30 CFR § 77.900
Low- and medium-voltage circuits serving portable or mobile three-phase alternating current equipment; circuit breakers.
Low- and medium-voltage circuits supplying power to portable or mobile
three-phase alternating current equipment shall be protected by suitable
circuit breakers of adequate interrupting capacity which are properly tested
and maintained and equipped with devices to provide protection against
undervoltage, grounded phase, short circuit, and over-current.