A current transformer has a rating of 4000:5; if 1000 amps load is on the primary, the secondary output would produce

• A. 0.25 amps
• B. 5 amps
• C. 2.5 amps
• D. 1.25 amps

Answer: (D)

A current transformer scales down large primary currents into a smaller secondary current in proportion to its turns ratio. A rating of 4000:5 simplifies to 800:1, meaning for every 800 A flowing in the primary, about 1 A appears in the secondary. With 1000 A on the primary, the secondary current is 1000 divided by 800, which equals 1.25 A. So the secondary output would be about 1.25 A. This assumes an ideal transformer; real CTs have small inaccuracies, but the proportional relationship holds.