For a current transformer rated 4000:5, what is the secondary current when the primary current is 2000 amps?

• A. 0.5 A
• B. 1.25 A
• C. 2.5 A
• D. 5 A

Answer: (C)

Current transformers scale down large currents using their turns ratio. In an ideal CT, ampere-turns balance: N_p I_p = N_s I_s, so the current ratio is the inverse of the turns ratio. The rating 4000:5 means 4000 A on the primary corresponds to 5 A on the secondary, a 800:1 primary-to-secondary current ratio. Thus the secondary current is the primary current divided by 800. For a primary current of 2000 A, the secondary current is 2000 ÷ 800 = 2.5 A. In practice, factors like burden can affect accuracy, but this is the correct ideal result.