What is the formula for single-phase current?

I = P ÷ (V × PF). A 5,000W load at 230V, PF 0.9 draws I = 5,000 ÷ (230 × 0.9) ≈ 24.15A.

What is the formula for three-phase current?

I = P ÷ (√3 × V × PF). A 10,000W load at 400V, PF 0.85 draws I = 10,000 ÷ (1.732 × 400 × 0.85) ≈ 16.98A.

Why is three-phase current lower than single-phase for the same power?

Three-phase power is delivered across three conductors; the √3 factor reflects that shared delivery, giving lower per-line current for the same total power.

What is the difference between primary and secondary current?

Primary current is at the source-connected winding; secondary is at the load-connected winding. Their ratio is the inverse of the voltage/turns ratio.

How do I calculate motor current from kW rating?

Convert kW to W, then I = P ÷ (√3 × V × PF) for three-phase. A 15kW motor at 415V, PF 0.85 draws ≈24.55A.

Why does power factor affect calculated current?

Lower power factor means more current is needed for the same real power, since PF appears in the denominator of the current formula.

How do I choose single-phase or three-phase mode?

Single-phase for residential/light commercial loads (110-240V). Three-phase for industrial motors and commercial buildings (380-480V).

How does this relate to cable and breaker sizing?

This gives base continuous current. Real sizing applies a 1.25× continuous-load margin and ambient/derating factors on top.

Transformer Current Calculator

Calculate transformer primary and secondary current for single-phase and three-phase systems. Free online calculator with instant results and power factor support.

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Transformer Current Calculator

Calculate transformer primary and secondary current for single-phase and three-phase systems with instant results and power factor support.

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System Parameters

Power

Voltage

Phase Configuration

Power Factor: 0.9

0.50.751.0

Typical range: 0.8-0.95 for most loads

Common System Examples

What Is a Transformer Current Calculator?

A transformer current calculator computes the current a transformer's winding or circuit carries from its rated power, voltage, and power factor. It supports both single-phase (I = P ÷ (V × PF)) and three-phase (I = P ÷ (√3 × V × PF)) systems, returning current alongside apparent power for cable, breaker, and protection device sizing.

The formula differs meaningfully between single-phase and three-phase systems — three-phase introduces the √3 factor accounting for the 120-degree phase relationship between line conductors, and using the single-phase formula on a three-phase system produces a current figure that's off by that same factor. This tool applies the correct formula for whichever configuration you select, and shows both primary and secondary current derived from the same power and voltage inputs.

Built for electricians sizing cables and overload protection, electrical engineers verifying transformer and motor current ratings, and facility managers assessing circuit loading. Includes six built-in presets spanning residential to heavy industrial systems, full step-by-step derivation, and text and JSON export — free and entirely browser-based.

Transformer Current Formulas

Core Formulas

Single-phase: I = P ÷ (V × PF)

Three-phase: I = P ÷ (√3 × V × PF)

Apparent Power: S = P ÷ PF

Example (3-phase): P = 10,000W, V = 400V, PF = 0.85

I = 10,000 ÷ (1.732 × 400 × 0.85) = ≈16.98A

√3 ≈ 1.7321 — used only for three-phase calculations, not single-phase
Lower PF → higher current — power factor sits in the denominator, so lower PF means more current for the same power
Primary vs. secondary current — related by the inverse of the transformer's voltage ratio
Use single-phase mode for most residential circuits; three-phase for industrial and commercial systems

How to Use the Transformer Current Calculator

Step-by-Step Guide

1Select single-phase or three-phase: Choose the system configuration matching your transformer or circuit — single-phase for most residential loads, three-phase for industrial and commercial systems.
2Enter power: Input the real power in watts (W) that the transformer or load draws.
3Enter voltage: Input the line voltage in volts (V) — line-to-neutral for single-phase, line-to-line for three-phase.
4Enter power factor: Input the load's power factor as a decimal between 0 and 1.
5Read the calculated current: View primary current, secondary current, and (for three-phase) line current, all derived from the same inputs.
6Apply a preset or export results: Use a built-in preset for common residential and industrial systems, or export the full calculation as text or JSON.

