Wire Routing and Support

The 7–40–72 Rule

The 7–40–72 rule of the ABYC’s standard E-11 is entitled Overcurrent Protection and states: “Ungrounded conductors (positive power feed conductor) shall be provided with overcurrent protection within a distance of 7 inches (175 mm) of the point at which the conductor is connected to the source of power measured along the conductor.” That means that the fuse or circuit breaker must be located no more than 7 inches from the battery. There are some exceptions to the 7–40–72 rule, as follows:

Starter motor circuits are exempted from circuit-protection requirements, and as already stated, these are the only circuits on board that are exempted.

If a wire is connected directly to a battery terminal and is contained throughout its entire length in a sheath or enclosure (such as a conduit, junction box, control box, or enclosed panel), the fuse or circuit breaker should be placed as close as practical to the battery but no farther than 72 inches (1.83 m) away. A common question that arises over this exception is just what constitutes a sheath.

The black corrugated-plastic tubing used by most production boatbuilders fills the bill for sheathing and is a good, inexpensive choice.

A wire not connected directly to the battery terminal can be protected with a fuse or a circuit breaker mounted as far as 40 inches (1.02 m) from the point of connection as long as the wire is contained in a sheath. For example, a large stud on the starter-motor solenoid is commonly used to connect various components on the engine. Wires so connected can have fuses or circuit breakers located as far away as 40 inches from the stud as long as the wire is in a sheath—hence the 7–40–72 rule.

Battery chargers (covered in chapter 6) add a degree of complexity to the basic rule. Built-in battery chargers, engine-driven alternators, and even solar panels are all considered battery chargers, and the rules go like this: “Each ungrounded (DC positive) conductor connected to a battery charger, alternator, or other charging source shall be provided with overcurrent protection within a distance of 7 inches (175 mm) of the point of connection to the DC electrical system or to the battery.” This means that if the charger is connected to the battery and sheathed or enclosed, the fuse or circuit breaker can be as far as 72 inches away (1.83 m) from the battery. If the wire is sheathed, the 40-inch rule applies as long as the wire is not connected directly to the battery.

One additional exception that applies to many newer powerboats is that no fuse or circuit breaker is required on self-limiting alternators. This applies to most alternators with internal voltage regulators as long as the connection is not at the battery, the conductor is in a sheath, and the wire is no more than 40 inches long. The protection rating must be based on the maximum rated output of the alternator. Figures 4-17, 4-18, and 4-19 on page 58 illustrate the various allowable protection locations and the exceptions.

The above recommendations are intended to minimize the chance of an electrical fire on your boat. But following these recommendations won’t do anything to protect the equipment itself. That’s why many electrical appliances will also have a built-in fuse. However, just because the device has a fuse built in doesn’t mean you can wire it into your boat without supplying an additional fuse or circuit breaker as close to the battery as you can get it.

Wire Routing and Support

The wire routing and support standards of the ABYC are intended to keep electrical wires from getting burned, chafed, or soaked by bilge water. It’s important to think dynamically when installing any new wiring or repairing old wiring. Conditions change dramatically when your boat is underway in a rough sea with engine parts spinning away and exhaust at full temperature. Improperly routed wires and bundles of wire collected into harnesses can be damaged by spinning pulleys and shafts; harnesses without a little slack to allow for flexing of the engine mounts can be pulled apart; and the insulation on wires and

SINGLE BATTERY (See E-9.11)(See E-11.12.1.2) Overcurrent protection device (fuse or circuit breaker)

Battery 72" max

Cranking motor conductors (no length restrictions) 7" or 40" max

Starter Conductors to various loads as needed (no length restriction)

Fig. 4-17. Figure 15 from section E-11 on battery supply circuits, showing the location of overcurrent protection. (© ABYC)

NO BATTERY SWITCH (See E-9.11)(See E-11.12.1.2)

Battery

Cranking motor conductors (no length restrictions) 72" max

7" or 40" max

Starter Conductors to various loads as needed (no length restriction)

Fig. 4-18. Battery supply circuits with no battery master switch installed. (© ABYC)

DUAL BATTERY (See E-9.11)(See E-11.12.1.2)

Overcurrent protection device (fuse or circuit breaker)

72" max

Cranking motor conductors (no length restrictions)

7" or 40" max

Conductors to various loads as needed (no length restriction)

Fig. 4-19. Circuit protection in a dual battery installation. (© ABYC)

cables near a hot exhaust manifold can melt away from the heat.

The rules for routing wires are simple, and common sense will go a long way toward keeping things right.

Harnesses, unless they run through a conduit or built-in channel, need to be supported at least every 18 inches (45.7 cm).

AC and DC conductors should never be bundled together unless separated by sheathing.

Grommets must be used when running wires through cutouts in bulkheads and fiberglass panels. Some acceptable types are illustrated in figure 4-20.

Wires must be kept as high above the bilge as possible, and those wires that must live there (such as the bilge pump wiring) should have waterproof