30 CFR § 75.900-2
Approved circuit schemes.
The following circuit schemes will be regarded as providing the necessary
protection to the circuit required by §75.900
(a) Ground check relays may be used for undervoltage protection if
the relay coils are designed to trip the circuit breaker when line voltage
decreases to 40 to 60 percent of the nominal line voltage.
(b) One undervoltage device installed in the main secondary circuit
at the source transformer may be used to provide undervoltage protection for
each circuit that receives power from that transformer.
(c) One circuit breaker may be used to protect two or more branch
circuits if the circuit breaker is adjusted to afford overcurrent protection
for the smallest conductor.
(d) Circuit breakers with shunt trip, series trip or undervoltage
release devices may be used if the tripping elements of such devices are
selected or adjusted in accordance with the settings listed in the tables of
the National Electric Code, 1968.