Prefabricated / Compact Substation Buying Guide
A compact substation buying guide covering construction types, when a prefabricated unit beats a switchroom, the specification checklist, sizing, layout and required test documentation.
Read MoreAn EV charging hub is a small distribution substation with a retail interface. The chargers get the attention, but the EV charging stations switchgear and the equipment behind it decide how much power the site can actually deliver, how quickly it can be commissioned, and whether the utility demand charge erases the margin on every session.
This guide follows the electrical path from the medium-voltage feeder to the charger terminals, shows how to size the station from charger count and simultaneity rather than nameplate totals, and sets out the protection, earthing and enclosure requirements that differ from ordinary commercial distribution. The range of distribution solutions for charging and similar public-facing infrastructure follows the same principles.
Why EV Charging Loads Stress the Distribution System
- Non-linear current: the AC-to-DC conversion stage in a fast charger draws a distorted current, so harmonic content is high and neutrals and transformers carry more current than a sinusoidal load of the same rating would produce.
- Continuous duty: fast charging loads are treated as continuous, which means feeders are sized above the charger’s rated input rather than at it.
- Peak-driven cost: the utility bills the highest monthly peak. A site that staggers sessions pays substantially less than one that does not.
- Public access and safety: the equipment sits where the public walks, so enclosure protection, tamper resistance and residual current protection are design requirements rather than options.
- Growth: almost every site plan expands. Infrastructure sized only for the first phase usually has to be rebuilt.
The Power Path: From MV Feeder to Charger
- Utility MV feeder: typically a 10 kV, 20 kV or 35 kV three-phase supply. For loads of the order of 1 MW and above this usually means a dedicated service and a formal interconnection study.
- Metering and disconnect: utility-owned revenue metering and a visible means of isolation at the point of connection.
- MV switchgear: either a ring main unit or a breaker-based panel, providing switching, isolation and transformer protection.
- Step-down transformer: medium voltage to the low-voltage distribution level, normally a delta-star vector group for the LV distribution system.
- Main low-voltage switchgear: incoming breaker, busbars, feeder breakers, power factor correction, surge protection and metering.
- Distribution cabinets: sub-distribution to groups of chargers, with dedicated protection per charger or power cabinet.
- Chargers: AC input converted internally to DC output at the vehicle connector.
Sizing the Station Before Choosing Equipment
The design load is not the sum of charger nameplates. It is the sum multiplied by a simultaneity factor and divided by efficiency and power factor, plus the auxiliary load, plus a margin.
Design apparent power ≈ (number of chargers × charger power × simultaneity factor) ÷ (charger efficiency × power factor) + auxiliary load, then apply a margin of at least 20% for expansion, harmonic derating and ambient temperature before rounding up to the nearest standard transformer rating.
| Number of chargers | Typical simultaneity factor | Practical effect |
|---|---|---|
| 2–5 | 0.75–0.90 | Sites with little or no load management |
| 6–10 | 0.60–0.75 | Managed sites where sessions rarely overlap fully |
| 11–20 | 0.50–0.65 | Highway and urban hubs with power sharing |
| 20+ | 0.45–0.55 | Mega hubs with active power allocation across stalls |
Because the transformer is the most expensive and least upgradable item on site, the sizing calculation deserves more attention than the charger selection. A station with intelligent power sharing can often run on a transformer one or two standard steps smaller than an unmanaged site, which changes both capital cost and the utility demand charge.
MV Side: Ring Main Unit or Breaker Panel
| Characteristic | Ring main unit | Circuit breaker panel |
|---|---|---|
| Primary switching | Load break switches, with fuses on the transformer feeder | Vacuum circuit breakers |
| Fault clearance | By MV fuses | By breaker and protection relay with adjustable settings |
| Protection features | Basic, determined by fuse characteristics | Overcurrent, earth fault and reclosing functions |
| Footprint and cost | Compact and typically substantially cheaper | Larger and more expensive |
| Best suited to | Stations with a single transformer and radial feeders | Large hubs with multiple transformers or a ring network |
For most sites with a handful of chargers and one transformer, a ring main unit with a fused transformer feeder is the standard answer, and it integrates neatly into a compact substation enclosure. Where the connection point has a high fault level, where several transformers must be coordinated, or where the utility requires adjustable protection settings, a breaker-based panel is the correct choice — the same reasoning that governs the medium and high voltage switchgear used in any distribution project. Insulation technology is a separate decision again, and the trade-offs are set out in our comparison of AIS and GIS switchgear.
