MIL-HDBK-1038
5.5.5.16 Protective Devices. Motor branch circuits are required to be protected
in accordance with NEC Table 430-152. There are exceptions in NEC Section 430-52
which permit ratings or settings exceeding NEC Table 430-152's maximums so as to
accommodate standard ratings of protective devices and motors which have excessive
starting currents. Standard ratings for fuses and circuit breakers are listed in
NEC Section 240-6. If a circuit breaker is used for this protection, it is
required in NEC Section 610-42(a) to be an inverse time type; an instantaneous
trip breaker is not acceptable. If an inverse time circuit breaker with
adjustable settings is used to protect a motor branch circuit, the breaker is
required to have a rating of at least 115 percent of the motor full load current;
this is addressed in NEC Section 430-110. The maximum rating for an inverse time
breaker is dealt with above; the code does not address the instantaneous trip
setting of an inverse time breaker. The maximum rating or setting of protective
devices is listed in NEC Table 430-152 as a percentage of motor full load current.
Full load current is listed in NEC Tables 430-147 and 430-150 for DC and AC
motors, respectively. Full load current is required to be used rather than the
nameplate amperes of a specific motor so that the motor can be replaced with
another motor, having the same horsepower rating, without necessitating the
replacement of overcurrent protective devices; this is addressed in NEC Section
430-6. Although a crane motor is considered in NEC Section 610-43 to be protected
from overload by a properly selected branch circuit overcurrent device, the use of
overload relay(s) is required by NCC. Unlike other protective devices, motor
nameplate amperes are used to select the heater element in overload relays. This
is done for the protection of a specific motor and may necessitate the replacement
of the heater elements if that motor is replaced with another motor. The overload
relay is required to be selected to trip at no more than 150 percent of motor
nameplate amperes.
Control circuit conductors are required to have overcurrent protection as
addressed in NEC Section 610-53; however, they are considered protected by an
overcurrent device having a rating not in excess of 300 percent of the conductors'
ampacity.
Overcurrent protection of transformers is required as addressed in NEC
Section 450-3(b). The primary of a transformer is required to be protected by an
overcurrent device rated or set at no more than 125 percent of the rated primary
current. If the secondary of a transformer is protected by an overcurrent device
rated or set at no more than 125 percent of the rated secondary current, the
primary overcurrent protection device can be rated or set at no more than 250
percent of the rated primary current. There are exceptions in NEC Section 450-
3(b) and, for motor control circuit transformers, in NEC Section 430-72(c)
regarding the rating or setting of overcurrent protective devices. These
arrangements only provide protection for the transformer; required primary and
secondary conductor protection may have to be provided by other devices. NEC
Section 240-3(i) considers the secondary conductors of a single-phase transformer
having a 2 wire (single voltage) secondary to be protected if the primary side of
the transformer is protected in accordance with NEC Section 450-3(b) and the
overcurrent protection device's rating does not exceed the value determined by
multiplying the secondary conductor ampacity by the secondary to primary
transformer voltage ratio.
On bridge cranes, the disconnecting means is required to be sized for
simultaneous operation of bridge, trolley, and hoist drives.
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