What causes transformers NOT to be 100% efficient

• A. I2R Loss
• B. Core Loss
• C. Dielectric Loss
• D. Stray Loss

Answer: (A)

The main idea is that some input power is turned into heat in the transformer’s windings due to their finite resistance. This resistive heating, called I^2R loss, occurs when current flows through the winding conductors: the power converted to heat is I squared times R. It reduces the power available as useful output, keeping efficiency below 100%. Other losses like core losses (hysteresis and eddy currents in the core), dielectric losses (in insulation), and stray losses (from leakage flux) also exist, but the direct dissipation in the windings from their resistance is the primary reason energy isn’t fully transferred.