Low Voltage Distribution and Protection
The LV side is where most site-specific requirements concentrate.
- Transformer type: dry-type cast-resin units with a suitable harmonic rating suit indoor and urban installations because they avoid oil containment; oil-immersed units remain common where the substation is outdoors and cost-driven.
- Residual current protection: DC fast charging requires type B residual current devices, because standard type A devices may not detect smooth DC fault currents from the charger.
- Surge protection: coordinated devices at the main LV panel and at charger inputs, appropriate to the lightning exposure of the site.
- Power quality: power factor correction or active harmonic filtering where the charger group cannot meet the utility’s requirements at the connection point.
- Earthing: a defined system with separate neutral and protective earth conductors, equipotential bonding across all accessible metalwork, and an earth electrode resistance kept low — commonly within a few ohms.
- Busbar and cable sizing: sized for continuous duty, with feeder runs long enough to matter checked for voltage drop so that chargers do not trip on under-voltage at start-up.
Prefabricated Substations for Compact and Fast Sites
Charging sites are frequently constrained by land, planning conditions and programme. Integrating the MV switchgear, transformer and LV distribution into a single factory-assembled, weatherproof enclosure removes most of the on-site electrical construction, shortens the critical path and makes the installation removable if the site lease ends. A compact ring main unit feeding the transformer, paired with LV assemblies and an outdoor distribution enclosure such as the outdoor low voltage distribution cabinet, is a common arrangement; the wider low voltage switchgear range covers the outgoing feeder protection and metering.
Site Selection and Grid Constraints
Power availability, not real estate, is usually the binding constraint. Before committing to a site, establish the available connection voltage and approved demand, the distance to the point of connection, the prospective fault level, and the utility’s lead time for a new service or upgrade — the last of these can run to many months and will govern the opening date.
Where the grid cannot deliver the required peak economically, a buffered arrangement using battery storage, on-site generation or solar canopies reduces the connection size and the demand charge; all of them add their own interconnection and protection requirements, so they belong in the design from the start rather than being retrofitted.
Conclusion
A charging station is designed from the load backwards: establish the real simultaneous demand, size the transformer with proper margin, choose ring main unit or breaker panel according to fault level and coordination needs, then build the LV side around continuous duty, type B residual current protection and clean earthing. Get that order right and the site will deliver its rated output on day one and accept additional chargers later without a rebuild.
If you are planning a charging hub, send us the charger schedule, simultaneity assumptions and available utility supply, and we will help size the MV and LV distribution and confirm the configuration that fits the site.
FAQ
What switchgear does an EV Charging Stations Switchgear need?
Medium-voltage switchgear — usually a ring main unit with a fused transformer feeder for smaller stations, or a circuit breaker panel where fault levels are high or multiple transformers must be coordinated — plus low-voltage switchgear for the incoming supply, charger feeders, power factor correction and surge protection.
Should I size the transformer on the sum of charger ratings?
No. Apply a simultaneity factor, divide by charger efficiency and power factor, add the auxiliary load, and allow a margin of at least 20%. Managed sites with power sharing can use a significantly smaller transformer than the arithmetic total suggests.
Why is type B residual current protection required for fast charging?
DC fast chargers can produce smooth DC fault currents that type A devices are not designed to detect. Type B residual current devices are required so that a DC earth fault is detected and the circuit is disconnected.
When should a breaker panel be chosen instead of a ring main unit?
Where the available fault level is high, where several transformers or transformer feeders need coordinated protection with adjustable settings, where a ring network is used, or where the utility requires relay-based protection and remote indication.
How can the utility demand charge be reduced?
By managing the site peak: sequencing sessions through power sharing and charge scheduling, buffering with battery storage, or adding on-site solar. All three reduce the highest monthly peak, which is the value the demand charge is based on.
Can a charging station be built in a container or compact enclosure?
Yes, and it is often the fastest option. A prefabricated enclosure integrating the MV switchgear, transformer and low-voltage distribution is factory assembled and tested, which reduces civil works and site time considerably.
