2026/09/11 00:12:39
A reliable Prefabricated Substation Manufacturer sizes a substation by more than simply matching the transformer nameplate to the connected load. The correct design must account for maximum demand, power factor, future expansion, short circuit level, cable capacity, thermal conditions, protection coordination, installation space, and local electrical regulations.
This guide explains how to select the transformer capacity and complete the main sizing checks for a prefabricated substation. It is intended for electrical contractors, project engineers, consultants, facility owners, procurement teams, and distributors comparing substation suppliers.
The first step is to create a complete load schedule. Include every load that will be supplied by the prefabricated substation, even if some equipment operates only occasionally.
For each load, record the rated power, voltage, phase, power factor, efficiency, starting current, operating schedule, and whether the load is continuous or intermittent.
Connected load is the sum of all equipment nameplate ratings. Maximum demand is the highest realistic power that the facility will use at one time. These values are rarely identical because different loads operate at different times.
Use the following basic method:
The basic apparent power formula is:
Required apparent power in kVA = Maximum demand in kW divided by power factor
For example, if the calculated maximum demand is 720 kW and the expected operating power factor is 0.90:
Required apparent power = 720 divided by 0.90 = 800 kVA
This 800 kVA result is the calculated operating requirement before adding growth capacity, special starting requirements, ambient derating, or redundancy.
For an existing facility, measured demand data is usually more reliable than a nameplate-only calculation. Review electricity bills, power quality records, supervisory control and data acquisition records, and power meter data.
Collect the following information:
Measured data can prevent both oversizing and undersizing. A transformer that is too large may operate inefficiently at low load, while a transformer that is too small may overheat or cause nuisance tripping.
After calculating the current demand, add a realistic allowance for future growth. Do not use an arbitrary margin without checking the project development plan.
A common calculation is:
Future demand = Current maximum demand x 1 plus growth rate
For an 800 kVA calculated requirement and a planned growth allowance of 20 percent:
Future transformer requirement = 800 x 1.20 = 960 kVA
The next commercially available transformer size may be 1000 kVA or another size accepted by the selected manufacturer and applicable standard.
Typical planning considerations include:
Transformer selection should balance thermal capacity, efficiency, voltage regulation, and operating cost. A transformer that runs continuously at its maximum rating may have limited reserve capacity and reduced service life.
Many projects target normal loading between approximately 60 and 80 percent of the transformer nameplate capacity. The correct target depends on the load profile, local climate, ventilation, duty cycle, and required reserve.
For example, if the expected future demand is 960 kVA and the target normal loading is 80 percent:
Recommended transformer capacity = 960 divided by 0.80 = 1200 kVA
However, this result must be checked against the actual load curve. A facility with a short daily peak may accept a higher loading level than a facility operating at full load continuously.
Large motors can create a temporary voltage drop when starting. A transformer with adequate steady-state kVA may still be unsuitable if it cannot support the starting event.
Review the following motor information:
If a large motor starts directly across the line, perform a voltage drop study. The solution may be a larger transformer, a soft starter, a variable frequency drive, a reduced-voltage starter, or a different operating sequence.
The transformer voltage ratio must match the utility supply and the facility distribution system. Confirm the nominal voltage, frequency, phase arrangement, and permissible voltage variation.
Specify at least the following parameters:
The selected voltage ratio must also match the incoming high voltage switchgear, outgoing low voltage switchgear, cables, protection devices, and downstream equipment.
The transformer type affects the enclosure design, fire protection, maintenance requirements, and total cost of the prefabricated substation.
The transformer type must be compatible with the substation enclosure, ventilation system, fire separation, environmental conditions, and local approval requirements.
Low power factor increases current and therefore increases transformer, cable, and switchgear loading. If the expected power factor is low, include power factor correction or select the transformer based on the actual apparent power requirement.
Nonlinear loads such as variable frequency drives, rectifiers, data center power supplies, and battery chargers can produce harmonics. Harmonics may cause additional heating and neutral conductor loading.
For projects with substantial nonlinear loads, request or calculate:
The prefabricated enclosure must provide sufficient space for the transformer, high voltage section, low voltage section, cable terminations, ventilation, inspection, and maintenance access.
Confirm the following dimensions before approving the layout:
A compact design is useful for transportation and land savings, but insufficient clearance can create installation delays and unsafe maintenance conditions.
The enclosure must remove heat produced by the transformer, switchgear, cables, and auxiliary equipment. Ventilation design must consider transformer losses, outdoor temperature, solar radiation, altitude, dust, humidity, and the enclosure protection rating.
Check the following:
Where the substation will be installed at high altitude, apply the required derating for reduced air density and reduced cooling performance. The manufacturer should confirm the permissible operating capacity at the actual site altitude.
