In a transformer, which statement about current on the high-voltage and low-voltage sides is correct?

• A. The high-voltage side carries less current and the low-voltage side carries more current
• B. The high-voltage side carries higher current
• C. No current flows on the high-voltage side
• D. Current is independent of voltage

Answer: (A)

In a transformer, power in roughly equals power out (ignoring losses), so the currents and voltages are tied by Vp Ip ≈ Vs Is. When the transformer steps voltage down (Vp higher, Vs lower), the current on the high-voltage side must be smaller than the current on the low-voltage side to keep power the same; Ip = (Vs/Vp) · Is and Vs/Vp < 1, so Ip < Is. That’s why the high-voltage side carries less current and the low-voltage side carries more. For example, a 1000 kVA transformer with HV at 13.8 kV and LV at 0.48 kV will have a much smaller current on the HV side than on the LV side. There is current on both sides, and current is not independent of voltage; the statement that HV side would carry more current (or that no current flows) is not correct.