Final answer:
The primary coil rating of a transformer is usually the higher of the two primary voltages listed. This is fundamental to the transformer equation that relates the ratio of voltages to the ratio of the turns in the coils of a transformer.
Step-by-step explanation:
The primary coil rating in a transformer refers to the voltage that the coil is designed to receive. When discussing a transformer, there can be two primary voltages listed; the one that the primary coil is actually rated for is typically the higher voltage. Understanding transformers involves the transformer equation, which shows that the ratio of the secondary voltage to the primary voltage is equal to the ratio of the number of turns in the secondary coil to the number of turns in the primary coil. For instance, in a step-up transformer, the primary voltage is lower, resulting in a higher secondary voltage. Conversely, a step-down transformer has a higher primary voltage than the secondary voltage.
Characterizing a transformer, like distinguishing between a step-up and step-down transformer, can be done by comparing the primary and secondary voltages or currents. A step-up transformer increases voltage and decreases current, whereas a step-down transformer decreases voltage and increases current.