Acceptable Locations for Fuses and Circuit Breakers

protection. So, why are motors different from anything else electrical found on board?

The answer to this question lies in the two major differences between motor circuits and other circuits. First, as already mentioned, motors will draw a considerable amount of additional current when they first start up. Once the motor is running, the current tapers off to a more reasonable and considerably lower level. This start-up current must be accounted for, but a more serious possibility is a locked-rotor condition that occurs when the motor is getting power but the armature is prevented from turning. This can be caused by corrosion in the motor housing or bearings, or it can happen when a bilge-pump motor gets a piece of debris jammed in the pickup or impeller of the pump. When this happens, the flow of electrons increases dramatically.

The ABYC standards provide that the circuit protection preclude a fire hazard if the motor circuit is energized for seven hours under any conditions of overload, including locked rotor. The best way to make sure you’re complying with this rule is to carefully follow the installation recommendations provided by the manufacturer. If you don’t have the printed recommendations for the motor, call the company and ask for them. The only alternative is to test amperage drawn by the motor while it’s in a locked-rotor condition, and this is not a very practical solution for the average boater.

Nonmotor Circuits

The ABYC standards address circuit protection for nonmotor loads more clearly than they do protection for motor circuits. Basically, the rating of the fuse or circuit breaker used on a nonmotor circuit must not exceed 150 percent of the maximum amperage of the smallest conductor feeding the appliance. Odds are good that when you determine the current of an appliance, you’ll discover that you can’t buy a fuse or breaker that falls at exactly the 150 percent value. The key words here are must not exceed. I generally work in the range of fuses and breakers that are between 115 to 150 percent of the total amperage-handling capabilities of the circuit I am trying to protect and can always find a match.

Distribution Panels

Distribution panels and switchboards fall into a slightly different category than regular on-board equipment. The protection ratings for distribution panels that supply multiple branch circuits are designed to protect not only the panel but also the primary-feed conductor to the board. This may be an important consideration if your boat’s original distribution panel has blank sockets where more equipment could be added.

In general, in order to save money and weight, boatbuilders try to use the smallest wire sizes that they can get away with. If your boat came through with, let’s say, three blank holes in the distribution panel where additional circuit breakers or fuses could be installed, you could have a problem. The boatbuilder may have rated the wire going from the battery to the panel for the loads he installed without any consideration for reserve capacity. As soon as you add anything to the panel, you risk exceeding the capacity of the feed wire.

The ABYC recommendations for dealing with this situation are clear: “A trip-free 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.” There is an exception to this rule that will apply to many newer boats: the fuse or circuit breaker for the wire that connects the battery to the distribution panel may be rated at up to 150 percent of the capacity of the wire if the panel is equipped with a submain circuit breaker rated at no more than 100 percent of the load on the panel. Figures 4-15 and 4-16 on page 56 show these possibilities and the allowable ratings.

The next consideration you’ll need to make if you’re adding electrical equipment to your boat is where to locate the fuse or circuit breaker. Not all fuses and breakers are mounted on the main distribution panel, so some rules for placement of these devices are needed.

MAIN AND BRANCH CIRCUIT PROTECTION

Battery Switch Main Circuit Protection Feeder

Main Breaker Main Breaker Main Breaker See E-9.11.3.1 E-11.7.1.2.1 Exception 2

Panelboard Sub-main Breaker Branch Breaker Branch Breaker Branch Breaker

Fig. 4-15. With this installation (submain breaker installed) the main circuit protector can be rated up to 150 percent of the feeder wire ampacity. (© ABYC)

Fig. 4-16. Without a submain breaker (this panel has none), the feeder protection should be rated at no more than 100 percent of the feeder ampacity. (© ABYC)

Feeders

Protection at Source of power

Distribution panel panelboard or switchboard

Source of power (battery)