What is the primary current required to produce a 1.25 A secondary current on a 4000:5 current transformer?

• A. 2000 A
• B. 500 A
• C. 1000 A
• D. 4000 A

Answer: (C)

Current transformers relate primary and secondary currents by their turns ratio. For a 4000:5 CT, the primary to secondary current ratio is 4000 to 5, so the secondary current equals the primary current multiplied by 5/4000 (which is 1/800). To produce a 1.25 A secondary, the primary must carry 1.25 × 800 = 1000 A. So the primary current required is 1000 A. This assumes an ideal CT with a suitable burden; real CTs introduce small errors due to magnetizing current and burden.