Electrical Estimating Methods, 4th Edition

CHAPTER 16 Transformers and Bus Duct

Transformers are devices with two or more coupled windings, with or without a magnetic core. They introduce mutual coupling between circuits and are used to convert a power supply from one voltage to another. Bus duct, or busway, is a prefabricated unit that contains and protects one or more busses. In this chapter, there are five sections on: transformers, isolating panels, two types of bus duct, and Uninterruptible Power Supply (UPS) systems. Units for measure, material and labor requirements, and a procedure for takeoff follow each description.

TRANSFORMERS

Transformers are used in alternating current (AC) systems to convert from one voltage to another. They cannot be used in direct current (DC) systems. The total energy entering a transformer is equal to the energy leaving the transformer, except for minor losses. Thus, when the voltage is stepped down, the current changes in inverse proportion, specifically it increases. A transformer's capacity can therefore, be designated in terms of the product of the voltage (V) and the amperage (A), that is, as volt-amps (VA) at either side. Larger transformers are often designated in thousand volt-amps or kVA.

Transformers are used for four basic applications: (1) instrument transformers, (2) control transformers, (3) isolating transformers, and (4) power transformers. Their capacities range from fractional VA to thousands of kVA. The first two are single-phase types only, but the second two ...