Transformers
Pad mounted transformers
A pad mounted transformer stands at ground level with its connections in a lockable, tamper resistant cabinet. It serves underground distribution on North American style networks.
- Standard
- IEEE C57.12.34three phase, compartmental, self cooled
- Scope of that standard
- 10 MVA and smaller, 60 Hzhigh voltage up to 34.5 kV, low voltage up to 15 kV
- Feed
- loop or radialset by the network design
- Front
- commonly dead frontlive front on some older networks
- Placement
- outdoors on a pad
The unit sits on a concrete pad, with its high and low voltage connections behind locked doors in a compartmental cabinet. It is built to stand in public places without a separate enclosure, which makes it the standard arrangement for underground residential and commercial distribution on North American style networks.
The network the unit joins sets most of the specification. Loop or radial feed, the high voltage connection system, the fusing and the switching all have to match what the utility or site network already uses. Send an existing unit's nameplate and the network operator's standard.
The standard has grown with the market. IEEE's own record shows the 2004 edition covered units of 2500 kVA and smaller with a low voltage of 480 V and below. The 2009 edition reached 5 MVA, and the current 2022 edition 10 MVA with a low voltage up to 15 kV. A specification that cites an older edition is worth checking against the current one before it goes out.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Rating | Three phase, in kVA | The load, within the scope of the standard |
| Feed arrangement | Loop feed or radial feed | The network design |
| High voltage front | Dead front with separable connectors, or live front | The network's existing connection system |
| Switching | Internal load break switching where specified | How the network is operated |
| Fusing | Internal fuses, with current limiting fuses where specified | The protection philosophy of the network |
| Voltage | Primary and secondary to the network standard | The utility or site network |
| Insulating liquid | Mineral oil or ester | Environmental and fire requirements |
Applicable standards
| Standard | What it covers |
|---|---|
| IEEE C57.12.34 | Pad mounted, compartmental type, self cooled, three phase distribution transformers, 10 MVA and smaller, at 60 Hz, with high voltage up to 34.5 kV and low voltage up to 15 kV |
| IEEE C57.12.00 | The general requirements C57.12.34 builds on for liquid immersed units |
| IEEE C57.12.28 | Pad mounted equipment, enclosure integrity: conformance tests for the coating of cabinets that stand above grade, outdoors |
North American destinations also apply their own minimum efficiency rules for distribution transformers. Each quotation names the rule that applies at the destination.
What a pad mounted transformer request needs
- Loop or radial feed, dead front or live front, and the connector system the network uses
- Switching and fusing, as the network operates them
- The edition of IEEE C57.12.34 your specification cites
- The nameplate or model of an existing unit, where this is a replacement
Questions buyers ask
Can a pad mounted transformer be supplied to an IEC market?
Pad mounted units are designed around North American practice. On IEC networks a compact substation usually does the same job. Send the requirement for either, and we confirm which fits the destination.