Port Townsend Marine Electric -- Since 1985
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System Design

Major Components

Electrical System

The following was written to assist the architect and the builder of a 48 foot charter launched in 2002. It is included here for anyone designing the electrical system for a new boat or for rewiring an older boat.

The entire electrical system can be considered in three parts: AC, DC, and bonding. While there are connections among these systems, for the most part they stand alone.

AC System

The AC system begins at its sources, the shore power input receptacle and/or the genset, The shore power input receptacle should be located with the expected mooring in mind. The standard shore power cord is 50 feet long, although two cords can be joined if needed. Line loss becomes a concern beyond 100 feet. On larger boats, two shore power input receptacles may be desirable, with a switch selecting the receptacle in use. Input receptacles are best located where they are protected from rain and spray.

Some boats are wired with two shore power input receptacles such that both can be used simultaneously, thus doubling the power available. However, most marinas provide only one shore power receptacle for each slip.

The most common shore power configuration available is 120 volts/30 amps, which supplies 3600 watts of power. Other configurations seen are 120 volts/50 amps (6000 watts), 120/240 volts/50 amps (12,000 watts), 120/208 volts/50 amps, and 208 volts/three phase. In Port Townsend, there are a few locations where the higher power (120/208) is available. The voltage available at the expected mooring of the boat is an important consideration in choosing the shore power configuration, as well as the anticipated electrical demand of the boat while at the dock.

Following the shore power input receptacle, an isolation transformer may be installed.

The isolation transformer serves three functions. First, it eliminates all hard-wire connections between the boat and the dock (and, hence, other boats nearby), thereby significantly reducing the potential for galvanic corrosion. Second, an isolation transformer with multiple input and output taps can be used to produce 120/240 volts from either a 120 or a 240 volt source. Third, an isolation transformer with multiple input taps can boost low input voltage to a more acceptable level. Because an isolation transformer produces both heat and noise, it must be installed in a well-ventilated but unobtrusive location in reasonable proximity to the shore power circuit path.

Next, an inverter may be installed. Most inverters contain internal circuitry which automatically switches between the invert mode and the charge mode, depending upon whether AC power is available at the inverter AC input. If AC is available, the inverter charges those batteries to which it is connected while passing the remaining current through to supply AC loads through the AC Panel.

On a boat with a genset, however, the inverter is not installed in the incoming AC line, but is fed from the AC panel, so that the inverter can function as a battery charger using AC supplied by the genset. In such an installation, a second AC panel is required, or a single AC panel with a separate section for inverter-supplied loads.

Wherever the inverter falls in the circuit path, it must be installed in a well-ventilated location in close proximity (but not above) the batteries to which it is connected.

If a genset is installed, the next item in the AC system is the input selector switch, which determines whether shore power or the genset is the source for the AC panel. Frequently, AC meters (volts, amps, hertz) are installed in the same box as the selector switch. The switch and/or AC meters can be installed in the pilothouse, engine room, or some other convenient location.

Finally, an AC Panel or panels distributes the AC to the various receptacles and appliances around the boat. On a smaller boat, the AC panel is installed in the pilothouse or some other convenient location above deck. On a larger boat, the AC panel generally is installed in or near the engine room. An AC Sub-Panel may be installed in the pilothouse.


DC System

 The DC system begins with the batteries. There are numerous options for type and number of batteries, with the best choice being determined by the size and purpose of the boat, the size of the engine, and the preference of the operator. There are, however, a few general rules:

-- House loads are best supported by deep-cycle batteries.

-- Engine starting can be accomplished by either start or deep-cycle batteries, depending upon the size of the engine and the number of deep-cycle batteries installed.

-- A genset should have its own starting battery.

-- Batteries should be located as close as possible to the loads they will support. This is especially true of engine starting batteries and batteries supporting an inverter.

-- Boats used in charter service and most larger fishing boats must have a battery in the pilothouse dedicated to a VHF radio, a GPS or a loran, and other loads determined by the pertinent regulations.

After the batteries is the battery switch or switches. These must be located as close as possible to the batteries which they control while also being readily accessible. Since a battery switch functions in part as an emergency disconnect in case of fire, it is preferable to install it outside of engine spaces. A boat with a genset, an inverter, and a dedicated starting battery will have three battery switches:


1) genset battery (On/Off only)

2) starting battery (double-throw to enable paralleling with house batteries

3) house batteries (double-throw in order to isolate the inverter supply)


The following items are then installed in close proximity to the battery switches:

-- circuit breaker to protect the positive feed to the DC panel

-- fuse to protect the inverter and its positive feed wire

-- circuit breaker to protect the electric windlass (if installed) and its feed wire

-- fuse to protect the positive wire to the alternator if the alternator wiring is separate from the engine wiring harness

-- isolator or combiner to supply charge current to a pilothouse battery (if installed)

Finally, a DC Panel or panels is/are installed. On a smaller boat, a single DC panel is sufficient, installed in a location convenient for the operator. On a larger boat, two or more DC panels may be preferable, with one installed in or near the engine room.

Several other panels may be installed which are part of the DC system:

-- Engine panel(s): These are generally installed near the helm, although on larger boats there is often an additional engine panel installed in the engine room.

-- Genset engine panel: installed either near the genset or in the pilothouse

-- Genset output panel: installed either near the genset or in the pilothouse

-- Alarm panel: installed near the helm. Alarm stations include: engine alarms, bilge(s), engine room fire, galley fire, and refrigeration malfunction.


Bonding System

For many years the common practice was to connect all underwater metal in one low resistance circuit intended to keep all metal at the same potential., thus reducing galvanic corrosion. However, that approach has fallen out of favor, especially for wooden boats. Now, we protect only that metal which, due to its placement on the galvanic scale, must be protected, and we protect it with its own zinc anode without connection to a common circuit. However, the following connections still are required, not to prevent corrosion, but to reduce the hazards of fire and electric shock.

-- All battery negatives are connected.

-- The DC negative is connected to the AC ground. If no isolation transformer is installed, a galvanic isolator is installed to interrupt the DC path from boat to dock to other boats.


Specifications in greater detail than those provided in the discussion above are found in the ABYC guidelines.

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Port Townsend Marine Electric
PO Box 261 Chimacum, WA 98325
ph: (360) 732-4085 ¶ inthewoods@olympus.net