Circuit models are often used when a transformer is part of a larger circuit such as a utility network. In this case, only the terminal characteristics of the transformer are usually of interest. This chapter concerns transformers operating at steady-state power frequencies, that is, generally 50 or 60 Hz. It considers a single-phase core and transformer. Usually, 3-phase cores and transformers can be analyzed as three separate single-phase units. Interconnections among the phases can be added later. Transformer impedances, along with other quantities such as voltages and currents, are often expressed in the per-unit (p.u.) system. In this system, these quantities are expressed as a ratio with respect to the transformer's nominal or rated phase quantities. Since the base voltages, currents, and impedances transfer across the ideal transformer in the same manner as their corresponding circuit quantities, the p.u. values of the circuit quantities are the same on both the primary and secondary sides.