The most common type of underground transformer in the distribution system is

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Underground distribution typically uses pad-mounted transformers because they are designed for ground-level, outdoor installations and provide a secure, weatherproof enclosure for the transformer and connections. This makes maintenance and service access straightforward without exposing energized equipment to the public. The unit sits on a concrete pad, keeping it accessible for technicians while protecting it from moisture and tampering, and it can be connected directly to underground primary and secondary cables.

Pole-mounted transformers are used for overhead systems, not underground, so they aren’t the common choice here. Vault transformers require larger underground vaults and can be more expensive and less common for standard underground distribution. Padless configurations aren’t a practical option in typical distribution design.

So, the pad-mounted transformer is the standard choice for underground distribution because it combines safety, accessibility, and suitability for underground cable connections.