Key facts
- Apparent load demand is Pn / (η × cos φ) when Pn is the load’s output power. Keep efficiency and power factor separate. [1]
- Utilisation applies to individual loads; coincidence applies to groups. The designer must justify the group factor. [2]
- In the example below, assumed demand and growth give 211.8 kVA, leading to a 250 kVA rating in the EIG table. [4; example assumptions]
- No-load energy estimates below use published S11 and S13 losses, assuming 8,760 energised hours per year. [6, 7; example assumption]
Build a load list with a clear power basis
List the consumers and identify whether each kW entry is electrical input or useful output. Schneider Electric’s Electrical Installation Guide gives apparent demand as Pa = Pn / (η × cos φ), where efficiency and power factor both matter when starting from output power. It defines power factor as kW divided by kVA. Do not silently mix output ratings with electrical-input demand. [1]
For each entry, record the power basis and the utilisation factor you intend to use. Group the loads that contribute to the same transformer demand, and document the assumed coincidence between them. The guide distinguishes utilisation per load from coincidence per group, with the group factor no greater than unity and its determination the designer’s responsibility. [2]
Use utilisation and coincidence as explicit assumptions
The EIG gives approximately 0.75 as an average motor utilisation factor in industrial installations and unity for incandescent lighting. These are examples within a design method, not universal settings. It also gives example group factors for lighting, heating and air conditioning, and socket outlets. Choose factors that the designer can explain for the actual load schedule. [2]
Keep the growth allowance separate from the coincidence factor. The worked example below uses an owner-assumed extension allowance; it is not a standard percentage. If the planned loads change, update the load list and calculation rather than retaining an unexplained margin.
Worked example: demand, power factor and planned extension
Example assumptions: treat the motor and lighting kW entries as electrical-input loads for this simplified calculation. Motors total 200 kW, with utilisation 0.75; lighting totals 30 kW, with utilisation 1. Apply an assumed group coincidence factor of 0.8, an initial power factor of 0.85 and an owner’s 25 % allowance for planned extension. These choices describe this example only. [1, 2, 4; example assumptions]
Motors: 200 × 0.75 = 150 kW. Lighting: 30 × 1 = 30 kW. Together they give 180 kW; applying the assumed group factor gives 180 × 0.8 = 144 kW. At power factor 0.85, demand is 144/0.85 = 169.4 kVA. Adding the planned extension gives 211.8 kVA. The next rating in the EIG table is 250 kVA. [1, 2, 4; example assumptions]
Now assume power-factor correction to 0.95 with the same active demand. The result is 151.6 kVA, or 189.5 kVA after the same extension allowance. A 200 kVA rating is present in our S11 and S13 ranges, although it is absent from the cited EIG rating table; 250 kVA remains another published choice. Review the complete application before deciding between them. [4, 5, 6, 7; example assumptions]
The EIG rating table gives approximately 352 A at 410 V for 250 kVA. Its three-phase current formula is In = kVA × 10³ / (√3 × U), with U the line-to-line voltage. Use the intended secondary voltage when checking current rather than carrying the example voltage into another country or installation. [4]
Compare no-load energy before choosing extra capacity
No-load loss is drawn whenever the transformer is energised. To illustrate that part of the energy comparison, assume continuous energisation for 8,760 hours in the example year and calculate no-load W × hours / 1,000. The values below use the published product losses; the annual energy is calculated from them rather than a separate product guarantee. [6, 7; example assumption]
These figures cover no-load energy only. They do not establish a complete annual-loss comparison or a payback period. Use the load cycle and actual installation requirements when choosing the transformer, and ask for the relevant loss guarantees in the offer. The EIG explicitly includes load cycle, future extensions and cooling in the selection process. [5]
| Transformer | No-load loss → annual energy |
|---|---|
| S11 200 kVA | 340 W → approximately 2,978 kWh/year |
| S11 250 kVA | 400 W → approximately 3,504 kWh/year |
| S13 250 kVA | 290 W → approximately 2,540 kWh/year |
Complete the specification before requesting a quotation
Tell the manufacturer the altitude and ambient conditions, installation arrangement, load cycle and planned extensions. The EIG calls for adequate cooling and temperature-rise provisions to avoid premature ageing, and identifies installation constraints, power-factor improvement and standard ratings as selection considerations. Ask the designer to resolve these alongside the demand calculation. [4, 5]
Confirm the primary and secondary voltages and the utility’s vector-group requirement. Our Nigeria and Indonesia market pages provide starting points for those discussions, but obtain the receiving utility’s specification for the particular connection. Keep the selected rating, voltage arrangement and required evidence together in the enquiry. [8, 9]
Step-by-step checklist
- List every consumer and distinguish electrical-input kW from useful-output kW. Include efficiency where the calculation starts with output power. [1]
- Apply justified utilisation per load and coincidence per group; record who selected each assumption. [2]
- Convert active demand to kVA using the chosen power factor and add the owner’s planned extension allowance. [1, 4, 5]
- Select a published candidate rating, then check current at the actual secondary voltage and review the load cycle and cooling. [4, 5]
- Confirm site conditions, utility voltage and vector group, then compare the relevant published losses before ordering. [5, 6, 7, 8, 9]
Frequently asked questions
Can I size the transformer directly from installed kW?
First account for the power basis, efficiency where applicable, utilisation, coincidence and power factor. Installed power and maximum kVA demand are different quantities. [1, 2, 3]
Is a 25 % growth allowance compulsory?
No. It is the owner’s assumption in this worked example. The EIG advises considering anticipated extensions without prescribing that example percentage. [4, 5]
Does correcting power factor change the example rating?
At power factor 0.95, the example gives 189.5 kVA including the assumed extension. Our ranges include 200 kVA; the cited EIG table moves from 160 to 250 kVA. Review the application before selection. [4, 5, 6, 7]
Are the annual energy figures total transformer losses?
No. They are no-load loss multiplied by an assumed 8,760 energised hours, converted to kWh. They exclude load-loss energy. [6, 7; example assumption]
Related markets and transformers
Sources
Bracketed references correspond to this list. Product figures refer to Jinying’s published specifications; worked-example factors are identified as assumptions.
- Schneider Electric EIG — Installed apparent power — https://www.electrical-installation.org/enwiki/Installed_apparent_power_(kVA) · accessed 2026-10-24
- Schneider Electric EIG — Estimation of actual maximum kVA demand — https://www.electrical-installation.org/enwiki/Estimation_of_actual_maximum_kVA_demand · accessed 2026-10-24
- Schneider Electric EIG — Application of utilisation and coincidence factors — https://www.electrical-installation.org/enwiki/Example_of_application_of_factors_ku_and_ks · accessed 2026-10-24
- Schneider Electric EIG — Choice of transformer rating — https://www.electrical-installation.org/enwiki/Choice_of_transformer_rating · accessed 2026-10-24
- Schneider Electric EIG — Choice of MV/LV transformer — https://www.electrical-installation.org/enwiki/Choice_of_MV/LV_transformer · accessed 2026-10-24
- Jinying — published S11 specifications — https://jinyingtransformer.com/oil-immersed/s11/ · accessed 2026-10-24
- Jinying — published S13 specifications — https://jinyingtransformer.com/oil-immersed/s13/ · accessed 2026-10-24
- Jinying — Nigeria market specifications — https://jinyingtransformer.com/markets/nigeria/ · accessed 2026-10-24
- Jinying — Indonesia market specifications — https://jinyingtransformer.com/markets/indonesia/ · accessed 2026-10-24
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