What are the transformer amps?
Enter the kVA rating and the primary and secondary voltages. The transformer calculator shows the turns ratio and the full-load current on each side; add the primary turns to get the secondary turns.
- Turns ratio (primary ÷ secondary)
- 2
The turns ratio is 2:1 (Step-down); at full load the primary carries 52.0833 A and the secondary 104.167 A.
- Primary full-load current (A)
- 52.0833
- Secondary full-load current (A)
- 104.167
- Impedance ratio
- 4
- Type
- Step-down
Turns ratio (primary ÷ secondary): 2. The turns ratio is 2:1 (Step-down); at full load the primary carries 52.0833 A and the secondary 104.167 A.
How to calculate
Works out a transformer’s turns ratio from its primary and secondary voltages, the full-load current on each side from its kVA rating (single or three-phase), and the secondary turns from the primary turns.
Example with the default inputs (Supply Single-phase, Rating (kVA) 25, Primary voltage 480 V, Secondary voltage 240 V): The turns ratio is 2:1 (Step-down); at full load the primary carries 52.0833 A and the secondary 104.167 A.
Method: a = Vp ÷ Vs = Np ÷ Ns. Full-load current: I = kVA × 1,000 ÷ V (single-phase) or ÷ (√3 × V) (three-phase) on each side. Ns = Np × Vs ÷ Vp. Impedance ratio = a².
- An ideal transformer: no losses, so the kVA in equals the kVA out, and the voltages are in the turns ratio.
- Three-phase voltages are line-to-line and the load is balanced.
- Full-load current is the current at the full kVA rating; real loads may draw less.
Worked examples
Each example is checked against the calculator on every build.
- Supply Single-phase, Rating (kVA) 25, Primary voltage 480 V, Secondary voltage 240 V, Primary turns 400 gives Turns ratio (primary ÷ secondary) 2, Primary full-load current (A) 52.083333, Secondary full-load current (A) 104.166667, Secondary turns 200, Impedance ratio 4, Type Step-down.Source: OpenStax, University Physics Volume 2, §15.6 Transformers (V_S ÷ V_P = N_S ÷ N_P; I_P V_P = I_S V_S for an ideal transformer), https://openstax.org/books/university-physics-volume-2/pages/15-6-transformers (retrieved 2026-10-05)
- Supply Three-phase, Rating (kVA) 75, Primary voltage 480 V, Secondary voltage 208 V gives Turns ratio (primary ÷ secondary) 2.307692, Primary full-load current (A) 90.21098, Secondary full-load current (A) 208.179184.Source: OpenStax, University Physics Volume 2, §15.6 Transformers (V_S ÷ V_P = N_S ÷ N_P; I_P V_P = I_S V_S for an ideal transformer), https://openstax.org/books/university-physics-volume-2/pages/15-6-transformers (retrieved 2026-10-05); Wikipedia, "Three-phase electric power" (balanced load: S = √3 × line voltage × line current), https://en.wikipedia.org/wiki/Three-phase_electric_power (retrieved 2026-10-05)
- Supply Single-phase, Rating (kVA) 1, Primary voltage 120 V, Secondary voltage 12,000 V gives Turns ratio (primary ÷ secondary) 0.01, Primary full-load current (A) 8.333333, Secondary full-load current (A) 0.083333, Type Step-up.Source: OpenStax, University Physics Volume 2, §15.6 Transformers (V_S ÷ V_P = N_S ÷ N_P; I_P V_P = I_S V_S for an ideal transformer), https://openstax.org/books/university-physics-volume-2/pages/15-6-transformers (retrieved 2026-10-05)
How it works
For an ideal transformer (no losses):
- turns ratio a = Vp ÷ Vs = Np ÷ Ns
- secondary turns Ns = Np × Vs ÷ Vp (shown when you give Np)
- impedance ratio = a²
The full-load current on each side comes from the kVA rating S:
- single-phase: I = S × 1,000 ÷ V
- balanced three-phase: I = S × 1,000 ÷ (√3 × V), with V the line-to-line voltage
Because the kVA is the same on both sides, Is ÷ Ip = a.
Rules
- Voltages from 1 mV to 10,000 kV; rating from 0.000001 to 10,000,000 kVA; primary turns a whole number from 1 to 10,000,000.
- Type is step-down when a > 1, step-up when a < 1, and 1:1 when a = 1.
- Single-phase steps are exact on the typed values; three-phase uses √3 as a float. Values are shown to 6 significant figures; the secondary turns are not rounded to a whole number.
Worked examples by hand
25 kVA, single-phase, 480 V to 240 V, 400 primary turns. a = 480 ÷ 240 = 2 (step-down). Ip = 25,000 ÷ 480 = 52.0833 A. Is = 25,000 ÷ 240 = 104.167 A. Ns = 400 × 240 ÷ 480 = 200. Impedance ratio = 2² = 4.
75 kVA, three-phase, 480 V to 208 V. a = 480 ÷ 208 = 2.30769. Ip = 75,000 ÷ (1.7320508 × 480) = 75,000 ÷ 831.384 = 90.2110 A. Is = 75,000 ÷ (1.7320508 × 208) = 75,000 ÷ 360.267 = 208.179 A.
1 kVA, single-phase, 120 V to 12 kV. a = 120 ÷ 12,000 = 0.01 (step-up). Ip = 1,000 ÷ 120 = 8.33333 A. Is = 1,000 ÷ 12,000 = 0.0833333 A.
Other questions people ask
How do I calculate a transformer’s turns ratio?
Divide the primary voltage by the secondary voltage. In an ideal transformer this equals the primary turns divided by the secondary turns. A 480 V to 240 V transformer has a ratio of 2, so it has twice as many turns on the primary as on the secondary.
How do I find a transformer’s full-load current?
Single-phase: amps = kVA × 1,000 ÷ volts. Three-phase: amps = kVA × 1,000 ÷ (√3 × line volts). A 25 kVA single-phase transformer carries 52.1 A on its 480 V side and 104.2 A on its 240 V side.
Why is the current higher on the low-voltage side?
An ideal transformer passes the same power through: volts times amps on one side equals volts times amps on the other. When the voltage halves, the current doubles. This is why the low-voltage winding uses thicker wire.
What is a step-down and a step-up transformer?
A step-down transformer has a lower secondary voltage than primary voltage (a ratio above 1), like the ones that feed homes. A step-up transformer raises the voltage (a ratio below 1), like those at power stations. A 1:1 transformer isolates a circuit without changing the voltage.
What is the impedance ratio?
A load on the secondary looks larger from the primary by the square of the turns ratio. With a ratio of 2, an 8 Ω load looks like 32 Ω. This is how audio and radio transformers match a load to a source.
Does the calculator size a transformer for a load?
Indirectly. Enter a kVA rating and check that the full-load currents are above what your load draws. Use the kVA calculator to turn a load’s volts and amps into kVA, and choose a standard rating above it with some margin.