Sizing a transformer substation by rating: table from 100 to 1600 kVA

The transformer rating drives almost the whole substation: the fuses in the transformer feeder, the main LV device, the cable cross-sections, the enclosure size, the oil sump and the metering method. Below we summarise these relationships in two tables for a container substation from 100 to 1600 kVA.

All values were calculated by the selection engine behind the Energeks station configurator: walk-in container substation with an operating corridor, 24 kV MV switchgear without F-gases, MarkoEco2 oil-immersed transformer meeting Ecodesign Tier 2, 6 LV outgoing feeders. Polish distribution network practice (15 and 20 kV). This is a typical selection and a starting point for design. The final equipment is decided by the designer, the grid connection conditions and the switchgear manufacturer's tables.

Start with the target rating and the current

The transformer is sized to the demand of the site with a margin for growth. If the investor plans an extension, it pays to fix the target rating now: the enclosure, the oil sump and the connections must be able to take it, even if a smaller transformer is installed first. The rating gives the currents on both sides:

I = S / (√3 x U)

For a 630 kVA transformer this is 24.2 A on the 15 kV MV side and 909 A on the 400 V LV side. The LV current determines the main device, the busbars, the cable cross-section and the terminals on the transformer.

MV side: current and fuses in the transformer feeder

The transformer feeder typically uses a switch-disconnector with HV HRC fuses (HH). At higher ratings the fuse reaches the limit of typical 24 kV switchgear and the engine switches to a circuit breaker (feeder V instead of T in the station code).

24 kV fuses with striker, typical selection; final choice according to the MV switchgear manufacturer's table (EN 62271-105) and with selectivity to the LV protection. From 1250 kVA fuses can be chosen after checking that table.

LV side: main device, connections and enclosure

Initial short-circuit current on the LV busbars calculated for a stiff MV network according to EN 60909-0. KSW enclosures with operating corridor and cable basement, external dimensions. Up to 630 kVA the oil sump is sized for the full oil volume of a 630 kVA transformer, as required by Polish network operators; above that it grows with the transformer oil.

Five thresholds where the substation changes

1. MV metering instead of LV. The configurator switches to MV metering by default from 400 kVA. LV metering at higher ratings is an exception that the network operator must approve, and the thresholds differ between operators. An MV metering panel adds one panel to the switchgear and can enlarge the enclosure.
2. From 630 to 800 kVA. The enclosure grows from 2.55 to 2.96 m width (KSW 2.96 x 4.56 m, approx. 28 t instead of approx. 20.7 t), the oil sump from 409 to 557 l, and the main LV device becomes a circuit breaker. Transformers from 800 kVA have flat LV terminals; Energeks TOC M12, M20 and M30 clamps are intended up to 630 kVA.
3. 1000 kVA at 15 kV. A 63 A HV fuse is at the limit of typical 24 kV switchgear, so the engine uses a circuit breaker in the transformer feeder by default. At 20 kV the same rating still fits 50 A fuses.
4. 1600 kVA. The transformer is connected to the LV switchboard with a 2500 A busway instead of cables, and the enclosure is multi-segment, designed individually with the enclosure manufacturer.
5. Above 1600 kVA. This is where the single-transformer container substation range ends. Higher ratings, up to 2500 kVA, are handled in the configurator's line for e-mobility, and two transformers from 2 x 400 to 2 x 1000 kVA fit in a container substation with an operating corridor.

What the table does not cover

• Grid connection conditions: they define the ownership boundary, the metering type and often the CT ratio.
• The operator standard when the substation is handed over to the network operator: colours, locks, balance metering, number of panels and feeders. In the configurator you select it in one field (PGE Dystrybucja, TAURON Dystrybucja, Enea Operator, Energa-Operator, Stoen Operator).
• Selectivity of MV and LV protection and settings: always the designer's responsibility.
• Location: fire distances, REI 120 walls next to buildings, crane access.

How to calculate it for your substation

In the station configurator you set the rating, voltage, metering, number of feeders and operation type, and the engine immediately swaps fuses, devices, cross-sections, enclosure and oil sump. From each configuration you can download, without registration, a technical design for adaptation by the designer and a data sheet in PDF, SVG drawings (plan, elevations, sections, earthing, foundation), a single-line diagram, a CSV bill of materials and JSON data. The configurator interface is in Polish.

Each configuration has its own code, e.g. ENG-B630-15S-LPT-06-K: line B (container), 630 kVA, 15 kV network, solid-air insulation, panels L, P and T, 6 feeders, operating corridor. A link with the code opens the configurator in exactly this setting: 630 kVA substation with MV metering, 1000 kVA substation with a circuit breaker in the T feeder. Ready-made sets are also in the shop: 630 kVA with operating corridor and 1000 kVA.

Sources

• Energeks station configurator selection engine, version 0.8 (October 2026).
• EN 60909-0, short-circuit currents in three-phase AC systems; EN 62271-105, switch-fuse combinations; EN 62271-202, prefabricated substations.
• Regulation (EU) 2024/573 on fluorinated greenhouse gases, Article 13(9) (MV switchgear up to 24 kV without F-gases from 1.01.2026).
• Commission Regulation (EU) No 548/2014 as amended by Regulation (EU) 2019/1783 (transformer losses, Tier 2).
• PGE Dystrybucja, standards for indoor MV/LV transformer substations (in force from 16.07.2025); Enea Operator, Booklet 1, compact prefabricated MV/LV substations up to 630 kVA (version 06.2024): oil sump for the full oil volume of a 630 kVA transformer.

See also: walk-in container transformer substation in the Energeks catalogue (range, dimensions, drawings).