Transformer Sizing Calculator
Size a distribution transformer for a building, estate or plant: enter the connected load with its diversity and power factor (or a maximum demand you already have), add a future-growth allowance, and the calculator returns the required kVA, the recommended standard IEC rating, the full-load currents on both the 11/33 kV and 415 V sides, and the utilisation now and at the growth horizon.
Establish the demand first with the Load Schedule Calculator, then size the LV incomer with the Cable Sizing and Voltage Drop Calculator.
Transformer selection CALC-07 · REV 2026-07
| Connected load | – |
|---|---|
| Diversified maximum demand | – |
| Design demand with growth | – |
| Required rating at target loading | – |
| Recommended standard rating | – |
| Full-load current, LV side | – |
| Full-load current, HV side | – |
| Utilisation at present demand | – |
| Utilisation at growth horizon | – |
| Vector group / connection | – |
Guidance tool only. A customer substation design must also cover fault level and impedance selection, HV protection and metering, motor-starting voltage dip, harmonic content (K-factor for VFD or data loads), earthing, and KPLC connection requirements — work for a registered engineer.
How this calculator works
- Maximum demand. Connected kW × diversity ÷ power factor gives the demand in kVA — the quantity a transformer is rated in. Load-type presets supply typical diversity and power factor (residential 0.50/0.90 through heavy industrial 0.85/0.80), all editable; or enter a known demand directly.
- Growth and utilisation. A growth allowance (typically 20%) gives the design demand, and the target loading (typically 80% of rating) adds thermal headroom — distribution transformers are most efficient around 50–75% loading, and running cooler extends insulation life.
- Standard rating. The required kVA is matched to the next standard IEC 60076 size (25–2500 kVA). KPLC network units are typically 25–630 kVA; larger ratings imply a customer substation with its own HV metering.
- Currents and checks. Full-load currents are reported for both windings (I = kVA ÷ (√3 × V)) — the LV figure feeds the incomer cable and main breaker sizing. Utilisation below 30% flags an oversized, loss-heavy unit; above 90% at the horizon flags thin headroom.
Assumptions and limits
- Standard Kenyan distribution arrangement assumed: Dyn11, 11 or 33 kV primary, 415/240 V secondary.
- Fault level and impedance selection, motor-starting voltage dip, harmonic (K-factor) derating, HV protection, earthing and the KPLC connection process are outside this tool and essential in the full design.
Planning a customer substation or a KPLC supply upgrade? See our consultancy services and design document library, or learn the method in a training program.