TryBuildCalcDistribution Transformer Sizing Calculator (Medium-Voltage Feeder Example)
Calculate your distribution 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: 800.00 kVA
IEC standard size, from 571.56 kVA base load + 15% safety margin
Sizing Calculation
Base load: 571.56 kVA
Safety margin: 15%
Load with margin: 657.29 kVA
Recommended standard size (IEC): 800.00 kVA
Headroom: 142.71 kVA (17.8%)
Utilization at recommended size: 71.4%
Full-Load Current at Recommended Size
At 11,000V, three-phase: 41.99 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.
Distribution transformer sizing example
Distribution transformers step medium-voltage utility feeders down to a utilization voltage, and are commonly specified using the same base-load-plus-margin-plus-standard-size method as any other transformer, just at a higher primary voltage.
This page is pre-set to an 11,000V, 30A three-phase feeder with a 15% margin on the IEC series — edit the values above to match your actual feeder study.
- 11,000V × 30A × 1.732 ÷ 1000 ≈ 571.6 kVA base load.
- With a 15% safety margin, that becomes about 657.3 kVA — rounded up to the nearest IEC standard size, 800 kVA.
- Medium-voltage primary sizing follows the identical formula as low-voltage sizing — only the voltage figure changes.
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 | 11,000V × 30.00A | Sets the base kVA load |
| Phase | Three-Phase (× 1.732) | Sets the phase multiplier |
| Series / Margin | IEC, +15% | Sets the standard-size catalog and margin |
Step 1 — Base Load
| Calculation | 11,000 × 30.00 × 1.732 ÷ 1000 |
| Base load | 571.56 kVA |
Step 2-3 — Safety Margin & Standard Size
| 571.56 × (1 + 15% ÷ 100) | 657.29 kVA (with margin) |
| Smallest IEC standard size ≥ 657.29 | 800.00 kVA |
Step 4 — Headroom, Utilization & Full-Load Current
| Headroom | 142.71 kVA (17.8%) |
| Utilization (base load ÷ recommended size) | 71.4% |
| Full-load current at recommended size | 41.99 A |
Therefore, for a 571.56 kVA base load with a 15% safety margin, you need a 800.00 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.