Transformer Current Chart by kVA and Voltage

A transformer current chart converts transformer capacity in kVA into amperes at a stated voltage and phase. The calculated current is used as an input for feeder and branch-circuit conductor sizing, overcurrent-protection review, disconnect and switchgear selection, raceway fill planning, voltage-drop calculations, and transformer secondary layout.

The current value is not determined by kVA alone. A 15 kVA transformer can produce 18 A, 36 A, 62.5 A, or another value depending on whether the listed voltage is primary or secondary, single-phase or three-phase, and line-to-line or line-to-neutral where applicable.

Every chart row should therefore identify:

Transformer size in kVA
Phase assumption
Primary or secondary side
Voltage at that side
Calculated line current in amperes

Transformer Current Formula

Calculate transformer full-load current with:

\(\displaystyle I = \frac{\text{kVA} \times 1000}{m \times V}\)

Where:

(I) = current in amperes
kVA = transformer apparent-power rating
(V) = voltage for the specific winding being calculated
(m) = phase multiplier

For a single-phase transformer:

\(\displaystyle I = \frac{\text{kVA} \times 1000}{V}\)

For a three-phase transformer:

\(\displaystyle I = \frac{\text{kVA} \times 1000}{\sqrt{3} \times V}\)

For three-phase calculations, use line-to-line voltage. The result is the line current on the winding being calculated.

15 kVA Transformer Current Chart

The following examples keep each voltage and phase assumption visible. Primary and secondary values remain in separate rows so the current is not mistaken for the opposite side of the transformer.

Transformer rating	Winding side	Phase	Voltage	Calculated current
15 kVA	Primary	Three-phase	480 V	18.0 A
15 kVA	Secondary	Three-phase	208 V	41.6 A
15 kVA	Primary	Single-phase	480 V	31.3 A
15 kVA	Secondary	Single-phase	240 V	62.5 A
15 kVA	Secondary	Single-phase	120 V	125 A

15 kVA Three-Phase Example

For a 15 kVA, three-phase transformer with a 480 V primary:

\(\displaystyle I_{primary} = \frac{15 \times 1000}{\sqrt{3} \times 480} = 18.0\text{ A}\)

For the same 15 kVA transformer with a 208 V three-phase secondary:

\(\displaystyle I_{secondary} = \frac{15 \times 1000}{\sqrt{3} \times 208} = 41.6\text{ A}\)

The secondary current is higher because the same transformer capacity is delivered at a lower voltage. Use the winding-side current associated with the conductors, disconnect, overcurrent protective device, bus, or raceway under review.

1500 kVA Transformer Current Chart

Large transformer current values can quickly move a design from standard AWG conductors into parallel kcmil conductors, bus duct, or purpose-designed switchgear connections. The arithmetic still follows the same kVA-to-amps formula.

Transformer rating	Winding side	Phase	Voltage	Calculated current
1500 kVA	Primary	Three-phase	13,800 V	62.8 A
1500 kVA	Primary	Three-phase	4,160 V	208.2 A
1500 kVA	Secondary	Three-phase	480 V	1,804.2 A
1500 kVA	Secondary	Three-phase	600 V	1,443.4 A
1500 kVA	Secondary	Three-phase	208 V	4,163.8 A

1500 kVA Three-Phase Example

For a 1500 kVA transformer supplied from a 13.8 kV, three-phase primary:

\(\displaystyle I_{primary} = \frac{1500 \times 1000}{\sqrt{3} \times 13{,}800} = 62.8\text{ A}\)

For a 480 V, three-phase secondary:

\(\displaystyle I_{secondary} = \frac{1500 \times 1000}{\sqrt{3} \times 480} = 1{,}804.2\text{ A}\)

A 1,804 A secondary result affects much more than conductor ampacity. It also affects parallel-conductor layout, raceway count and fill, gutter and enclosure space, termination provisions, equipment bus ratings, pull-box dimensions, conductor bending space, and available installation paths.

Primary and Secondary Current

A transformer has the same rated kVA on both windings, but it does not have the same current on both sides. Current changes inversely with voltage.

For an ideal transformer:

\(\displaystyle \frac{I_{primary}}{I_{secondary}} \approx \frac{V_{secondary}}{V_{primary}}\)

A chart column labeled only “amps” is incomplete. It can lead to selecting a primary feeder from a secondary-current value, or vice versa.

Use a clear labeling format such as:

kVA	Side	Voltage	Phase	Amps
15	Primary	480 V	3Ø	18.0 A
15	Secondary	208 V	3Ø	41.6 A

Do not place 18.0 A and 41.6 A into one unlabeled 15 kVA row. Both values are correct, but each applies to different conductors and equipment.

Calculator Inputs and Results

Use the Transformer Current Calculator when the transformer kVA, voltage, and phase are known and the required result is line current. Enter the kVA rating and the voltage for the winding side being reviewed, then select the applicable phase arrangement.

Use the Transformer Primary Current Calculator when reviewing the supply-side current. The voltage input must be the primary winding voltage.

Use the Transformer Secondary Current Calculator when reviewing output-side current. The voltage input must be the secondary winding voltage.

Conductor and Equipment Review

Transformer current is the starting value for electrical design, not the finished conductor or equipment selection.

After calculating primary or secondary current, review the actual installation conditions that affect the final design:

Conductor ampacity at the applicable terminal rating and insulation temperature rating
Adjustment factor for the number of current-carrying conductors
Ambient-temperature correction factor
Copper or aluminum conductor material
AWG or kcmil conductor size and any parallel-conductor arrangement
Raceway fill, pull length, and conductor bending space
Feeder or branch-circuit voltage drop
Transformer inrush, motor loads, continuous loads, and downstream load characteristics
Transformer nameplate data, manufacturer instructions, protection study results, adopted-code requirements, and AHJ requirements

The chart calculation provides rated winding current. It does not establish final overcurrent-protective-device sizing, conductor size, available fault current, interrupting rating, or transformer protection settings.