1500 kVA Transformer Amps

A 1500 kVA transformer has a fixed apparent-power rating of 1,500,000 VA, but its ampere value changes with the voltage and phase arrangement. A high-voltage primary may carry only tens of amps, while a low-voltage secondary can carry more than 1,800 amps.

That current value is used during feeder and secondary-conductor planning, bus and switchgear review, raceway layout, voltage-drop calculations, overcurrent-protection coordination, and available-fault-current evaluation. At this transformer size, the secondary-current result can drive parallel conductor quantities, large kcmil conductor selections, busway decisions, and equipment space requirements.

Transformer Current Formula

The calculation uses the transformer rating in VA, the line voltage for the side being calculated, and the selected phase multiplier.

For a three-phase transformer:

\(I = \frac{VA}{\sqrt{3} \times V_{LL}}\)

For a single-phase transformer:

\(I = \frac{VA}{V}\)

For a 1500 kVA transformer:

\(\displaystyle 1500\text{ kVA} = 1{,}500{,}000\text{ VA}\)

The calculator divides 1,500,000 VA by the entered primary or secondary voltage and the phase multiplier:

Three-phase multiplier: \(\sqrt{3}\), approximately 1.732
Single-phase multiplier: 1

Use the line-to-line voltage for a three-phase calculation. Do not substitute a line-to-neutral voltage into a three-phase transformer-amp formula unless the transformer configuration and calculation method specifically call for it.

1500 kVA Three-Phase Example

A common distribution arrangement is a 1500 kVA, three-phase transformer with a 13.8 kV primary and a 480 V secondary.

Transformer side	Line voltage	Calculation	Full-load current
Primary	13,800 V	(1{,}500{,}000 \div \(1.732 \times 13{,}800\))	62.8 A
Secondary	480 V	(1{,}500{,}000 \div \(1.732 \times 480\))	1,804.2 A

The primary-to-secondary voltage ratio changes current in the opposite direction. The transformer delivers essentially the same kVA rating on each side, subject to normal transformer losses, but the 480 V secondary must deliver that power at a much higher current.

A 62.8 A primary current may appear modest beside a 1,804 A secondary current, but both values represent the same 1500 kVA transformer rating at different voltages.

Primary and Secondary Current

Calculate each transformer side independently using that side’s rated line voltage.

A 1500 kVA three-phase transformer has the following approximate full-load currents at several common voltages:

Line voltage	1500 kVA three-phase current
34.5 kV	25.1 A
13.8 kV	62.8 A
12.47 kV	69.5 A
4.16 kV	208.2 A
600 V	1,443.4 A
480 V	1,804.2 A
240 V	3,608.4 A
208 V	4,164.1 A

The calculated ampere value is transformer full-load current, not an automatic conductor ampacity or overcurrent-device selection. Conductor sizing can require additional evaluation of terminal ratings, insulation temperature rating, ambient-temperature correction factors, adjustment factors for current-carrying conductors, installation method, raceway fill, and the equipment’s listed limitations.

Secondary Feeder Considerations

A 1500 kVA transformer at 480 V produces approximately 1,804 A at full load. That magnitude commonly requires a secondary design using multiple parallel conductor sets, bus duct, busway, or other listed distribution equipment appropriate for the installation.

The ampere result is a starting point for reviewing:

Secondary feeder ampacity and parallel conductor configuration
kcmil conductor size, insulation type, and terminal temperature limitations
Number of current-carrying conductors in each raceway
Raceway fill, pulling tension, bending space, and termination space
Secondary-main equipment rating and bus rating
Voltage drop between the transformer secondary and downstream distribution equipment
Motor loads, transformer loading characteristics, and calculated demand
Grounding and bonding configuration
Available fault current and interrupting-rating requirements

For example, a 480 V secondary current of 1,804 A does not mean that an 1,800 A switchboard, feeder, or overcurrent protective device is automatically suitable. The actual equipment rating, calculated load, transformer impedance, conductor arrangement, and applicable design requirements must be evaluated as a complete system.

Field Verification

Use the transformer nameplate as the controlling source for rated kVA, primary voltage, secondary voltage, phase, winding configuration, taps, impedance, and temperature-rise data. A nominal utility voltage or a building distribution voltage may not match the actual nameplate rating used for equipment selection.

Large-transformer work also requires review of the primary protection, secondary protection, utility requirements, conductor terminations, bus ratings, grounding, fault current, and utility/AHJ requirements. A qualified electrical engineer should review the final design where the transformer supplies service equipment, critical loads, high fault duty, or large parallel secondary conductors.

Transformer Current Calculators

Use the applicable Elecatrix calculator based on the current value needed: