Larger transformers are about _____% efficient.

• A. 95
• B. 97
• C. 98
• D. 99

Answer: (C)

Large transformers run very efficiently because their design minimizes the main loss sources while delivering near full rated power. At full load, the losses are mainly core losses (iron losses) which stay fairly constant and are a small fraction of the rated power, and copper losses (I^2R) which are kept as low as possible with large, low-resistance windings and good cooling. When you add these losses together, they typically amount to about 2% of the output, which puts the efficiency around 98%. Some units can approach 99% under ideal conditions, but 98% is the common practical figure used for large transformers. Lower efficiencies (in the mid-to-upper 90s) are more typical of smaller or older designs where losses aren’t as well minimized.