Transformers are bought in fixed kVA steps and live for decades, so a sizing mistake is expensive in both directions. An undersized unit runs hot, loses life early and trips on peaks. An oversized one costs more, sits at a low load where efficiency and power factor are poor, and carries a higher fault level than your switchgear may be rated for. The method below works for a factory, workshop or commercial building fed at 11 kV or 22 kV and stepped down to 415 V, 50 Hz.
The sizing method, step by step
Step 1: Build the load list
List every load that will be connected to the transformer: motors, lighting, HVAC, welding sets, furnaces, UPS, compressors, EV chargers, and future additions. Write the rating in kW (motor shaft output is not the same as input; divide by efficiency to get input kW) and the power factor of each group. For motors, use the nameplate efficiency and power factor, and remember that a 75 kW motor lightly loaded draws less than 75 kW but its power factor falls.
Separate loads into continuous, intermittent and standby. A standby pump that never runs together with its duty pump should not be added twice.
Step 2: Apply a demand factor
Not everything runs at once at full load. The demand factor is the ratio of maximum demand to total connected load. Typical values depend entirely on the process: a continuous process plant may be close to 0.8 to 0.9, while a general engineering workshop is often well below that. Do not guess. Use your own recorded maximum demand from the DISCOM bill or energy meter if you have an existing plant. For a new plant, use the process engineer’s load study.
Step 3: Convert kW to kVA
Transformers are rated in kVA because their limit is current and voltage, not power factor.
kVA = demand kW ÷ power factor
If your power factor is low, the kVA demand rises. Improving it with power factor correction reduces the kVA drawn from the transformer, which can free capacity or avoid a bigger unit.
Step 4: Add growth and a loading margin
Allow for planned expansion, typically 15 to 25 percent, unless the site is truly fixed. Also decide the maximum loading you are willing to run at. Many engineers aim for a normal loading of roughly 60 to 80 percent of rated kVA, which keeps the unit efficient and leaves headroom for motor starting and peaks. A transformer running continuously at 100 percent will age faster, especially in a hot ambient.
Step 5: Check motor starting
A direct-on-line motor draws 6 to 7 times its rated current for a few seconds. If the largest motor is a significant fraction of the transformer kVA, the voltage dip during starting can trip contactors or stall other loads. As a rough rule, check whether the largest DOL motor exceeds around 10 to 15 percent of the transformer rating; if so, consider a star-delta starter, soft starter or a VFD or motor starter and ask your engineer to run the voltage dip calculation.
Step 6: Select the standard rating
Round up to the nearest standard size. Common distribution ratings in India are:
| kVA rating | Full-load current on 415 V side (approx.) | Full-load current on 11 kV side (approx.) |
|---|---|---|
| 315 | 438 A | 16.5 A |
| 500 | 696 A | 26.2 A |
| 630 | 877 A | 33.1 A |
| 1000 | 1391 A | 52.5 A |
| 1600 | 2226 A | 84.0 A |
| 2000 | 2783 A | 105 A |
The formula is I = kVA × 1000 ÷ (1.732 × V). Other standard sizes include 100, 160, 200, 250, 400, 800, 1250 and 2500 kVA.
Worked example
- Connected load: 650 kW (input basis).
- Demand factor: 0.75, so maximum demand is 650 × 0.75 = 487 kW.
- Power factor: 0.90, so apparent power is 487 ÷ 0.90 = 541 kVA.
- Growth allowance 20 percent: 541 × 1.2 = 650 kVA.
- Nearest standard size above this: 800 kVA.
Now compare. If the plant corrects its power factor to 0.97, the same maximum demand becomes 487 ÷ 0.97 = 502 kVA, and with 20 percent growth about 602 kVA, which fits a 630 kVA unit. That is why power factor and transformer size are linked, and why the power factor calculator and the transformer sizing calculator are best used together. For the 800 kVA option, the LV full-load current is about 1113 A.
Check impedance and fault level
Percentage impedance (%Z) decides how much the voltage drops under load and how large the short-circuit current is on the LV side. The approximate maximum fault current, with an infinite HV source, is full-load current ÷ per-unit impedance. A 1000 kVA, 5 percent unit gives 1391 ÷ 0.05 = about 27.8 kA at the LV terminals. Your LV panel, ACB and MCCB short-circuit ratings must be higher than this. A lower impedance transformer gives better voltage regulation but a higher fault level, and the reverse. Confirm the %Z and the resulting fault level with a qualified engineer before finalising the breaker ratings, and see our circuit breakers range.
Parallel operation, tap range and cooling
- If you plan to parallel two transformers later, specify identical vector group, ratio, similar impedance and kVA ratio not exceeding about 2:1. Decide this at the ordering stage.
- Decide the tap range (for example plus or minus 5 percent in steps of 2.5 percent) to handle incoming voltage variation.
- Choose oil-immersed or dry-type by location and fire norms.
- Derate or specify a higher rating for high ambient temperature or high altitude sites.
Common mistakes
- Adding connected kW straight to kVA without a power factor.
- Ignoring harmonic loads such as VFDs and UPS, which add heating beyond what the kVA shows.
- Choosing the next size up “for safety” without checking the effect on fault level and no-load loss.
- Forgetting that kVA on a DG set, transformer and UPS are different things and are not interchangeable.
What to send us for a quotation
Send us the maximum demand, connected load list, HV and LV voltage, preferred kVA, impedance, vector group, cooling type, indoor or outdoor location and any specification. We will respond with a quotation within 24 hours through request a quote, or you can speak to us via contact us. For anything safety-critical, such as fault level and breaker coordination, confirm with a qualified electrical engineer. You can also browse all transformers.
Frequently asked questions
How do I calculate the kVA of a transformer for my factory?
Take the maximum demand in kW, divide by the power factor to get kVA, add a growth margin of around 15 to 25 percent, and round up to the next standard rating. Use recorded demand from your electricity bill for an existing plant. Check motor starting and fault level before you finalise the size.
What size transformer do I need for a 500 kW load?
At 0.9 power factor, 500 kW is about 556 kVA. Allowing normal loading headroom, a 630 kVA transformer is a typical choice, or 800 kVA if you expect significant growth. If your power factor is corrected to 0.97 or more, 630 kVA gives more margin. Confirm with a load study.
Is it bad to oversize a transformer?
Yes, within reason. An oversized unit costs more, has higher no-load losses that run all year, operates at low load where power factor is poor, and gives a higher fault level that your switchgear must withstand. Aim for normal loading in the 60 to 80 percent range.
How does power factor affect transformer size?
Transformer capacity is limited by kVA, and kVA equals kW divided by power factor. At a lower power factor the same kW needs more kVA. Raising the power factor with capacitors reduces the kVA drawn, which can allow a smaller transformer or free spare capacity on an existing one.
