Transformer Current Calculator

A transformer current calculator converts a transformer’s apparent power rating in kVA into estimated primary and secondary line current. The result establishes the electrical loading basis for the transformer supply and output conductors, feeder review, disconnect and overcurrent-protection evaluation, voltage-drop calculations, raceway-fill planning, and equipment load coordination.

The calculator reports the current associated with the entered Transformer size, Primary voltage, Secondary voltage, and Phase. It calculates electrical current only. It does not select conductors, AWG or kcmil sizes, breakers, fuses, disconnects, bus ratings, or other equipment.

Primary and Secondary Current

A transformer transfers apparent power from one voltage level to another. When voltage decreases, current increases proportionally for the same kVA; when voltage increases, current decreases.

For a step-down transformer, the secondary current is normally higher than the primary current. That relationship is visible immediately in the calculated values and helps establish the expected feeder and secondary-conductor loading.

The calculator provides:

Result	Electrical use
Primary current	Estimated line current on the transformer supply side
Secondary current	Estimated line current available on the transformer output side
Apparent power	Entered transformer kVA expressed in VA
Phase multiplier	The multiplier used for the selected phase calculation

The current results are based on apparent power, not measured operating demand. Actual current can be lower when the connected load is below the transformer rating, or it can vary with load type and operating conditions.

Inputs and Calculation Method

Transformer size

Transformer size is the transformer apparent power rating or load basis, entered in kVA.

The calculator converts kVA to volt-amperes:

\(\text{Apparent power (VA)} = \text{Transformer size (kVA)} \times 1{,}000\)

A 75 kVA transformer therefore has an apparent power value of:

\(75 \times 1{,}000 = 75{,}000\ \text{VA}\)

Primary voltage and Secondary voltage

Primary voltage is the transformer input line voltage. Secondary voltage is the transformer output line voltage.

For balanced three-phase calculations, each voltage should represent the applicable line-to-line system voltage. The calculator uses the primary voltage to determine Primary current and the secondary voltage to determine Secondary current.

Phase

The Phase field chooses between single-phase and balanced three-phase current math.

Phase selection	Current formula	Phase multiplier
Single-phase	\(I = \frac{VA}{V}\)	1
Balanced three-phase	\(I = \frac{VA}{\sqrt{3} \times V}\)	1.7321 x

For balanced three-phase systems, the calculator uses the line-current relationship:

\(\text{Current (A)} = \frac{\text{Apparent power (VA)}}{\sqrt{3} \times \text{Line voltage (V)}}\)

The displayed Phase multiplier of 1.7321 x is the rounded value of \(\sqrt{3}\).

Calculation Example

Use the following entered values:

Input	Value
Transformer size	75 kVA
Primary voltage	480 V
Secondary voltage	208 V
Phase	Balanced three-phase

First, convert the transformer size to apparent power:

\(75\ \text{kVA} \times 1{,}000 = 75{,}000\ \text{VA}\)

Calculate primary current at 480 V:

\(I_{primary} = \frac{75{,}000}{1.7321 \times 480} = 90.211\ \text{A}\)

Calculate secondary current at 208 V:

\(I_{secondary} = \frac{75{,}000}{1.7321 \times 208} = 208.1792\ \text{A}\)

Result

Result	Calculated value
Primary current	90.211 A
Secondary current	208.1792 A
Apparent power	75000 VA
Phase multiplier	1.7321 x

The 75 kVA transformer has a lower calculated line current at its 480 V primary than at its 208 V secondary because the same apparent power is delivered at a lower voltage on the secondary side.

Using Transformer Current in Design

Transformer current is commonly carried into several downstream electrical calculations:

• Establishing the ampere basis for primary and secondary feeder conductors.
• Reviewing conductor ampacity after applying applicable adjustment factors and correction factors.
• Checking whether conductor terminal ratings and insulation temperature ratings support the final conductor selection.
• Estimating voltage drop on the transformer primary feeder or secondary conductors.
• Evaluating raceway fill and conduit layout after the conductor count, insulation type, and AWG or kcmil size are known.
• Reviewing downstream panelboard, switchboard, disconnect, and bus current ratings.
• Comparing transformer secondary capacity with calculated branch-circuit and feeder loads.
• Coordinating motor loads, continuous loads, and other connected load categories in the broader distribution design.

A calculated secondary current of 208.1792 A does not automatically mean that a 208 A conductor, breaker, or fuse is selected. The final design must account for the installation and equipment requirements that govern the circuit.

Field Verification

Use the transformer nameplate and installation documents to verify the electrical basis before applying the calculated current to a design. Confirm the actual transformer kVA rating, primary and secondary voltage configuration, phase arrangement, tap position where applicable, and whether the load is balanced three-phase or single-phase.

Conductor and overcurrent-protection decisions require separate review of applicable code requirements, conductor ampacity, terminal limitations, current-carrying conductors, ambient temperature, raceway conditions, voltage drop, available fault current, transformer impedance, equipment listings, and AHJ requirements. The calculator’s Primary current and Secondary current results are the kVA-to-ampere starting values for that work.