How to Select a Distribution Transformer for a Factory?

Selecting a transformer for a factory is an engineering decision with a long tail. Choose too small and production trips under peak load; choose too large and you pay for capacity and no-load losses you never use.

This guide walks through the selection sequence we use with customers: load, rating, voltage, construction, cooling, protection, then testing.

Selecting a transformer for a factory is an engineering decision with a long tail. Choose too small and production trips under peak load; choose too large and you pay for capacity and no-load losses you never use.

This guide walks through the selection sequence we use with customers: load, rating, voltage, construction, cooling, protection, then testing.

List every significant load: motors with their ratings and starting method, drives, heaters, welders, compressors, lighting, HVAC, IT and any process equipment. Note each item’s kW, power factor and expected running pattern.

Do not skip the small loads. Lighting and small power can be 15–25% of a factory’s connected load.

Not everything runs at once. A diversity factor, the ratio of actual maximum demand to total connected load, typically falls between 0.5 and 0.8 for factories depending on the process. Using connected load without diversity will badly oversize the transformer.

Divide the diversified kW by the expected power factor to get kVA. Then add growth margin, 20–30% is typical, more if a specific expansion is planned.

Also check the largest motor starting condition. Direct-on-line starting of a large motor causes a voltage dip; if the dip exceeds the tolerance of other equipment, you need a larger transformer or a soft starter / VSD.

Your transformer primary must match the voltage the utility offers, normally 11 kV, 22 kV or 33 kV. Confirm the declared fault level at the point of connection as well, because it sets the required short-circuit withstand.

Outdoor substation, higher rating, best overload capability → oil immersed. Indoor substation inside a building, fire-regulated environment, low maintenance → dry type cast resin or VPI.

At minimum: winding temperature indication with alarm and trip. For oil filled units add Buchholz relay, oil level and pressure relief. On the network side, protection is provided by the MV switchgear fuses or relay, the transformer impedance must be compatible with the protection settings.

Require routine tests on every unit: winding resistance, ratio and vector group, impedance and load loss, no-load loss and current, dielectric tests and functional checks. Type tests apply for new designs or where the specification demands them.

Not sure which rating fits your project? Send us your load list and we will size it for you, or email info@xsdfftransformer.com, call +86 158 6789 7761 or WhatsApp +852 5416 2620.