How to Size a Distribution Transformer for a Factory
Reviewed by the Fengguang engineering team · Last updated 7 Nov 2026
Size a factory transformer from maximum demand, diversity, voltage drop and motor starting, not from the sum of connected loads. A practical checklist for 10 kV and 20 kV projects.
Start from maximum demand, not connected load
The connected load on a factory drawing is almost never the load the transformer sees. Add the nameplate ratings of every machine and the number will be two or three times the real maximum demand. Start instead from a measured or calculated maximum demand in kVA, then apply a realistic diversity factor and a load-growth allowance.
For a new plant, split the load into continuous process load, intermittent load and future expansion. Keep the transformer sizing calculation with the single-line diagram so that the basis is visible to the protection and procurement teams.
Check voltage drop and motor starting
A transformer that is correct on thermal load can still be wrong on voltage. Large motors, welding equipment and VFD-driven lines can produce a voltage dip at the LV busbar. Check the transformer impedance, the motor starting current and the upstream fault level together. If the dip is outside the process tolerance, a larger transformer or a different starting method may be needed.
Apply the site conditions
Ambient temperature, altitude, enclosure and ventilation change the usable capacity. A unit rated for 40 °C in an open room may need derating in a 50 °C enclosure or at 2,000 m altitude. State the site conditions in the enquiry; they are design inputs, not afterthoughts.
| Input | What to confirm |
|---|---|
| Maximum demand | kVA, not kW; include power factor |
| Diversity / simultaneity | Which loads run together |
| Voltage drop | Motor starting and sensitive loads |
| Fault level | Transformer impedance and protection |
| Ambient / altitude | Derating and cooling margin |
| Future expansion | Spare capacity and connection space |
FAQ
Should I size for 80% or 100% load?
Most industrial transformers run below 80% for thermal margin and future growth, but the correct value depends on the load profile, ambient temperature and tariff. A 100% continuous load leaves no margin for overload or high ambient.
Do I need a 10 kV or 20 kV transformer?
Use the voltage of the site distribution network. A 20 kV transformer cannot be applied to a 10 kV system; the insulation level, bushings and tap range are different.
Can I use a dry-type transformer outdoors?
A standard cast-resin unit is intended for indoor, ventilated rooms. Outdoor use needs an IP enclosure, anti-condensation heaters and a corrosion class; many outdoor factory sites are better served by an oil-immersed unit.
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Need this applied to a project?
Send the application, voltage class and site conditions through the inquiry form. The team will check the transformer family and the document scope.
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