DC Overcurrent Protection (Fuses or Breakers)

[GordMay]

Excerpted from ABYC Section E-11
AC and DC ELECTRICAL SYSTEMS ON BOATS(Effective 2004)

11.12. OVERCURRENT PROTECTION

11.12.1. FOR DC SYSTEMS

11.12.1.1. Battery Charging Sources

11.12.1.1.1. Each ungrounded conductor connected to a battery charger, alternator, or other charging source, shall be provided with overcurrent protection within a distance of seven inches (175mm) of the point of connection to the DC electrical system or to the battery.

EXCEPTIONS: 1. If the conductor is connected directly to the battery terminal and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box or enclosed panel, the overcurrent protection shall be placed as close as practicable to the battery, but not to exceed 72 inches (1.83m).

2. If the conductor is connected to a source of power other than a battery terminal and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box or enclosed panel, the overcurrent protection shall be placed as close as practicable to the point of connection to the source of power, but not to exceed 40 inches (1.02m). Overcurrent protection is not required in conductors from self-limiting alternators with integral regulators if the conductor is less than 40 inches (1.02m), is connected to a source of power other than the battery, and is contained throughout its entire distance in a sheath or enclosure.

11.12.1.1.2. In addition to the provisions of E-11.12.1.1.1, the ungrounded conductor shall be provided with overcurrent protection within the charging source, or within seven inches (175mm) of the charging source, based on the maximum output of the device.

EXCEPTION: Self-limiting devices.

11.12.1.2. Overcurrent Protection Device Location - Ungrounded conductors shall be provided with overcurrent protection within a distance of seven inches (175mm) of the point at which the conductor is connected to the source of power measured along the conductor. (See FIGURE 15*)

EXCEPTIONS: 1. Cranking motor conductors.

2. If the conductor is connected directly to the battery terminal and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box or enclosed panel, the overcurrent protection shall be placed as close as practicable to the battery, but not to exceed 72 inches (1.83m).

3. If the conductor is connected to a source of power other than a battery terminal and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box or enclosed panel, the overcurrent protection shall be placed as close as practicable to the point of connection to the source of power, but not to exceed 40 inches (1.02m).

NOTE: See Section E- 11.16.4, Installation.

11.12.1.3. Motors or motor operated Equipment - Motors and motor operated equipment, except for engine cranking motors, shall be protected internally at the equipment, or by branch circuit overcurrent protection devices suitable for motor current. The protection provided shall preclude a fire hazard if the circuit, as installed, is energized for seven hours under any conditions of overload, including locked rotor.

NOTES: 1. It may be necessary to use thermally responsive protection devices on the equipment or system if the motor is not capable of operating continuously at maximum possible loading.

2. It may be necessary to test as installed in order to assure compliance with the locked rotor requirement. Voltage drop, due to wire size, and delay characteristics of the overcurrent protection device may have to be adjusted to protect the motor.

11.12.1.4. Non-motor Loads - The rating of overcurrent protection devices used to protect a load other than a DC motor shall not exceed 150 percent of the ampacity of its supply conductor. (See TABLE IV.)

11.12.1.5. Branch Circuits

11.12.1.5.1. Each ungrounded conductor of a branch circuit shall be provided with overcurrent protection at the point of connection to the main switchboard unless the main circuit breaker or fuse provides such protection.

11.12.1.5.2. Each fuse or trip-free circuit breaker shall be rated in accordance with E-11.12.1.3 and E-11.12.1.4 and shall not exceed 150 percent of the conductor ampacity in TABLE IV. (See FIGURE 15*)

11.12.1.6. Panelboards and Switchboards - A tripfree circuit breaker or a fuse shall be installed at the source of power for panelboards and switchboards, and shall not exceed 100 percent of the load capacity of that panel, or 100 percent of the current carrying capacity of the feeders.

EXCEPTION: The trip-free circuit breaker or fuse at the source of power may be rated at up to 150 percent of the conductor ampacity if there is a submain circuit breaker or fuse in the panelboard or switchboard that is rated at no more than 100 percent of the load capacity, or the feeder ampacity, whichever is less. (See FIGURE 16.)

11.12.1.7. Circuit Breakers

11.12.1.7.1. Circuit breakers installed in spaces requiring ignition protection shall comply with SAE J1171, External Ignition Protection of Marine Devices, or UL 1500, Ignition Protection Test for Marine Products. If internal explosion tests are required, the ignition of the test gas shall be created at four times the current rating of the device being tested.

11.12.1.7.2. Circuit breakers shall

11.12.1.7.2.1. have a DC voltage rating of not less than the nominal system voltage, and

11.12.1.7.2.2. be of the trip-free type, and

11.12.1.7.2.3. be capable of an interrupting capacity according to TABLE V, and remain operable after the fault,

EXCEPTION: Integral overcurrent protection in electrical devices.

NOTES: 1. A fuse in series with, and ahead of the circuit breaker, may be used to comply with TABLE V.

2. Consult the circuit breaker manufacturer to determine the fuse size and the type of fuse.

11.12.1.7.2.4. be of the manual reset type except as provided in E-11.12.1.9.

11.12.1.8. Fuses

11.12.1.8.1. Fuses shall have a voltage rating of not less than the nominal system voltage.

11.12.1.8.2. Fuses installed in spaces requiring ignition protection shall comply with SAE J1171, External Ignition Protection for Marine Devices, or UL 1500, Ignition Protection Test for Marine Products. If internal explosion tests are required, the ignition of the test gas shall be created at four times the rating of the fuse.

11.12.1.9. Integral Overcurrent Protection Devices - Integral overcurrent protection devices without a manual reset may be used as an integral part of an electrical device provided the rest of the circuit is protected by a trip-free circuit protection device(s) or a fuse(s).

11.12.1.10. Pigtails - Pigtails less than 7 inches (175mm) in length are exempt from overcurrent protection requirements.

* Figure 15 pictured at: http://cruisersforum.com/photopost//...php?photo=1650

Old Figures 11 & 12 from former Section E-9 (essentially the same)
http://cruisersforum.com/photopost//...php?photo=1648
http://cruisersforum.com/photopost//...php?photo=1649

thanks