Motors and transformers
Ideal transformer current ratio
- where
- primary current (A)
- secondary current (A)
- secondary voltage (V)
- primary voltage (V)
- secondary turns
- primary turns
Explain this equation
Gives the primary current for a known secondary load current in an ideal transformer. Use it to estimate how much a step-down load will draw from the supply side.
V_p and V_s are the primary and secondary winding voltages, and N_p and N_s the turns. I_s is the secondary load current and I_p the matching primary current. Note that the currents go the opposite way to the voltages.
An ideal transformer passes power through without loss, so V_p × I_p equals V_s × I_s. If the voltage is stepped down, the current is stepped up by the same ratio. The calculator works out I_p = V_s × I_s / V_p.
230 V to 23 V with 10 A on the secondary draws only 1 A from the primary. A 10:1 step-down means one-tenth of the current on the high-voltage side.
Watch out: a real transformer also draws magnetising current and has losses, so the primary current is slightly higher, and some current flows even with no load.
For an ideal transformer, a reduction in voltage is accompanied by a greater secondary current for the same transferred apparent power. Current ratio is inverse to voltage ratio when comparing the same sides.
Use I_p = V_s × I_s/V_p to estimate the primary load-current component. Real transformers also take magnetising current and incur losses.
Enter your values to see the result.
Common mistake
Do not multiply primary current by the voltage step-down ratio to find secondary current.
Limits
Ideal single-phase model, negligible losses and magnetising current. It does not predict actual no-load current or permission to exceed winding ratings.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.