If you are laying out an electrical room, the first question is rarely “which transformer” and almost always “will it fit”. Dry type transformer sizes are reasonably predictable: for the common 10 kV / 0.4 kV cast resin family, a 30 kVA unit occupies roughly 900 × 600 mm at floor level, a 1000 kVA unit roughly 1680 × 1030 mm, and a 2500 kVA unit roughly 2150 × 1250 mm — before any enclosure is fitted. Around those footprints you then need working clearance, ventilation height, a route to get the unit in, and a floor that will carry it.
This guide gives you the dimension chart from 30 to 2500 kVA, the clearance rules from both the IEC/GB tradition and the NEC tradition, and a room-sizing formula you can apply to a real project in a couple of minutes.
Three numbers decide whether a room works: the footprint (length × width), the height, and the weight. Weight matters as much as footprint, because it drives both the floor slab and the rigging plan — a 2500 kVA dry-type unit weighs around 5.2 tonnes, and it has to arrive through a doorway before it can sit on its plinth.
Two cautions apply to every chart you will find, including the one below. First, published dimensions usually describe the bare unit without an enclosure; adding an IP20 or IP23 metal enclosure typically increases each horizontal dimension by 100–300 mm and raises the top clearance requirement. Second, manufacturers build to slightly different core and winding designs, so a 10–20% spread between catalogues is normal. Treat the chart as a planning tool and the manufacturer’s outline drawing as the authority.
Dry-type transformers are built in preferred steps rather than continuously. In the 10 kV class the usual ladder runs 30, 50, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000 and 2500 kVA. If your load calculation lands between two steps, the decision is usually between the next standard size up and a custom rating — and standard is almost always cheaper and faster, even when it means a little unused capacity.
The chart below shows typical bare-unit dimensions and weights for 10 kV / 0.4 kV cast resin dry-type transformers, with the rail gauge needed for the foundation channels. Where a cell shows a range in practice, the values here reflect common mainstream designs.
| Rating | Length × width × height (mm) | Total mass (kg) | Rail gauge (mm) |
|---|---|---|---|
| 30 kVA | 900 × 600 × 950 | 380 | 550 × 550 |
| 50 kVA | 950 × 650 × 1000 | 480 | 550 × 550 |
| 80 kVA | 1000 × 680 × 1050 | 580 | 660 × 660 |
| 100 kVA | 1050 × 700 × 1100 | 650 | 660 × 660 |
| 125 kVA | 1100 × 720 × 1150 | 750 | 660 × 660 |
| 160 kVA | 1150 × 750 × 1200 | 850 | 820 × 820 |
| 200 kVA | 1200 × 780 × 1250 | 950 | 820 × 820 |
| 250 kVA | 1250 × 800 × 1300 | 1100 | 820 × 820 |
| 315 kVA | 1300 × 830 × 1350 | 1250 | 820 × 820 |
| 400 kVA | 1350 × 860 × 1400 | 1400 | 820 × 820 |
| 500 kVA | 1430 × 900 × 1450 | 1600 | 820 × 820 |
| 630 kVA | 1500 × 940 × 1500 | 1900 | 820 × 820 |
| 800 kVA | 1580 × 980 × 1550 | 2200 | 820 × 820 |
| 1000 kVA | 1680 × 1030 × 1600 | 2600 | 1000 × 1000 |
| 1250 kVA | 1780 × 1080 × 1680 | 3100 | 1000 × 1000 |
| 1600 kVA | 1880 × 1130 × 1750 | 3700 | 1000 × 1000 |
| 2000 kVA | 2000 × 1180 × 1850 | 4400 | 1000 × 1000 |
| 2500 kVA | 2150 × 1250 × 1950 | 5200 | 1000 × 1000 |
Clearance rules come from two different families of standard, and international projects often have to satisfy both. The IEC/GB tradition comes from installation codes such as GB 51348 and expresses distances from the enclosure or bare unit to walls and doors. The North American tradition comes from NEC Article 450 and separates working clearance from separation from combustible material.
| Requirement | IEC / GB practice | NEC practice |
|---|---|---|
| Side and rear wall clearance, up to 1000 kVA | ≥ 0.6 m (600 mm) | Working clearance 36 in (914 mm) at the operating face |
| Side and rear wall clearance, 1250–2500 kVA | ≥ 0.8 m (800 mm) | Per manufacturer or authority having jurisdiction |
| Clearance to the door / operating face | ≥ 0.8 m up to 1000 kVA; ≥ 1.0–1.2 m above | 36 in (914 mm) |
| Separation from combustible material | Handled through enclosure IP rating | 6 in (152 mm) up to 112.5 kVA; 12 in (305 mm) 113–1000 kVA; 18 in (457 mm) 1001–2000 kVA |
| Unit-to-unit spacing, same room | ≥ 0.8 m between enclosures | 36 in (914 mm) up to 112.5 kVA; 48 in (1219 mm) 113–1000 kVA |
| Top clearance for ventilation | ≥ 600 mm to slab, tray or pipework | 12 in (305 mm) minimum |
| Bare unit versus enclosed | Open units need a 1.8 m guard fence with ≥ 0.6 m to the unit | Guarding per local code |
Numbers in a table are a starting point, not a design. Where a local authority imposes stricter distances, or where the room also contains switchgear, the binding figure is the largest one that applies. If your layout has to be signed off by a consultant, our dry type transformer specification guide sets out the associated environmental, climatic and fire classes that usually accompany the clearance drawing.
