# What are the transformer amps?

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.

- Page: https://www.acalculator.org/physics/transformer-calculator
- JSON spec: https://www.acalculator.org/physics/transformer-calculator.json
- Version: 5da527535838

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| ph | Supply | Single-phase, or three-phase with line-to-line voltages. |
| kva | Rating (kVA) | The transformer’s rated apparent power in kilovolt-amperes, from its nameplate. |
| vp | Primary voltage | The voltage on the supply side (line-to-line for three-phase). |
| vs | Secondary voltage | The voltage on the load side (line-to-line for three-phase). |
| np | Primary turns | The number of turns on the primary winding, if you know it. Leave empty to skip. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| ratio | Turns ratio (primary ÷ secondary) | Vp ÷ Vs, which equals Np ÷ Ns for an ideal transformer. |
| ip | Primary full-load current (A) | The current on the primary side at the full kVA rating. |
| is | Secondary full-load current (A) | The current on the secondary side at the full kVA rating. |
| ns | Secondary turns | Np × Vs ÷ Vp. |
| zRatio | Impedance ratio | a²: a load impedance on the secondary looks a² times as large from the primary. |
| kind | Type | Step-down, step-up, or 1:1 (isolation). |

## 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.

## Assumptions

- 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

1. ph = single, kva = 25, vp = 480, vs = 240, np = 400 gives ratio = 2, ip = 52.083333, is = 104.166667, ns = 200, zRatio = 4, kind = 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).
2. ph = three, kva = 75, vp = 480, vs = 208 gives ratio = 2.307692, ip = 90.21098, is = 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).
3. ph = single, kva = 1, vp = 120, vs = 12,000 gives ratio = 0.01, ip = 8.333333, is = 0.083333, kind = 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).

## FAQ

### 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.

## Sources

- OpenStax, University Physics Volume 2, §15.6 Transformers: V_S ÷ V_P = N_S ÷ N_P, and 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": for a balanced load, S = √3 × line voltage × line current. https://en.wikipedia.org/wiki/Three-phase_electric_power (retrieved 2026-10-05)