A common mistake is to assume that a 100 kVA transformer can drive 100 kW of equipment. It cannot, and the difference between the two units is not a rounding error: kVA is apparent power, while kW is real power, and they are linked by the power factor. Sizing a transformer correctly requires the total connected load, an estimate of how much of it runs at the same time, the power factor of the load, and a loading margin for starting currents and future growth. This article explains the method step by step, with reference to IEC 60076-1 for power transformers and GB/T 1094.1, the Chinese equivalent standard.
Step 1: Survey the Loads and Apply a Demand Factor
Not all installed equipment runs at full power simultaneously. The demand factor is the ratio of the maximum power actually drawn to the total connected power. For a workshop with continuous production lines, the demand factor is high, often 0.8 to 0.9; for a machine shop with motors that cycle on and off, 0.5 to 0.7 is typical; for a mixed office or residential building, 0.4 to 0.6. Multiply the total connected power by the demand factor to get the calculated load, and note any large motors separately, because their starting current can briefly reach five to eight times the running current.
| Load type | Typical demand factor |
|---|---|
| Continuous production lines | 0.8 - 0.9 |
| Machine shops, cyclic motor loads | 0.5 - 0.7 |
| Offices, mixed small loads | 0.4 - 0.6 |
Step 2: Convert Real Power to Apparent Power
Transformers are rated in kVA, so the calculated kW load must be divided by the power factor. The formula is S = P / cos phi, where S is the apparent power in kVA, P is the real power in kW and cos phi is the power factor. Motors and induction equipment typically run at 0.7 to 0.85 power factor, and a poor power factor makes the transformer larger than the kW figure suggests. Where the plant has many motors, installing a capacitor bank to correct the power factor to 0.9 or higher often reduces the required transformer size and the electricity bill together.
Step 3: Choose the Loading Rate and the Margin
A transformer should not be run at 100 percent of nameplate rating. Standard practice is to select a unit that operates at 60 to 80 percent of rated capacity under normal conditions, leaving margin for starting currents, voltage fluctuation, ambient temperature and future expansion of 15 to 25 percent. The formula that combines everything is: required capacity S = P_total x demand factor / (power factor x loading rate). A worked example makes the method clear. A workshop has 200 kW of connected equipment, a demand factor of 0.7 and a power factor of 0.85. The calculated load is 200 x 0.7 = 140 kW. With a loading rate of 0.8, the required capacity is 140 / (0.85 x 0.8) = 206 kVA, so a 250 kVA transformer is selected, not the 200 kVA that the kW figure alone suggests. This is the same conclusion as the older rule of thumb that the kVA rating should be about 1.25 times the kW load, but the formula is transparent and can be adjusted for any plant.
Step 4: Check Voltage, Connection and Impedance
Capacity is only one part of the selection. The primary and secondary voltages must match the utility supply and the plant distribution voltage; the connection group (such as Dyn11 or Yyn0) must suit the earthing system; and the impedance voltage, usually 4 to 6 percent for distribution transformers, affects the short-circuit current that downstream switchgear must withstand. For large or critical installations, these points are checked together with the fault level of the network, and the final choice is confirmed against IEC 60076-1 or GB/T 1094.1 data sheets.
Oil-Immersed or Dry-Type
| Consideration | Oil-immersed | Dry-type |
|---|---|---|
| Typical location | Outdoor or dedicated substation | Indoor, close to the load |
| Fire and environmental risk | Oil needs containment and fire protection | No oil, lower fire risk |
| Maintenance | Oil testing and sealing checks | Low routine maintenance |
| Typical standard | IEC 60076-1 | IEC 60076-11 |
Choose the oil-immersed type for outdoor substations and high-capacity plants, and the dry-type for indoor installations where fire safety, space and maintenance matter most. Both are selected with the same capacity method described above.
FAQ
Can a 100 kVA transformer drive 100 kW of equipment?
No. At a power factor of 0.8 the transformer can deliver 80 kW at full load, and at the recommended 80 percent loading rate only about 64 kW of continuous load is good practice.
What is the demand factor and why does it matter?
The demand factor is the ratio of the actual maximum power drawn to the total connected power. Using it prevents oversizing a transformer for machines that never run at the same time.
What loading rate is recommended for a distribution transformer?
60 to 80 percent of nameplate rating is the usual design range, leaving margin for starting currents, voltage dips, temperature and 15 to 25 percent future expansion.
Does a low power factor require a bigger transformer?
Yes. A lower power factor means more kVA for the same kW, so the transformer must be larger. Power factor correction capacitors often reduce the required size.
What is the formula for transformer sizing?
Required capacity in kVA equals the total connected power in kW times the demand factor, divided by the power factor times the loading rate. Round up to the nearest standard rating.
Oil-immersed or dry-type transformer: which should I choose?
Use oil-immersed for outdoor and high-capacity substations, and dry-type for indoor locations where fire safety and low maintenance dominate; apply the same capacity calculation to both.