Use the unit’s footprint plus the applicable clearances. The relationships are simple.
Bare unit 1680 × 1030 × 1600 mm, mass 2600 kg. Apply 600 mm to the rear and sides and 800 mm at the operating face, with 600 mm above for ventilation.
Bare unit 2150 × 1250 × 1950 mm, mass 5200 kg. Apply 800 mm to the rear and sides and 1200 mm at the door, with 600 mm above.
Add 20–25% to the floor area if you expect a future capacity increase or need space for cable trays and conduit bends. Resizing a room later costs far more than the extra square metres.
Distributed load is simply mass divided by footprint area. The 2500 kVA example above gives 5200 kg over 2.69 m², roughly 1930 kg/m², so the slab and plinth must be designed for that plus a safety factor for rigging, jacking and dynamic effects during placement.
The access path fails more projects than the room itself. Check the narrowest point on the route — lift door, staircase landing, corridor corner, roller shutter — against the unit’s real envelope, including lifting lugs and any removable panels.
| Access point | Typical opening | Practical limit |
|---|---|---|
| Standard single door | 2030 × 915 mm | Roughly up to 300 kVA three-phase |
| Double door | 2030 × 1830 mm | Roughly up to 2000 kVA |
| Roll-up or goods entrance | 3050 mm and above | All standard sizes, subject to weight capacity |
| Freight elevator | Varies, check nameplate | Limited by rated load, not just by door size |
Clearance and ventilation are the same problem seen from two directions. A cast resin transformer rejects heat by convection, so a room with inadequate air change will see winding temperatures climb even when the load is within rating. Common practice for naturally ventilated rooms is to provide inlet and outlet openings sized from the transformer’s total loss, with the outlet at high level so hot air can leave rather than recirculate. Where natural ventilation cannot achieve this, forced ventilation is added — and if the room is enclosed, a forced-air-cooled unit is often the more honest choice than pretending natural convection will cope.
Keep the design ambient at or below 40 °C, with relative humidity controlled enough to avoid condensation on epoxy surfaces. Basement rooms with persistent dampness need dehumidification, because moisture on insulation surfaces is a slow, silent route to reduced dielectric strength.
Clearance planning should also stay in step with the electrical design. If the transformer is going into a plant with high harmonic content, the sizing decision changes, and our guide to dry type transformers for industrial applications covers how derating and K-factor selection interact with physical layout.
Dry type transformer sizes run from about 900 × 600 mm at 30 kVA to 2150 × 1250 mm at 2500 kVA for bare 10 kV cast resin units, with weights from roughly 380 kg to 5.2 tonnes. Add the applicable clearances, a 600 mm ventilation band above the unit, a 20–25% area margin and a workable access route, and you have a room that will not need redesigning. Confirm the final envelope against the manufacturer’s outline drawing before foundations are cast — that drawing, not a catalogue table, is what the site will build to.
If you are still choosing between technologies for a constrained indoor space, the comparison in our SCB series dry type transformer technical data and our guide to selecting the right dry type transformer both give you the decision framework, including what can be customized in a dry type transformer when a standard footprint does not fit the room you have.
Standard three-phase dry-type ratings step through 30, 50, 100, 160, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000 and 2500 kVA in the 10 kV class, with 630 kVA and 1000 kVA among the most frequently specified.
In IEC/GB practice, at least 0.6 m from the side and rear walls for units up to 1000 kVA and 0.8 m above that, with at least 0.8–1.2 m at the operating face. NEC practice applies a 36 in (914 mm) working clearance at the operating face and 6–18 in from combustible material depending on rating.
For a bare 1680 × 1030 × 1600 mm unit with 600 mm side and rear clearance and 800 mm at the door, plan around 2.9 × 2.4 m of floor area and 2.2 m clear height. Add roughly 20–25% if you expect future capacity growth.
Allow at least 600 mm between the top of the unit and any slab, cable tray or pipework so rising hot air can escape. NEC practice sets a 12 in (305 mm) minimum, but 600 mm is the more practical figure where heat accumulates.
Not always. Open units are permitted where access is restricted and a guard fence with adequate clearance is provided. An IP20 or IP23 enclosure is usual in accessible plant rooms, and IP54 where dust or moisture is present.
Roughly 380 kg at 30 kVA, 1100 kg at 250 kVA, 2600 kg at 1000 kVA and 5200 kg at 2500 kVA for bare 10 kV cast resin units. Enclosed versions and 35 kV primary units are heavier, so always confirm the as-built mass before designing the floor.
Yes, with the correct enclosure — typically IP23 or IP54 with weather protection. Standard indoor cast resin units are not weather-rated, and installing one outdoors without an appropriate enclosure voids the intent of the specification.
