Transformer Sizer

Sizes a new transformer — or an N-1 redundant group — from connected load, growth/safety margins, ambient + altitude derating (IEC 60076-1/2/11-aligned reference conditions), and standard ANSI/IEEE or IEC preferred kVA ratings. For analyzing losses/efficiency of an existing transformer, see the Generator & Transformer Analysis calculator instead. Fully

AAJappsEngineering Calculation Report

Transformer Sizer

IEC 60076-1/2/11

Generated

For preliminary engineering use only — always verify results against project-specific standards and a qualified engineer's review before use in a real design.

Sizes a new transformer \u2014 or an N-1 redundant group \u2014 from connected load, growth and safety margins, ambient and altitude derating, and standard kVA ratings. Distinct from the Generator & Transformer Analysis calculator's Transformer tab, which analyzes an existing unit's losses/efficiency from test data rather than sizing a new one from load.

Standards followed

IEC 60076-1 §4.2 (reference conditions)IEC 60076-2 §5.1/§6.3.2 (temperature rise & altitude)IEC 60076-11 (dry-type altitude)IEC 60076-1 R10 preferred sizesANSI/IEEE C57.12.00-style ratings

What it can do

Sizes from kW+PF or direct kVA, with growth and safety margins.
Ambient derating above the IEC 40°C reference and altitude derating above the IEC 1000m reference, with separate rates for naturally- vs. forced-cooled types.
N-1 redundant group sizing — any (N−1) of N identical units carries the full load.
Rounds to either ANSI/IEEE or IEC 60076-1 preferred standard kVA sizes.

Example problem

Size an N-1 redundant pair (N=2) of transformers for an 800kW, 0.9 PF load, with 15% growth margin, 10% safety margin, 45°C ambient, and 1000m altitude.

Design load = 800 / 0.9 = 888.9 kVA.
Margined load = 888.9 × 1.15 × 1.10 = 1124.4 kVA.
Ambient derate factor = 1 − (45−40)×1.25% = 0.9375 (5°C above the 40°C IEC reference).
Altitude derate factor = 1.0 (at the 1000m reference, no altitude derating applies).
Effective required capacity = 1124.4 / 0.9375 = 1199.4 kVA.
With N=2, any 1 of 2 units must carry the full 1199.4 kVA alone → per-unit requirement = 1199.4 kVA.
Round up to the nearest standard ANSI size → 1500 kVA per unit (3000 kVA total installed).

1500 kVA per unit, 3000 kVA total installed.

Want the full step-by-step version?

Worked Example: Sizing an N-1 Redundant Transformer Pair with Ambient Derating →

The 40\u00b0C ambient and 1000m altitude reference points are directly from IEC 60076-1/2, but the specific derating rates (%/\u00b0C, %/100m) are commonly published engineering rules of thumb, not literal standard values \u2014 always confirm against the manufacturer's own thermal/altitude rating curve.

Connected load

Load Basis

Connected Load

Power Factor

Margins & derating

Growth Margin

Safety Margin

Ambient Temperature(IEC ref. 40°C max)

°C

Site Altitude(IEC ref. ≤1000m)

Cooling Class

Standard sizes & redundancy

Standard Basis

Total Units Installed (N)

With N total units, any (N−1) must be able to carry the full design load alone — N=2 (the common 1-for-1 redundant pair) sizes each unit for 100% of the load; N=1 means no redundancy. Ambient/altitude derating rates above the IEC reference points are commonly published engineering rules of thumb — always confirm against the manufacturer's thermal/altitude rating curve for a final selection.

Sizing result

Design Load888.9 kVA

Margined Load1124.4 kVA

Ambient Derate Factor0.9375

Altitude Derate Factor1.0000

Combined Derate Factor0.9375

Effective Required Capacity1199.4 kVA

Per-Unit Required (÷1)1199.4 kVA

Recommended Standard Size1500 kVA per unit

Total Installed Capacity3,000 kVA