Switchgear current ratings must be based on transformer full-load current and system fault conditions.
For a three-phase transformer, the approximate full-load current is:
Full-load current = Transformer kVA x 1000 divided by square root of 3 x system voltage in V
For a 1000 kVA transformer with a 400 V secondary voltage:
Full-load current = 1000 x 1000 divided by 1.732 x 400 = approximately 1443 A
The low voltage main breaker, busbar, cable, and termination system must be selected to carry this current under the actual installation conditions.
For the high voltage side, select switchgear based on:
Transformer impedance strongly affects the available short circuit current on the low voltage side. A lower impedance generally allows higher fault current, while a higher impedance reduces fault current but may increase voltage drop.
A simplified estimate of transformer secondary fault current is:
Short circuit current = Full-load current x 100 divided by transformer impedance percentage
For a transformer with a full-load current of 1443 A and an impedance of 6 percent:
Estimated fault current = 1443 x 100 divided by 6 = approximately 24050 A
This value is only a preliminary estimate. The final short circuit study must include utility fault level, cable impedance, motor contribution, parallel transformers, and the complete distribution network.
Protection must isolate the faulted section without unnecessarily disconnecting the entire facility. Review the time-current curves for the high voltage fuse or breaker, transformer protection, low voltage main breaker, feeder breakers, and motor protection devices.
Protection coordination should address:
Do not select fuse and breaker ratings only from the transformer kVA. The ratings must also reflect cable ampacity, fault current, selectivity, inrush behavior, and local electrical regulations.
The prefabricated substation requires a complete grounding and bonding design. Bond the transformer tank, enclosure, doors, switchgear, cable screens, neutral equipment, and other exposed conductive parts according to the applicable earthing system.
Verify:
Ground resistance alone does not prove that the system is safe. The fault clearing time, bonding continuity, touch voltage, and protective device operation must also be checked.
Adding every equipment nameplate rating and choosing the next transformer size may produce an unnecessarily expensive design. Use demand factors and measured operating data while still accounting for future expansion.
Calculating transformer size from kW without converting to kVA underestimates current. Always use the expected operating power factor, especially for motor-heavy or power electronic loads.
An oversized transformer can have higher purchase cost, larger physical dimensions, higher no-load losses, and inefficient operation at low load. The reserve should be based on a documented growth plan and required operating profile.
A transformer may handle normal running current but still cause unacceptable voltage dips during motor starting. Check starting current and starting sequence before approving the rating.
Variable frequency drives, rectifiers, and data center equipment can increase transformer heating. Request harmonic data and verify whether a special transformer, larger neutral, or harmonic filter is needed.
The transformer, switchgear, cables, busbars, enclosure ventilation, foundation, and protection system must work together. A correct transformer rating does not guarantee a correct prefabricated substation design.
High altitude, high ambient temperature, coastal salt, industrial dust, flooding, and limited ventilation can require derating or upgraded enclosure protection. Provide accurate site information to the manufacturer.
Review the single line diagram, general arrangement, cable entry, lifting points, foundation interface, and maintenance clearances before production. Drawing approval is one of the most effective ways to prevent costly site modifications.
Purchasing teams often face uncertainty because suppliers quote different transformer ratings, protection systems, enclosure materials, and testing scopes. Compare equivalent technical specifications rather than comparing only the total price.
Every quotation should clearly state the load, power factor, growth margin, ambient temperature, altitude, transformer loading, and short circuit assumptions used for sizing. This allows the buyer to compare quotations fairly and identify hidden exclusions.
Clarify whether the supplier is responsible for load calculations, protection coordination, grounding design, utility approval support, civil foundation drawings, factory testing, site testing, and commissioning. Written responsibility limits reduce disputes during installation.
Send the supplier a complete technical brief containing the transformer capacity target, load schedule, voltage levels, site conditions, layout restrictions, cable directions, protection requirements, and applicable standards. Incomplete information often leads to provisional pricing and later design changes.
The selected supplier should review the load calculation and confirm that the transformer rating is compatible with the switchgear, cables, thermal design, foundation, and operating conditions. The design should be approved through drawings and a documented technical clarification process.
Juhonkia can support the selection and configuration of a complete prefabricated substation by coordinating transformer capacity, high voltage switchgear, low voltage distribution, enclosure construction, ventilation, protection, grounding, testing, and delivery requirements. Contact Juhonkia with your load schedule and site data so the final design can be checked before manufacturing.
The most reliable approach is to calculate present demand, convert it to kVA, add justified future capacity, verify motor and harmonic performance, size all associated equipment, and complete the protection and thermal studies. A qualified Prefabricated Substation Manufacturer should then confirm the assumptions and provide a coordinated, tested, and installation-ready solution.
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