Transformer Full Load Amps Calculator (Primary & Secondary FLA)

Enter nameplate kVA, phase, and both winding voltages for primary and secondary FLA together. Default: 100 kVA, 480→208 V, 3φ → ~120 A / ~278 A. Prefer this over generic kVA to amps for transformer nameplates.

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. Enter the operating load from the project load study or measured demand; this calculator does not assume a typical loading range.

About this calculator

Transformer nameplate FLA for both windings in one screen. Generic one-side conversion: kVA to Amps. Capacity from load: Transformer Size. Small voltage correction (208→240): buck-boost sizing. Hub: power calculator · transformer.

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

Approximate secondary FLA. Open the calculator with matching primary/secondary defaults where useful.

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

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. Small utility correction (208→240): buck-boost chart.

Use the right tool

Goal	Tool
Primary + secondary FLA from nameplate kVA	This page
Any single-side kVA ↔ amps (not transformer-specific)	kVA to Amps
208→240 / buck-boost unit kVA	Buck-boost sizing
SCCR / available fault vs marked rating	SCCR Calculator

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.

Why not use the generic kVA to amps calculator?

Same SI formula, but this page owns both windings, transformer charts, and the nameplate workflow. Use kVA to amps for generic converters not tied to a transformer nameplate.