Electrical Systems - Manufacturer Requirements

Each ungrounded conductor used to carry current must be protected by a fuse or a circuit breaker. If used, a circuit breaker must be of the manually reset type rather than the automatic reset type. Additionally, the circuit breaker must be of the tripfree type, which means that the breaker will open the circuit even if the handle is held in the "on" position.

FIGURE 23 - Overcurrent Protection

FIGURE 23 - Overcurrent Protection (continued)

FIGURE 24 - Overcurrent Protection Exceptions

 

The circuit breaker or fuse required must be located in one of three places:

1. at the point where a conductor receives its power (see Figure 25a);
2. at the point where a smaller conductor is connected to a larger conductor (see Figure 25b); or
3. at the point where a circuit initially receives its power if the breaker or fuse is sized to protect the smallest conductor in the circuit. (See Figures 25c and 25d)

The schematic in Figure 25e shows these three methods of overcurrent protection.

Locations 1 and 2 actually describe a single location. The point in a circuit where a smaller conductor is connected to a larger conductor becomes the source of power for the smaller conductor. Basically, each conductor, noting the permitted exceptions, must be provided with overcurrent protection. This overcurrent protection may be located at a conductor’s power source or at the point where the circuit receives its power, as long as the current rating of the overcurrent protection device protects the smallest conductor. Circuits in which there is no reduction in conductor size must be protected at the circuit’s source of power, again noting the permitted exceptions.

For shore power installations, the source of power is the dockside or shore located receptacle. The shore power cable and the main power feeders from the shore power inlet on the boat to the main distribution panel are protected by overcurrent protections at the dockside or shore locate connection.

Circuits using conductors of different insulation temperature ratings resulting in the use of smaller gauge conductor to carry the same or larger rated current must have overcurrent protection sized to protect the conductor with the lowest amperage rating. For example, if a 10 AWG supply conductor (60° C) is connected to a 14 AWG (125° C), the conductor amperage capacity is 40 amps in both cases and therefore one circuit breaker at the source end of the 10 AWG conductor is all that is needed.

FIGURE 25 - Location of Overcurrent Protection

FIGURE 25 - Location of Overcurrent Protection (continued)

NOTE:
Up to 40" is allowed if the conductor, throughout this distance, is contained in a sheath or enclosure such as a junction box, control box, or enclosed panel.

A circuit breaker or fuse must be sized so as not to exceed the permitted percentages of the current (ampere) rating of the conductor it is to protect as permitted in this section of the regulation. The current rating of the conductors must be taken from Table 5, including correction factors as applicable, as referred to in 183.425 and 183.435. The voltage of the circuit must be known in order to properly size the overcurrent protection.

(1) FOR LESS THAN 50 VOLTS

The breaker or fuse rating cannot exceed the Table 5 value by more than 150% (i.e. 1.5 times the table value). See Example A below.

(2) FOR 50 VOLTS OR MORE

The breaker or fuse should correspond exactly to the Table 5 value (See Example B). If, however, a standard rating of breaker or fuse does not correspond to the Table 5 value, then the next larger size breaker or fuse may be used, provided it does not exceed 150% (1.5 times) the Table 5 value (see Example C).

EXAMPLE A - Less Than 50 Volts

For a No. 10 AWG 90° C conductor in an engine room, Table 5 gives a value of 45.1 amperes (55 X .82 correction factor). 150% of this value is 67.65 (45.1 X 1.5). The MAXIMUM breaker current rating would be 67.65 amperes. In practical terms, a 60 or 65 ampere device would be chosen.

EXAMPLE B - 50 Volts or More

For a No. 6 AWG 105° C conductor in an engine room, Table 5 gives a value of 102 amperes (120 X .85 correction factor). If a standard size breaker or fuse is available in this rating, it should be used.

EXAMPLE C - 50 Volts or More

For a No. 6 AWG 80° C conductor run with two other current carrying conductors, in circuits of 50 volts or more, in an engine room, Table 5 gives a value of 54.6 amperes (100 X .70 correction factor X .78 correction factor). As the next larger standard breaker or fuse rating is 60 amperes, this value may be used provided it does not exceed 150% of the table value. The 150% value would be 81.9 amperes (54.6 X 1.5) and, as 60 amperes is LESS than this, then a 60 ampere breaker or fuse is the maximum that may be used. Of course, a lower rated circuit breaker or fuse may be used, as could a larger conductor.The boat manufacturer will have to determine the circuit loads in the boat in order to properly size conductors and the related overcurrent protection. If a conductor is supplying a single load, sizing the conductor and its overcurrent protection is simple. If, however, a single conductor is supplying multiple or cumulative loads, such as a distribution panel, the boat manufacturer does not have to size the conductor to carry the summation of all the loads connected to the panel, but only a percentage of these loads, due to loading factors. The regulation does not require that a conductor be sized to carry the full load of a distribution panel, but does require that proper overcurrent protection be provided for the size of conductor chosen.

The boat manufacturer has a choice between the extremes of providing a conductor that can carry the full load, or, using a smaller conductor that is adequate because of load factors, but still large enough to avoid nuisance tripping of its overcurrent protection.

Of course, whichever size conductor is used, it must be protected with the proper size overcurrent protection. It must also be noted that 183.425(b) restricts the maximum load a conductor may carry to that specified in Table 5, except for intermittent higher currents as discussed under 183.425(b).

In addition to a current (ampere) rating, a circuit breaker or fuse also has a voltage rating. A circuit breaker or fuse must have a voltage rating equal to, or preferably greater than, the nominal voltage of the circuit it is protecting. This is to ensure that the device will operate properly when an overcurrent (too many amperes) situation occurs. See Figure 26.

NOTE:
Nominal circuit voltage means the named voltage — i.e. a 12 volt DC system may charge at about 13.8 volts, but the nominal circuit voltage is 12 volts DC.

FIGURE 26 - Voltage Rating of Overcurrent Protection

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