the singlephase loads at 277 Volts, a voltage which is 57.7% of the line voltage. Common arrangements are 480Y/277Volts and 208Y/120Volts. Singlephase loads can be connected linetoline as well, if adequately insulated at both ends and protected by twopole, commontrip circuit breakers. A singlephase load connected in a linetoneutral configuration only loads the transformer phase that it is connected across. A singlephase load connected linetoline loads two transformers with a derating penalty of about 14%. (If these loads are balanced linetoline across all three phases, this penalty cancels out.) Singlephase loads are almost always served linetoneutral instead of linetoline. The common neutral connection provides a convenient place to apply groundfaultcurrent detection or protection. The grounding conductor at the neutral carries current only if current returns outside the normal four wires.
