The turns ratio tells you which way the voltage changes — and the two cases are easy to confuse, so it is worth stating plainly.
Secondary voltage is higher than primary. Secondary current is lower.
Example: 208 V → 600 V (≈ 1:2.88)
Secondary voltage is lower than primary. Secondary current is higher.
Example: 480 V → 120 V (4:1)
Voltage steps up where secondary turns exceed primary turns. Current moves in the opposite direction — a step-up in voltage is a step-down in current, and vice versa.
Ip ÷ Is = Ns ÷ Np
Current moves in the opposite direction to voltage.
Power is conserved across the ideal transformer:
Vp × Ip = Vs × Is.
A step-up in voltage is a step-down in current,
and vice versa.
Vp / Vs = Np / Ns. The side with more turns has the higher voltage.
Ip / Is = Ns / Np. Higher voltage means lower current — the two are inversely related.
A transformer that raises the voltage (step-up) reduces the current by the same ratio.
In an ideal transformer, VA in = VA out. Vp × Ip = Vs × Is — the turns ratio changes voltage and current, not total power.
The turns ratio relates the primary and secondary windings through a single proportional relationship:
Np ÷ Ns = Vp ÷ Vs = Is ÷ Ip
Where N is the number of winding turns, V is voltage, and I is current, with the subscript p for primary and s for secondary. In an ideal transformer, the voltage ratio equals the turns ratio exactly.
In practice you usually know the voltages and want the ratio. A transformer rated 480 V primary and 120 V secondary has a turns ratio of 4:1. A 120 V to 12 V transformer has a ratio of 10:1. The transformer turns ratio calculator performs this division for you, whether you enter voltages or the actual winding turn counts.
The calculator finds the turns ratio and classifies the transformer as step‑up or step‑down.
Enter the actual number of winding turns to find the ratio and voltage relationship.
The turns ratio lets you find the full‑load current on either side once you know the transformer rating. Since apparent power is conserved, a lower‑voltage side carries proportionally more current.
The relationship between primary and secondary current follows directly from the turns ratio:
Since apparent power is conserved in an ideal transformer (Vp × Ip = Vs × Is), the side with the lower voltage carries the higher current. This is why secondary conductors are almost always larger than primary conductors in step‑down applications.
A 1 kVA transformer stepping 240 V down to 24 V has a turns ratio of 10:1. The primary carries about 4.2 amps and the secondary about 41.7 amps. The secondary voltage is one‑tenth of the primary, so the secondary current is ten times the primary current — exactly the 10:1 ratio at work.
This matters for conductor and protection sizing. The low‑voltage secondary almost always carries the larger current, which means thicker cable and a larger breaker. Getting the current ratio right is what keeps the whole installation safe.
The turns ratio is a starting point, not a finished design. Once you know the ratio and the currents, two further questions follow naturally.
Is the transformer correctly sized for the load it feeds? That means applying demand and diversity factors, converting kW to kVA, and selecting the next standard rating.
What fault current can the transformer deliver? The percent impedance, not the turns ratio, governs the available short‑circuit current — but the transformer's rating and voltage feed directly into that calculation.
Shandong Huali Electromechanical Co., Ltd. manufactures diesel generator sets, dust-suppression equipment, and industrial automation systems, with over 25 years of experience and exports to more than 20 countries. We publish free electrical calculators to help engineers and buyers specify power systems correctly.
A: Divide the primary turns by the secondary turns: Np ÷ Ns. Equivalently, divide primary voltage by secondary voltage, since Vp ÷ Vs equals the turns ratio in an ideal transformer. A 480 V to 120 V transformer is 4:1.
A: The turns ratio is Np ÷ Ns = Vp ÷ Vs = Is ÷ Ip. The voltage ratio equals the turns ratio, and the current ratio is its inverse: Ip ÷ Is = Ns ÷ Np.
A: A step-up transformer has more turns on the secondary, raising voltage and lowering current. A step-down transformer has more turns on the primary, lowering voltage and raising current. The transformer turns ratio calculator classifies each automatically.
A: Rearrange the ratio formula: Ns = Np × (Vs ÷ Vp), or Np = Ns × (Vp ÷ Vs). If you know one winding’s turns and the voltage ratio, you can solve for the other.
A: Current is inversely proportional to turns and voltage: Ip ÷ Is = Ns ÷ Np. A 10:1 step-down transformer carries ten times more current on the secondary than on the primary.
A: Find the full-load current from the rating and voltage on one side using I = kVA × 1000 ÷ V (three-phase divides by √3 × V), then apply the current ratio to find the other side. Power is conserved: Vp × Ip = Vs × Is.
