Most buildings, commercial, industrial, institutional, and residential, receive their power from a public utility. Usually, the customer is given a choice of voltages.
For example, 240/120-V single-phase, three-wire service is, very common in suburban and rural areas. This service comes from a single-phase, 240-V transformer, with one wire from each end of the secondary coil and with the neutral from the midpoint of its secondary coil. The voltage between the end terminal connections is 240 V and between each end wire and the neutral, 120 V (Fig. 15.3b).
In large cities, the service to large buildings can be 208/120 V, three-phase, four-wire, with 208 V available between phase wires and 120 V between a phase wire and the neutral (Fig. 15.3c). Another choice is 480/277 V, three-phase, fourwire, with 277 V available between a phase leg and the neutral. It is more economical to use the higher voltage, 480/277 V, for motors and industrial lighting.
The lower voltage 208/120 V is required for residential or commercial lighting and appliances.
In some areas, the utility will provide both voltage services on separate meters to a large building. But in other areas, the customer must choose one or the other voltage from only one meter and then use transformers to provide the second voltage service.
Transformers
Transformers may be dry or liquid-immersed type. The liquid-immersed type is used for large installations. If the liquid is mineral oil, special fire-protection precautions are needed. Any liquid-filled transformer requires means for containing the liquid if the transformer tank should leak.
All transformers are rated in kVA, with primary and secondary voltages. Taps may be provided on the primary to compensate for variations in utility voltage as much as 10% below and 5% above nominal voltage, in 21â„2% increments. The manufacturer can also make available to the engineer the reactance and resistance of the coils and the noise rating. Noise can be minimized by use of vibration isolation mountings.
The power losses in a transformer create heat, which must be dissipated. Drytype transformers are cooled by circulating air in the spaces enclosing the transformers.
For liquid-filled transformers, which usually have very high capacity, the liquid may be circulated through coolers to transfer heat from the coils. Average losses in transformers used in buildings are about 2% of the rated capacity.
