Sizing a compact substation wrong is one of the most expensive mistakes in a distribution project. Undersize the unit and you get nuisance tripping, transformer overheating and no room to expand; oversize it and you pay for copper, switchgear and civil works your load will never use. This guide explains exactly how our engineers at Chuanli Electric size a compact substation — from connected load and demand calculation, through transformer kVA selection, to the enclosure, protection and layout — using IEC-based methods and a worked 500 kVA example.

What Is a Compact Substation (and When Is It the Right Choice)?

A compact substation — also called a prefabricated substation, box-type substation, package substation или unit substation — is a factory-assembled, skid- or cabin-mounted unit that combines three functions inside one enclosure:

• Medium-voltage switchgear — a ring main unit (RMU), load-break switch or vacuum circuit breaker for the incoming MV supply, typically rated 12 kV to 40.5 kV.
• A compact substation transformer — usually a distribution transformer from 100 kVA to 2500 kVA, oil-immersed or dry-type.
• Low-voltage distribution — an LV board with outgoing feeders, protection and metering.

Because the unit is assembled and type-tested in the factory to IEC 62271-202, site work is reduced to a foundation, cable terminations and commissioning. That is why compact substations are the default choice for solar and wind farms, mining sites, EV charging hubs, water-treatment plants, temporary construction power and any location where a conventional brick-and-mortar substation is too slow or too costly.

If you are still comparing architectures, start with our compact substation product range, which covers both European and American enclosure styles from 12 kV to 40.5 kV.

Step 1 — Collect and Group Your Connected Load

Sizing starts with data, not guesswork. Build a load schedule listing every load with its rated power (kW), quantity, voltage, power factor and duty cycle. Group the loads by type, because different loads behave very differently at the design stage:

Step 2 — Calculate Demand Load and Apparent Power (kVA)

Connected load is not the load the transformer actually sees. Apply a demand (diversity) factor to each group, then convert kW to kVA. The two formulas you need are:

Typical demand factors for planning purposes are shown below. Always confirm them against the actual operating regime of the facility.

This is the heart of any substation capacity calculation: get the demand load and the power factor right, and the kVA follows.

Step 3 — Select the Transformer kVA Rating

With the required kVA in hand, add spare capacity and round up to the next standard rating. We normally recommend 20–30% spare capacity for future expansion and to keep the transformer below its continuous thermal limit.

Standard distribution transformer ratings are 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000 and 2500 kVA. The table below maps common ratings to the load they can serve.

Do not forget derating. A transformer rated for 40°C ambient and 1000 m altitude may need to be upsized if the site runs hotter or higher. Cooling class (ONAN/ONAF) and temperature-rise limits also change the real continuous capacity. Our power and distribution transformers are supplied with full IEC 60076 test data so you can verify these margins before ordering.

Step 4 — Configure the Enclosure, Switchgear and Protection

Once the kVA is fixed, configure the internal architecture. The two mainstream enclosure families are:

• European type (IEC) — SF6 or vacuum RMU with a compact footprint, ideal for cable-fed urban and industrial networks. See the ZBW-12/0.4 European-type outdoor prefabricated substation.
• American type — pad-mounted design with a larger compartment, common where overhead or loop-feed distribution is standard.

Inside the enclosure you are specifying an MV switching device (load-break switch, RMU or circuit breaker), MV fuses or relays, the transformer, and an LV board with main breaker, metering and outgoing feeders. Protection coordination to IEC 62271-200 и IEC 61439 should be verified against the prospective short-circuit current at the point of connection. If a turnkey cabin assembly suits your project better, review our prefabricated cabin-type substation.

Step 5 — Plan Layout, Ventilation, Cabling and Installation

The final step is to confirm the physical envelope and site interfaces:

• Enclosure rating — IP23 to IP54 depending on indoor, outdoor or coastal exposure, with corrosion class to ISO 12944 for aggressive environments.
• Ventilation — natural or forced ventilation sized to the transformer losses; inadequate ventilation is the number-one cause of premature transformer ageing.
• Cable entry — bottom, front or side entry; confirm cable type, bending radius and gland plate positions early.
• Foundation and earthing — concrete pad or steel skid, with the earth mat bonded to the enclosure.

