Transformer

The Full Picture

In building power, a transformer lets the utility deliver power at a higher voltage and the building use it at a lower one. A common arrangement steps a 480 V feeder down to 208Y/120 V for lighting and receptacle panelboards. The size is given in kVA, the apparent power the unit can carry. For a three-phase load, kVA is volts times amps times the square root of 3, divided by 1,000. This page is about the electrical device, not the AI model type of the same name.

Transformers come in several constructions, and the drawing or spec names which. Federal regulations define a liquid-immersed distribution transformer as one where the core and coil assembly is immersed in an insulating liquid, and a dry-type as one with a gaseous or dry-compound medium. The same definition places distribution transformers at an input line voltage of 34.5 kV or less. Indoor oil-insulated units have vault requirements in OSHA's rules, so the type affects the room and enclosure scope as well as the unit price.

The transformer is shown on the one-line diagram with its kVA, primary and secondary voltage, and the primary and secondary protection. The equipment schedule or electrical specification lists impedance, taps, temperature rise, enclosure and mounting. On site plans, pad-mounted units show their pad location and the primary and secondary conduit routes. The estimator counts each transformer EA, described by kVA, voltage, phase and type.

Items that are easy to miss sit around the transformer. The primary and secondary protective devices, the feeders on both sides, the grounding connection, the concrete pad or housekeeping base for a pad-mounted or floor-mounted unit, and vibration isolation for a dry-type in an occupied area each need their own line. Utility-owned transformers are often outside the contractor's scope, so the estimator must read whether the utility furnishes them or whether the contractor buys them. Because larger units are made to order, lead time is flagged.