30 CFR § 77.509 - Transformers; installation and guarding.
(a) Transformers shall be of the totally enclosed type, or shall be placed at least 8 feet above the ground, or installed in a transformer house, or surrounded by a substantial fence at least 6 feet high and at least 3 feet from any energized parts, casings, or wiring.
(b) Transformer stations shall be enclosed to prevent persons from unintentionally or inadvertently contacting energized parts.
(c) Transformer enclosures shall be kept locked against unauthorized entry.
The following state regulations pages link to this page.