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Pad mounted transformer sizes
The twelve standard three phase ratings under ANSI/IEEE C57.12.34. Figures are typical for liquid-filled units at 65 C rise and vary by manufacturer, so confirm against the approved drawing before a pad is poured.
| Rating | Dimensions, in | Weight, lb | Impedance | 480 V amps | Typical use |
|---|---|---|---|---|---|
| 45 kVA | 48 x 46 x 52 | 1,350 | 3.5% | 54 | Small retail pad, well pump, cell site |
| 75 kVA | 52 x 50 x 55 | 1,750 | 4.0% | 90 | Strip retail, small office, restaurant |
| 112.5 kVA | 55 x 52 x 57 | 2,050 | 4.2% | 135 | Medium retail, light industrial shop |
| 150 kVA | 58 x 54 x 60 | 2,400 | 4.5% | 180 | Office building, grocery, small school |
| 225 kVA | 62 x 58 x 63 | 3,000 | 5.0% | 271 | Mid-rise office, larger school |
| 300 kVA | 66 x 60 x 66 | 3,500 | 5.0% | 361 | Light manufacturing, apartment block |
| 500 kVA | 72 x 66 x 72 | 4,900 | 5.5% | 601 | Manufacturing plant, large retail |
| 750 kVA | 80 x 72 x 78 | 6,600 | 5.75% | 902 | Heavy industrial, small data hall |
| 1000 kVA | 86 x 76 x 82 | 8,200 | 5.75% | 1,203 | Industrial plant, data centre feed |
| 1500 kVA | 96 x 84 x 90 | 11,500 | 6.0% | 1,804 | Large industrial, campus feed |
| 2000 kVA | 104 x 90 x 96 | 14,500 | 6.0% | 2,406 | Large data centre, heavy plant |
| 2500 kVA | 112 x 96 x 102 | 17,500 | 6.0% | 3,007 | Top of the C57.12.34 range |
How to pick a rating
Secondary amperage is the fastest check. Divide the connected load in kVA by 1.732 times the secondary voltage in kV. At 480 volts a 300 kVA unit gives roughly 361 amps, which is why the table above carries that column.
Size to the actual demand with headroom, not to the sum of every nameplate on the job. Oversizing wastes money twice over, once on the unit and again on losses that run every hour for thirty years. Undersizing is worse, because a transformer at its limit runs hot and a hot transformer dies early.
What the columns mean
- Dimensions are the cabinet, not the pad. The concrete is larger on every side and the serving utility usually specifies it. More on dimensions
- Weight decides the rigging. Past about 3,000 lb a forklift is no longer the right tool and a crane goes in the freight quote.
- Impedance drives fault current and voltage drop, and any coordination study will ask for it. It climbs with rating.
- 480 V amps is full load secondary current at 480Y/277, the standard US commercial service.
Why these twelve
ANSI/IEEE C57.12.34 covers three phase pad-mounted units up to 2500 kVA, and the ratings in it are the ones manufacturers actually tool for. Asking for a non standard rating means a custom build, a longer lead time and a price that reflects both. If a job lands between two standard sizes, taking the larger one is almost always cheaper than a special.
Figures here are typical and every manufacturer differs by a few inches and a few hundred pounds. A few inches is enough to make a poured pad wrong, so get the approved drawing for the unit actually being supplied before concrete is ordered. Send us the load and we will size it and quote it.