Transformer Calculator
Calculate primary and secondary full-load currents, turns ratio, and determine whether a transformer is step-up, step-down, or isolation for single-phase and three-phase systems.
Results
Single Phase Step Down Transformer
SINGLE PHASE
Primary Full-Load Current (I₁)
31.25 A
Secondary Full-Load Current (I₂)
125.00 A
Turns Ratio (N₁ / N₂)
4.00 : 1 (4.00)
Transformer Type
Step Down Transformer
For a 15 kVA Single Phase transformer operating at 480 V primary and 120 V secondary, primary full-load current is 31.25 A and secondary full-load current is 125.00 A with a turns ratio of 4:1.
Transformer Formulas & Calculations
A transformer operates on electromagnetic induction. The full-load primary and secondary currents depend on the transformer's total power rating and rated voltages. The turns ratio indicates the relationship between windings and determines whether the unit steps voltage up or down.
Formula:
1Ø I = VA / V | 3Ø I = VA / (1.732 × V) | Turns Ratio = N₁ / N₂ = V₁ / V₂ = I₂ / I₁ Variables & Parameters
Rating (VA / kVA / MVA) Apparent Power capacity of the transformer
V₁ (Primary Voltage) Input voltage supplied to the primary winding in Volts (V), kV, or MV
V₂ (Secondary Voltage) Output voltage delivered by the secondary winding in Volts (V), kV, or MV
I₁ (Primary Current) Full-load current drawn by the primary winding in Amperes (A)
I₂ (Secondary Current) Full-load current supplied to the connected load in Amperes (A)
Turns Ratio (N₁ / N₂) Ratio of primary to secondary turns (equivalent to V₁ / V₂)
Frequently Asked Questions
How to size a transformer?
To size a transformer: (1) Determine the total load in Amperes and system voltage. (2) For a single-phase system, calculate kVA = (Volts × Amps) / 1,000. For a three-phase system, calculate kVA = (Volts × Amps × 1.732) / 1,000. (3) Per NEC 450.3 and NEC 215.2, multiply by 1.25 (125%) for continuous loads running 3 hours or longer. (4) Select the next standard commercial kVA rating (e.g., 15, 30, 45, 75, 112.5, 150 kVA).
How do you calculate primary and secondary full-load currents of a transformer?
For a 1 phase transformer sizing calculator, the formula is: Primary Current I₁ = (Rating in VA) / V₁ and Secondary Current I₂ = (Rating in VA) / V₂. For a 3 phase transformer sizing calculator, the formula is: Primary Current I₁ = (Rating in VA) / (1.732 × V₁) and Secondary Current I₂ = (Rating in VA) / (1.732 × V₂).
How do you size step down, step up, buck boost, control, and auto transformers?
When using this tool as a step down transformer sizing calculator (e.g., 480V to 120V) or step up transformer sizing calculator (e.g., 120V to 480V), enter your primary and secondary voltages to instantly determine full-load currents and turns ratio. For a buck boost transformer sizing calculator or control transformer sizing calculator, calculate the required boost percentage and VA rating needed to handle relay and contactor inrush currents. For an auto transformer sizing calculator, calculate currents based on shared winding voltage ratios.
How is the transformer turns ratio calculated?
The turns ratio is the ratio of primary turns to secondary turns: Turns Ratio = N₁ / N₂ = V₁ / V₂ = I₂ / I₁. When using a transformer sizing calculator kva, if V₁ > V₂ the unit functions as a step down transformer; if V₁ < V₂ it functions as a step up transformer.
How do you convert between VA, kVA, and MVA in transformer calculations?
1 kVA = 1,000 VA, and 1 MVA = 1,000,000 VA. For voltage: 1 kV = 1,000 V and 1 MV = 1,000,000 V. This transformer calculator automatically handles all unit conversions across single-phase and three-phase systems.