Because compact substations arrive pre-assembled and type-tested, on-site work is typically measured in days rather than weeks. For a real-world example of integrating a compact substation in a remote, challenging location, see our field guide on installing a mobile compact substation at a mountain mining site.

Compact Substation Sizing: Worked 500 kVA Example

A small manufacturing plant has a connected load of 400 kW and a plant-wide power factor of 0.88.

1. Demand load: 400 kW × 0.70 demand factor = 280 kW
2. Apparent power: 280 kW ÷ 0.88 = 318 kVA
3. Add 25% spare: 318 kVA × 1.25 = 398 kVA
4. Select standard rating: round up to a 500 kVA compact substation

This is exactly why the 500 kVA compact substation is one of the most common specifications worldwide: it comfortably serves a 300–400 kW demand while leaving room for expansion. Choosing a 400 kVA unit here would leave almost no margin; choosing an 800 kVA unit would waste capital on oversized switchgear and civil works.

Common Compact Substation Sizing Mistakes to Avoid

• Using connected load as demand load — this overstates the transformer by 30–50%.
• Ignoring power factor — at 0.7 PF you need 43% more kVA than at 1.0 PF for the same kW.
• No spare capacity — leaves no headroom for a second shift or added equipment.
• Forgetting derating — high ambient, altitude and enclosure class all reduce usable kVA.
• Mismatching protection — an oversized fuse or under-set relay protects nothing.
• Overlooking harmonics — VFD and rectifier loads can require a larger transformer and K-rated design.

Frequently Asked Questions (FAQ)

What size compact substation do I need?

Total your connected load in kW, apply a demand factor (typically 0.6–0.9), divide by the power factor to get kVA, add 20–30% spare, then round up to the next standard transformer rating. A plant with a 280 kW demand at 0.88 PF needs about 318 kVA, so a 500 kVA compact substation is the practical selection.

How do I calculate kVA for a compact substation?

Use the formula kVA = kW ÷ power factor. For example, 280 kW at a power factor of 0.88 equals 318 kVA. Always use the demand load, not the sum of all connected equipment, as the numerator.

What is the difference between a compact substation and a prefabricated substation?

They describe the same equipment. “Compact substation”, “prefabricated substation”, “box-type substation” and “package substation” are all factory-assembled units covered by IEC 62271-202, combining MV switchgear, a transformer and LV distribution in one enclosure.

What is a 500 kVA compact substation used for?

A 500 kVA unit typically serves commercial buildings, small factories, water-treatment plants and solar farm auxiliary loads with a demand up to roughly 400 kW. It is the most common mid-range rating because it balances cost, footprint and spare capacity.

Should I add spare capacity for future expansion?

Yes. Add 20–30% spare kVA at the design stage. Upsizing one step at the transformer is far cheaper than replacing the entire substation later, and it keeps the transformer below its continuous thermal limit during peak demand.

What voltage levels are available for compact substations?

Compact substations are commonly built for 12 kV, 24 kV, 36 kV and 40.5 kV primary voltages, with a low-voltage side of 400/415 V or 480 V depending on the regional standard. The MV switchgear is type-tested to IEC 62271-200.

Сколько времени занимает установка компактной подстанции?

Because the unit is pre-assembled and factory-tested, on-site work is usually completed in one to three days: setting the unit on its foundation, connecting MV and LV cables, earthing and commissioning. Complex or remote sites may take longer.

Why Trust This Guide

Chuanli Electric is a Chinese manufacturer of medium- and high-voltage switchgear and substations, supplying products built to IEC 62271, GB/T and ISO 9001 standards. We are a qualified supplier to China State Grid and have contributed equipment to major national projects including the Three Gorges hydropower scheme and the West-East Power Transmission programme. Every compact substation we ship is type-tested and delivered with full electrical and temperature-rise test data.

Related Resources

Continue building your substation knowledge with these guides from Chuanli Electric:

• Fast Hoisting and Integrated Installation Steps for Mobile Compact Substation on Mountain Mining Sites
• Switchboard vs Switchgear: What Is the Difference?
• Power Transformer Harmonic Filter Matching Tips for Concentrated Solar Power Plants
• Internal Arc Fault Suppression Technology of Medium Voltage Assembled Switchgear Cabinet
• Explore the full compact substation product catalogue