30 CFR § 75.701-2
Approved method of grounding metallic frames, casings and other enclosures receiving power from single-phase 110-220-volt circuit.
In instances where single-phase 110-220-volt circuits are used to feed
electrical equipment, the only method of grounding that will be approved is
the connection of all metallic frames, casings and other enclosures of such
equipment to a separate grounding conductor which establishes a continuous
connection to a grounded center tap of the transformer.