TryBuildCalcIEC Transformer Size Calculator (International Standard Sizes)
Calculate your IEC transformer size.
🕒 Last updated: August 31, 2026
Inputs
Load
ℹ️Design or measured load current, not one downstream device's individual nameplate rating.
Sizing
ℹ️ANSI/IEEE and IEC are two genuinely different, non-overlapping standard size catalogs — pick the one matching your region/supplier. Under ANSI/IEEE specifically, single-phase and three-phase units also use separate catalogs, so the phase you selected above changes the recommended size too.
ℹ️Applied on top of your base load before rounding up to a standard size. 20% is a common default; consider more for known near-term load growth.
Cost
Recommended Transformer Size: 250.00 kVA
IEC standard size, from 207.84 kVA base load + 20% safety margin
Sizing Calculation
Base load: 207.84 kVA
Safety margin: 20%
Load with margin: 249.41 kVA
Recommended standard size (IEC): 250.00 kVA
Headroom: 0.59 kVA (0.2%)
Utilization at recommended size: 83.1%
Full-Load Current at Recommended Size
At 400V, three-phase: 360.85 A
Assumptions Used
Recommended size rounds up to the nearest IEC standard distribution transformer kVA rating — never down. This is a reference sizing estimate; consult a licensed electrical engineer before ordering equipment.
Sizing the wire and breaker for this transformer's secondary feeder? Wire Size Calculator →
Checking the total connected load ahead of this transformer? Electrical Load / Panel Size Calculator →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Transformer kVA Calculator.
IEC standard transformer sizing
IEC standard distribution transformer kVA ratings (25, 50, 100, 160, 200, 250, 315, 400, 500, 630, 800...) are used across most of the world outside North America — a genuinely different catalog from the ANSI/IEEE list, not just a relabeling of it.
This page is pre-set to a 400V, 300A three-phase load rounded to the IEC series — edit the voltage and current above to match your actual project.
- 400V × 300A × 1.732 ÷ 1000 ≈ 207.8 kVA base load.
- With a 20% safety margin, that becomes about 249.4 kVA — rounded up to the nearest IEC standard size, 250 kVA.
- The same load rounds to a different recommended size (300 kVA) under the ANSI/IEEE three-phase series — switch the series selector to compare.
Transformer kVA Formula: How Is It Calculated?
Sizing mode computes a base load, adds a safety margin, then rounds up to a real standard transformer size. Current mode runs the same underlying relationship in reverse.
Step 1 — Base Load (kVA)
From Voltage & Current: Base kVA = V × I × 1.732 ÷ 1000 (three-phase) or V × I ÷ 1000 (single-phase)
From Power & Power Factor: Base kVA = kW ÷ Power Factor
Use whichever input matches the data you actually have — both feed the same base-kVA figure used in every step below.
Step 2 — Add Safety Margin
kVA with Margin = Base kVA × (1 + Safety Margin % ÷ 100)
A 10-25% margin is standard engineering practice, covering operational fluctuation and near-term load growth; 20% is this calculator's default.
Step 3 — Round Up to a Standard Size
Recommended kVA = smallest standard size in the selected series ≥ kVA with Margin
ANSI/IEEE and IEC are separate, genuinely different standard size catalogs — the same load can round up to a different recommended size depending which series you select. The result always rounds up, never down.
Step 4 — Headroom, Utilization & Full-Load Current
Headroom = Recommended kVA − kVA with Margin
Utilization % = Base kVA ÷ Recommended kVA × 100
Full-Load Current = (Recommended kVA × 1000) ÷ (V × 1.732 or V)
Headroom is the spare capacity created by rounding up to a discrete standard size, on top of your chosen safety margin. Full-load current at the recommended size (not the smaller base load current) is what downstream protection and conductors should be sized against.
Current Mode — Full-Load Current from a Known kVA
Full-Load Current = (Transformer kVA × 1000) ÷ (V × 1.732) for three-phase
Full-Load Current = (Transformer kVA × 1000) ÷ V for single-phase
Given a known transformer nameplate rating, this runs the base-load formula in reverse to find its full-load current at any voltage you specify.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Load | 400V × 300.00A | Sets the base kVA load |
| Phase | Three-Phase (× 1.732) | Sets the phase multiplier |
| Series / Margin | IEC, +20% | Sets the standard-size catalog and margin |
Step 1 — Base Load
| Calculation | 400 × 300.00 × 1.732 ÷ 1000 |
| Base load | 207.84 kVA |
Step 2-3 — Safety Margin & Standard Size
| 207.84 × (1 + 20% ÷ 100) | 249.41 kVA (with margin) |
| Smallest IEC standard size ≥ 249.41 | 250.00 kVA |
Step 4 — Headroom, Utilization & Full-Load Current
| Headroom | 0.59 kVA (0.2%) |
| Utilization (base load ÷ recommended size) | 83.1% |
| Full-load current at recommended size | 360.85 A |
Therefore, for a 207.84 kVA base load with a 20% safety margin, you need a 250.00 kVA IEC transformer.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.