Transformer sizing begins with the actual coincident operating load—not the sum of every connected nameplate and not a rule-of-thumb margin alone. Convert the expected demand into kVA, then verify starting duty, harmonics, ambient conditions, voltage regulation, short-circuit requirements, redundancy and credible future growth.
1. Define the transformer duty
First establish whether the transformer supplies a building, process line, workshop or mixed load; whether it operates continuously or intermittently; whether it must start large motors; and whether another transformer provides redundancy. The same connected kW can require a different transformer when the operating sequence changes.
2. Build the load schedule
Group loads by type and record connected rating, demand or simultaneity, power factor, motor efficiency where relevant, duty cycle and harmonic information. Do not apply one diversity factor without understanding which loads can operate together.
| Load group | Record | Why it matters |
|---|---|---|
| Lighting and small power | Connected kW, demand factor, PF | Usually aggregated, but operating hours and nonlinear share may differ. |
| Motors | Shaft kW, efficiency, PF, starting method | Running input and starting voltage dip both require review. |
| UPS, VFD and rectifier loads | kW/kVA, true PF, harmonic data | Harmonic current can increase heating and neutral current. |
| Future loads | Rating and planned date | Provides a traceable growth allowance instead of an arbitrary percentage. |
3. Convert demand to kVA
Required kVA = demand kW / power factorMotor stated by mechanical shaft outputMotor input kVA = shaft kW / (efficiency × power factor)Add coincident contributions. Where groups have substantially different power factors, the electrical designer may use a complex-power calculation rather than simply adding kVA.
Worked preliminary example
A facility has 300 kW of general connected load with a justified demand factor of 0.75 and expected PF of 0.90. A 90 kW motor runs simultaneously at 94% efficiency and PF 0.88. A documented near-term expansion allowance of 15% is applied.
General demand: 300 × 0.75 / 0.90 = 250 kVA
Motor input: 90 / (0.94 × 0.88) ≈ 109 kVA
Present demand: 250 + 109 = 359 kVA
With 15% expansion: 359 × 1.15 ≈ 413 kVAA 500 kVA standard rating may be a candidate, but it is not final until the following checks are complete.
4. Check what the kVA total misses
Motor starting and voltage dip
Running kVA does not prove that a motor can start without an unacceptable dip. Review locked-rotor or starting current, acceleration time, starting method, transformer impedance, upstream source impedance, cable impedance and the sensitivity of other connected loads.
Harmonic loads
UPS systems, drives, rectifiers and chargers can add winding and stray losses. Provide the load type, true power factor and current harmonic spectrum or THDi where available. Selecting the next standard kVA alone does not resolve a harmonic-duty question.
Ambient and installation
State temperature, altitude, indoor/outdoor location, enclosure, ventilation, direct sun, dust, salt, humidity and seismic conditions. A dry-type enclosure or restricted airflow can change thermal capability; non-standard conditions may require a design adjustment.
Growth and redundancy
Tie growth margin to a real expansion plan. One large transformer may simplify the installation, while two units can provide staging or redundancy. Compare part-load losses, switching arrangement, fault level, civil space and the load that must remain available during an outage.
5. Complete the electrical specification
| Item | Confirm |
|---|---|
| Voltage and frequency | Rated HV/LV, highest system voltage where applicable, and 50/60 Hz duty. |
| Vector group and neutral | Phase displacement, neutral availability, earthing and parallel compatibility. |
| Taps | Range, steps and whether off-circuit or on-load operation is required. |
| Impedance | Fault current, voltage regulation and parallel load sharing. |
| Construction and cooling | Oil-immersed, cast-resin dry type or project-specific arrangement and cooling stage. |
| Loss schedule | Guaranteed no-load and load loss for the exact rating, reference temperature and applicable standard. |
| Documents | Applicable standards, drawings, test records and destination-market requirements. |
6. Avoid common sizing mistakes
- Adding every nameplate at 100% when loads are mutually exclusive.
- Applying diversity again to a measured maximum demand that already reflects simultaneity.
- Using motor shaft kW as electrical input kW.
- Ignoring motor starting, harmonics or ambient conditions.
- Adding vague future margin without checking the switchboard, cables and protection.
- Selecting by transformer series number instead of the approved parameter and loss schedule.
Information for a technical inquiry
Catalogue ranges support initial screening. The approved quotation-stage datasheet and project studies should determine the final procurement rating.