46 CFR §111.50-3

1. (a)Purpose. The purpose of overcurrent protection for conductors is to open the electric circuit if the current reaches a value that will cause an excessive or dangerous temperature in the conductor or conductor insulation. A grounded conductor is protected from overcurrent if a protective device of a suitable rating or setting is in each ungrounded conductor of the same circuit.
2. (b)Overcurrent protection of conductors. Each conductor must be protected in accordance with its current carrying capacity, except a conductor for the following circuits which must meet the following listed subparts:
   1. (1)Propulsion circuits, subpart 111.35.
   2. (2)Steering circuits, subpart 58.25 of this chapter.
   3. (3)Motor circuits, subpart 111.70.
   4. (4)Flexible cord and fixture wire for lighting circuits, subpart 111.75.
   5. (5)Switchboard circuits, subpart 111.30.
3. (c)Fuses and circuit breakers. If the allowable current-carrying capacity of the conductor does not correspond to a standard rating for fuses or circuit breakers that meets Section 240.6 of NFPA 70 (incorporated by reference; see 46 CFR 110.10-1), then the next larger such rating is acceptable, except that:
   1. (1)This rating must not be larger than 150 percent of the current-carrying capacity of the conductor; and
   2. (2)The effect of temperature on the operation of fuses and thermally controlled circuit breakers must be taken into consideration.
4. (d)Parallel overcurrent protective devices. An overcurrent protective device must not be connected in parallel with another overcurrent protective device.
5. (e)Thermal devices. No thermal cutout, thermal relay, or other device not designed to open a short circuit may be used for protection of a conductor against overcurrent due to a short circuit or ground, except in a motor circuit as described in Article 430 of NFPA 70 or in IEC 60092-202:2016 (both incorporated by reference; see § 110.10-1 of this subchapter).
6. (f)Ungrounded conductors. A fuse or overcurrent trip unit of a circuit breaker must be in each ungrounded conductor. A branch switch or circuit breaker must open all conductors of the circuit, except grounded conductors.
7. (g)Grounded conductor. An overcurrent device must not be in a permanently grounded conductor, except:
   1. (1)An overcurrent device that simultaneously opens all conductors of the circuit, unless prohibited by § 111.05-17 for the bus-tie feeder connecting the emergency and main switchboards; and
   2. (2)For motor-running protection described in Article 430 of NFPA 70 or in IEC 60092-202:2016 (both incorporated by reference; see § 110.10-1 of this subchapter).