What This Calculator Provides

✓Real-time calculation as you type
✓Single-phase and three-phase mode support
✓Primary, secondary, and line current results
✓Apparent power (VA) calculation
✓Full step-by-step derivation
✓Six built-in presets (residential, industrial, motor)
✓Calculation history (saved locally)
✓Export as text or JSON file
✓No signup required

Real-World Use Cases

Residential Single-Phase Circuit Sizing

An electrician calculating current for a 5,000W single-phase load at 230V with power factor 0.9 uses the calculator to find I = 5,000 ÷ (230 × 0.9) ≈ 24.15A — used to confirm the circuit needs at least a 25A or 32A breaker with appropriately rated cable.

US Residential Panel Load Check

An electrician verifying a 2,000W load at 120V with power factor 0.9 calculates I = 2,000 ÷ (120 × 0.9) ≈ 18.5A — confirming the circuit fits within a standard 20A residential branch circuit with margin.

Industrial Three-Phase Transformer Current

An engineer sizing cables for a 10,000W three-phase load at 400V with power factor 0.85 calculates I = 10,000 ÷ (1.732 × 400 × 0.85) ≈ 16.98A per line, used to select appropriately rated three-phase cable and protection.

Commercial Building Supply Current

A facility manager verifying a 15,000W three-phase commercial supply at 415V, power factor 0.9, calculates I ≈ 23.4A per line — cross-checking this against the building's installed switchgear rating to confirm adequate capacity.

Three-Phase Motor Full-Load Current

An electrician sizing an overload relay for a 7,500W three-phase motor at 400V with power factor 0.8 calculates I ≈ 13.53A full-load current, setting the overload relay to approximately 104% of this figure per standard practice.

Large Industrial Load Cable Selection

An engineer sizing cable for a 50,000W heavy industrial load at 415V, power factor 0.85, calculates I ≈ 81.85A per line — used to select cable with sufficient ampacity plus the standard continuous-load safety margin.

Tips & Common Mistakes

Pro Tips

💡Apply the standard 1.25× continuous-load safety factor on top of this calculator's result when sizing cables and breakers for loads that run continuously for three hours or more.
💡Use the motor's rated power factor from its nameplate, not an assumed value, for full-load current calculations used in overload relay and cable sizing — power factor varies meaningfully between motor types and sizes.
💡For three-phase systems, always confirm whether your voltage figure is line-to-line (which this calculator expects) or line-to-neutral — using the wrong one skews the result by a factor of √3.
💡Cross-check calculated current against a clamp meter reading when commissioning new equipment — a significant mismatch usually points to an incorrect power factor assumption or measurement error.
💡When comparing single-phase and three-phase options for the same total power, remember three-phase draws less current per line — useful context when deciding whether to specify a three-phase supply for a large load.

Common Mistakes to Avoid

✕Using the single-phase formula for a three-phase system, or vice versa. This produces a current figure off by a factor of √3 (about 1.732) — always match the formula to the actual system configuration.
✕Forgetting power factor entirely and using I = P ÷ V. This calculates the current as if power factor were 1.0, understating actual current draw for any load with a power factor below unity.
✕Assuming this calculator's result already includes cable derating and safety margins. It computes the base continuous current from power, voltage, and power factor — additional factors for ambient temperature, grouping, and continuous-load margin need to be applied separately per applicable electrical codes.
✕Confusing primary and secondary current when working with a transformer rather than a simple load. Primary current relates to secondary current by the inverse of the turns/voltage ratio, not by the same formula applied twice with the same voltage.
✕Using nominal voltage when actual supply voltage differs meaningfully. Since current is inversely proportional to voltage, a 5% low supply voltage increases actual current draw by roughly 5% for the same power.

