Transformer Full Load Amps Calculator

Enter nameplate kVA, phase, and both winding voltages to calculate primary and secondary FLA together. Default: 100 kVA, 480 V → 208 V, three-phase.

Quick calculator

Use line-to-line voltage for three-phase. Results update instantly.

Transformer rating (kVA)

Phase

Primary voltage (V)

Secondary voltage (V)

Advanced calculator

Scale winding currents by operating load. Nameplate FLA still shown at 100%.

Operating load (%)

100% = nameplate FLA. Use 70–90% for typical continuous plant loading.

About this calculator

See our power calculators on the hub. This is a transformer nameplate task: it reports both winding currents in one result. For a generic one-side conversion, use kVA to Amps; for equipment capacity, use Transformer Size. Browse the power calculator hub.

Primary and secondary FLA

Result explanation

Primary FLA: 120.3 A @ 480 V · Secondary FLA: 277.6 A @ 208 V.

Assumptions: Screening only. Verify the nameplate, tap position, conductor conditions, OCPD rules, and available fault current before procurement.

Continue Your Calculation

After you know both winding FLA values, screen transformer %Z and equipment SCCR before preliminary protection review.

SCCR check →

Common three-phase transformer FLA chart

kVA	208 V	480 V	600 V
75	208 A	90 A	72 A
100	278 A	120 A	96 A
150	416 A	180 A	144 A
500	1,388 A	601 A	481 A

Transformer full-load current formula

Three-phase: FLA = kVA × 1,000 ÷ (√3 × line-to-line V). Single-phase: FLA = kVA × 1,000 ÷ V. PF is not used because transformer nameplate capacity is already kVA.

Example: 100 kVA, 480 V, 3φ: 100,000 ÷ (1.732 × 480) = 120.3 A. At 208 V secondary: 277.6 A.

Workflow: load → transformer kVA → this FLA tool → %Z fault screen → breaker screening.

Use the right tool

Generic kVA to Amps handles any load side. This page is for a transformer with known primary and secondary voltages. Use SCCR Calculator when you also need to compare a marked SCCR / kAIC rating.

Frequently asked questions

How do I calculate transformer full-load amps?

Use kVA and the voltage of the winding you are checking. For three-phase, divide kVA × 1,000 by √3 × line-to-line voltage.

How many amps is a 100 kVA transformer?

At 480 V three-phase it is about 120.3 A; at 208 V three-phase it is about 277.6 A. Voltage side matters.

Is primary current the same as secondary current?

No. The lower-voltage winding carries more current for the same kVA.

Does FLA choose the breaker size?

No. It is an input to preliminary protection review; code and manufacturer rules still apply.