The Basics of Your Boat's Circuitry

Regardless whether your boat is a runabout, a sailboat, or a motor yacht, the principles of a boat's AC electrical system are the same. In the simplest of terms, electricity is transmitted from a shore side power source to your boat through a shore cord that connects to the boat at an AC electrical inlet. In small boats, the basic system, the shore connection is a three conductor, 15 ampere, vinyl covered cord. The 15 ampere shore cord is little more than a 125 volt extension cord, usually supplying one device and having no branch circuits (Fig. 1)

Many mid-size boats use a 30 ampere, 125 volt electrical system. The 30 ampere shore cord and matching shore power inlet are fitted with threaded rings providing a watertight connection from the power source to the boat. From the shore power inlet, 30 ampere conductors lead to the boat's AC panel board, from which power is distributed through the boat by wiring systems called branch circuits (Fig. 2).

Some larger boats use a 50 ampere, 125 volt system. In both the 30 ampere and 50 ampere systems, the shore cord contains three conductors--the black conductor is ungrounded or "hot" and carries 125 volts of electricity; the white conductor is the grounded conductor or neutral, and the green conductor is the ground (Fig. 3)

In still larger yachts, a 50 ampere, 125/250 volt system is employed which has a shore cord containing four conductors--the white neutral conductor, the green grounding conductor, and red & black ungrounded conductors each carrying 125 volts. In this system, the two 125 volt conductors can be combined to provide 250 volts necessary for such large appliances as ranges and clothes dryers (Fig 4).

A simple equation enables us to clearly see the relationship between the size of the electrical system. This equation will approximate the work the system will be able to perform: volts x amperes = watts. A 15 ampere, 125 volt system has 1875 available watts, whereas the 50 ampere, 250 volt system has 12,500 available watts. A toaster oven uses approximately 1500 watts.
Because the boat's AC electrical system is polarized, all wires will be connected in the same relation to all terminals of the receptacles and fixed AC electrical equipment throughout the boat--white, neutral: green, ground: and the hot wire will be another color, either red or black. The polarized electrical system insures that the electricity in each circuit will flow in the same direction.
Most panel boards today are equipped with polarity indicator lights to assure that a reversed polarity situation does not exist at the shore side power source. The absence of priority indicator lights does not mean that your boat is not protected against reversed polarity. Check your boat's owner's manual to determine what polarity protection system you have.
  In a properly designed system, electricity enters the AC panel board through a main circuit breaker. The power is transferred to the various branch circuits by way of individual branch circuit breakers. The ampere rating of these branch circuit breakers is determined by the circuit that they control. Just as the size of the artery in your blood vascular system is determined by the organ it supplies. Typical breaker ratings are as follows. outlets, 15 ampere; refrigerator, 10 ampere: water heater, 20 ampere: range, 20 ampere: and battery charger, 15 ampere.

The circuit breaker automatically interrupts the flow of current, if the current exceeds the amount the circuit is designed to handle. In this way it prevents the generation of heat that could cause fire and it prevents shock if the circuit is shorted. Remember that a ''tripped" circuit breaker is an indication that a problem exists in that particular circuit. The problem could simply be that the circuit is overloaded--too many appliances operating on that circuit. On the other hand, a tripped circuit breaker may indicate a problem within an appliance, a piece of equipment, or within the circuit itself causing a short circuit. Determine the cause of the tripped breaker and correct the problem before attempting to reset the circuit breaker. Familiarize yourself with the circuit breakers on your panel board and know what appliances or pieces of AC equipment each controls.

It is becoming more common for boats twenty-six feet and larger to be equipped with an auxiliary generator that provides an alternate source of AC electrical power while cruising or at anchor.

The generator is powered by a small gasoline or diesel engine and the electrical power generated is measured in kilowatts. The larger the kilowatt capacity of the generator, the larger the engine required to power the unit. For this reason, the size and weight of the auxiliary generator together with the space available aboard the boat will be limiting factors in determining what capacity generator can be installed.

Boats equipped with an auxiliary generator will have a three position, rotary switch that permits the boat owner to select "shore power", ''generator", or an ''off" position that cuts all AC electrical power to the AC panel board. The AC generator is connected to the AC panel board, through the selector switch to prevent the same portion of the system from being energized by the generator and shore power source at the same time.

When switched to ''generator," the panel board is receiving its power from the auxiliary generator and the AC equipment and appliances aboard the boat can be operated as if using a shoreside AC power source. Remembering that the toaster oven we referred to earlier in this chapter uses approximately 1500 watts (l.5 kilowatts), it is easy to see that a 3 kW generator (3,000 watts) is using 50% of its capacity to provide power for the toaster oven.
Working with a boat's electrical system is something many boat owners refuse to do, because they consider it mysterious. More than any other system on your boat, the AC electrical system is standardized. By having the proper tools, following the safety tips outlined earlier, and using only the finest marine electrical equipment, wiring your boat is relatively easy.

Perhaps one of the most common wiring projects that a boat owner will encounter, is replacing shore power cord plugs or connector. MARINCO makes a variety of plugs and connectors in 15A. 2O A. 30A. and 50A. Your MARINCO dealer will help you select the plug and connector suited to your boat and shore power source.

As mentioned, replacing a plug or connector is not a complicated procedure. First, be sure the shore cord is disconnected from the shore power source and the boat. It is best to cut the shore power cord approximately six inches behind the old plug or connector to insure that the conductors have no evidence of moisture or corrosion. After stripping the insulation from the cord about two inches, strip about 1/2" of the insulation from each conductor. The conductors should be bright in color and corrosion free. Insert the white wire (neutral) into the silver pocket of the device and tighten the silver or nickel captive terminal screw. Repeat this procedure placing the black wire (hot) into the black pocket and tightening the black terminal screw. Remembering to leave the ground wire about a 1/2" longer , insert it into the green pocket and tighten the green terminal screw.
Along these same lines, you may wish to assemble a "Y" adapter if your boat is equipped with two shore power cords.
The most common application of the "Y" adapter uses a 50 ampere, 250 volt power source and splits to two 30 ampere, 125 volt shore power cords. This arrangement provides the boat's electrical system with 12,500 watts of usable power. Keep in mind that the total amperage drawn should not exceed the amperage rating of the lowest rated component of the adapter. For example, do not impose an amperage load greater than 30 amperes per leg on The "Y" adapter outlined above.
MARINCO offers several styles of ''Y" adapters as well as blank "Y" cords that can be fitted with the plug and connectors required for your dock and boat. We will discuss more about adapters and ''Y" adapters in the section dealing with marinas.

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