Common System Current Reference

System	Power / Voltage	Phase / PF	Current
Residential (EU)	5,000W / 230V	1-phase / 0.9	24.15 A
Residential (US)	2,000W / 120V	1-phase / 0.9	18.52 A
Industrial 3-phase	10,000W / 400V	3-phase / 0.85	16.98 A
Commercial 3-phase	15,000W / 415V	3-phase / 0.9	23.20 A
3-phase motor	7,500W / 400V	3-phase / 0.8	13.53 A
Large industrial load	50,000W / 415V	3-phase / 0.85	81.85 A

* Single-phase: I = P ÷ (V × PF). Three-phase: I = P ÷ (√3 × V × PF). Add safety margins for cable/breaker sizing per applicable codes.

Frequently Asked Questions

What is a transformer current calculator?

A transformer current calculator is a free online tool that computes the current a transformer's winding carries from its rated power, voltage, and power factor. It supports both single-phase (I = P ÷ (V × PF)) and three-phase (I = P ÷ (√3 × V × PF)) systems, and returns apparent power alongside the calculated current.

What is the formula for single-phase transformer current?

I = P ÷ (V × PF), where I is current in amps, P is real power in watts, V is voltage in volts, and PF is power factor. For example, a 5,000W load at 230V with power factor 0.9 draws I = 5,000 ÷ (230 × 0.9) = 5,000 ÷ 207 ≈ 24.15A.

What is the formula for three-phase transformer current?

I = P ÷ (√3 × V × PF), where √3 ≈ 1.7321 accounts for the three-phase relationship between line conductors. For example, a 10,000W load at 400V with power factor 0.85 draws I = 10,000 ÷ (1.732 × 400 × 0.85) = 10,000 ÷ 588.9 ≈ 16.98A.

Why is three-phase current lower than single-phase current for the same power?

Three-phase power is delivered across three conductors simultaneously rather than one, and the √3 factor in the denominator reflects that shared delivery — for the same total power and voltage, three-phase current per line is lower than the equivalent single-phase current would be, which is one reason three-phase distribution is more efficient for higher power loads.

What is the difference between primary current and secondary current?

Primary current flows in the winding connected to the power source; secondary current flows in the winding delivering output to the load. Their ratio is the inverse of the voltage ratio (Ip/Is = Ns/Np) — a step-down transformer has lower primary current and higher secondary current for the same power, since power is conserved across the turns ratio.

How do I calculate current for a three-phase motor from its kW rating?

Convert kW to W (multiply by 1,000), then apply I = P ÷ (√3 × V × PF). A 15kW motor at 415V with power factor 0.85 draws I = 15,000 ÷ (1.732 × 415 × 0.85) ≈ 24.55A — this full-load current figure is what's used for cable and overload relay sizing.

Why does power factor affect the calculated current?

Power factor separates real power (useful work) from apparent power (total current-carrying demand). A lower power factor means more current is needed to deliver the same real power, since PF appears in the denominator of the current formula — a load with PF 0.7 draws proportionally more current than the same wattage load at PF 0.95.

How do I choose between single-phase and three-phase mode?

Use single-phase mode for residential circuits and most household and light commercial equipment (typically 110-240V). Use three-phase mode for industrial motors, commercial buildings, and heavier equipment typically running at 380-480V line voltage across three conductors.

How does this calculator's current relate to cable and breaker sizing?

The calculated current is the continuous full-load current the winding or circuit must carry, but real-world cable and breaker sizing typically applies a safety margin (commonly 1.25× for continuous loads) on top of this base figure, and accounts for ambient temperature and installation method derating factors that this calculator doesn't include.

Is my data private when using this calculator?

Yes. All calculations run entirely in your browser using JavaScript. Your power, voltage, and power factor values are never transmitted to any server, stored in any database, or accessible to anyone other than you.