What is transformerin construction?

The electrical engineer sizes it from the connected load on the secondary panel with applicable demand factors, then rounds up to a standard rating (15, 30, 45, 75, 112.5, 150, 225, or 300 kVA for common dry-type units). The estimator does not size transformers; the takeoff prices exactly the kVA shown on the one-line. If a load study changes the size during design, the one-line revision is the trigger to reprice.

Dry-type transformers are air-cooled units installed indoors in electrical rooms, owned by the building, and priced in the electrical contractor's scope. Pad-mounted transformers are liquid-filled units sitting outdoors on a concrete pad at the service entrance, usually owned by the utility, with the contractor providing the pad, the conduit, and sometimes the secondary conductors per the utility's requirements. Confirming who furnishes what is a bid-day question worth asking twice.

A K-rated transformer is built to handle the extra heating caused by harmonic-rich loads such as variable frequency drives and electronic equipment, using heavier windings and an oversized neutral. The K-factor rating (K-4, K-13, and so on) appears in the specifications or on the one-line. It adds meaningfully to the unit price, so a takeoff that assumes standard units when the spec calls for K-rated ones will be low on every transformer line.