Transformer Secondary Current Calculator

A transformer secondary current calculation converts apparent power on the secondary side into amperes. The resulting current is commonly used as a starting value for feeder and secondary-conductor evaluation, raceway planning, transformer load review, voltage-drop review, and coordination with downstream distribution equipment.

The calculator produces Estimated secondary current from the entered Transformer rating, Secondary line voltage, and Phase model. It uses apparent-power arithmetic only. A 75 kVA transformer with a 208 V balanced three-phase secondary produces an estimated secondary current of 208.1792 A.

This current is not a conductor ampacity, an overcurrent-protection setting, or a transformer equipment selection. Those decisions require the actual installation details.

Secondary kVA-to-Amps Formula

The calculation begins by converting the entered Transformer rating from kVA to VA:

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

The current formula then depends on the selected Phase model.

Phase model	Secondary-current formula
Single phase	\(\displaystyle I = \frac{VA}{V}\)
Balanced three phase	\(\displaystyle I = \frac{VA}{\sqrt{3} \times V}\)

For Balanced three phase, the calculator uses a Phase multiplier of:

\(\displaystyle \sqrt{3} = 1.7321\)

The Secondary line voltage is the voltage basis used in the selected phase model. For balanced three-phase arithmetic, it is the line-to-line voltage.

Calculation Example

Enter the following values:

Field	Entered value
Transformer rating	75 kVA
Secondary line voltage	208 V
Phase model	Balanced three phase

First, convert the transformer rating:

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

Then calculate secondary line current:

\(\displaystyle I = \frac{75{,}000}{208 \times 1.7321}\)

\(\displaystyle I = 208.1792\text{ A}\)

The calculator result is therefore:

Result field	Value
Estimated secondary current	208.1792 A
Apparent power used	75,000 VA
Transformer rating used	75 kVA
Secondary voltage used	208 V
Phase multiplier	1.7321 x

For preliminary layout, 208.1792 A identifies the approximate secondary current level that must be carried from the transformer to the first downstream disconnect, panelboard, switchboard, or other distribution equipment.

Electrical Design Use

The calculated secondary current can support several early design and estimating tasks:

• Establishing a preliminary feeder-current basis before selecting conductor sizes in AWG or kcmil.
• Comparing transformer secondary loading against the expected connected load basis.
• Identifying whether a proposed raceway arrangement may become difficult because larger conductors, parallel conductors, or greater raceway fill could be required.
• Starting a voltage-drop review when secondary conductors extend a significant distance from the transformer.
• Checking the current basis used in downstream distribution, motor-load review, or service-equipment coordination.
• Comparing alternative secondary voltages or transformer ratings using the same apparent-power basis.

A lower secondary voltage produces more current for the same kVA rating. For example, under balanced three-phase conditions, a 75 kVA transformer at 208 V produces more secondary current than the same 75 kVA transformer at a higher line voltage because the denominator in the current formula is smaller.

Field Verification

Estimated secondary current is an arithmetic value based solely on apparent power, voltage, and phase multiplier. It does not determine final conductor ampacity or equipment ratings.

Final field and code decisions must be evaluated separately using the actual installation, including conductor insulation temperature rating, terminal rating, ambient-temperature correction factor, adjustment factor for current-carrying conductors, conductor material, raceway fill, grounding and bonding requirements, voltage drop, transformer secondary conductor arrangement, overcurrent protection, and manufacturer equipment limitations. Confirm the final design with the applicable electrical requirements and the AHJ.