Articles, Catamaran Electrical Systems

An electrical power system on a boat is comprised of various electrical circuits. A circuit can be thought of as a path that if followed completely leads back to the starting place. An electrical circuit refers to a complete path for electrical current flowing through wires. An electrical circuit is formed by connecting a voltage source such as a battery bank or AC power source to one or more electrical loads (appliances that use electricity) by means of a conductor, typically copper wire. In a DC battery circuit battery voltage pushes electrical current through various electrical loads when they are switched on, using some of the energy stored in the battery to perform useful work. In an AC circuit the voltage source (shore power or a gen-set) pushes electrical current in a similar manner, again using energy in the process.

There are primary or main circuits, which typically include the main power sources on board (either DC or AC). There are also secondary or branch circuits, which are complete electrical paths within the primary circuits. For example, all electrical loads on the boat are individual secondary circuits. And finally there are internal circuits within each electrical or electronic appliance on board.

All electrical circuits need to be protected from too much current flow, which can cause wire conductors to overheat and wire insulation to burn, and can damage internal circuits in individual appliances. Protection of an electrical circuit is in the form of an intentional weak link, typically a fuse or circuit breaker, which the industry refers to as a circuit protection device (CPD). Circuit protection on board a boat must be taken seriously. A potentially devastating electrical fire can result when too much amperage travels through a wire and enough heat is generated to melt and burn the wire insulation and surrounding materials.

CPD’s are meant to protect against unexpected problems, and shouldn’t preclude proper wire sizing and adherence to proper electrical system design and installation practices. Sizing wire is beyond the scope of this article, but the process is relatively easy: you simply match the maximum sustained amperage in a given circuit and the total length of the circuit wiring with a proper wire size that will be safe and also prevent an excessive voltage drop for the type of appliance used. There are different wire sizes required depending on whether or not the wire is in an engine space, and how much voltage drop is allowable. Detailed guidelines are given in section E-9 of the ABYC Standards.

Sizing wire correctly, however, can’t protect against accidental grounding-through wire chafe, equipment failure or accidentally grounding a circuit while performing system maintenance-that temporarily allows a dangerous amount of current to flow. CPDs handle unsafe levels of current by opening the circuit, either through thermal devices or devices that sense a magnetic field created by excess amperage.

Types of CPDs

Fuses are strictly thermal devices that melt at a predetermined amperage. They are reliable and relatively inexpensive, although total cost includes the purchase of a fuse mounting block and a protective cover of some type, spares since fuses must be replaced after each overcurrent condition, and some form of circuit disconnect. Circuit breakers can be thermal or magnetic devices, or a combination of the two. Circuit breakers are typically more expensive than fuses, especially for high load circuits, but they also serve as circuit disconnects and since they are resettable the need to carry spares is not as critical.

Class T Fuses

This is the fuse recommended by most inverter manufacturers. It has extremely fast short-circuit response and a 20,000 Ampere DC Interrupt Capacity, and is rated for up to 160 VDC.

ANL Fuses

ANL fuses have a 6000 Ampere DC Interrupt Capacity and meet ABYC requirements for main DC circuit protection on large battery banks with a voltage up to 32VDC.

SEA Fuses

SEA fuses that are an economical choice for circuit protection between 100 and 300 amperes. They have a 2000 Amperes DC Interrupt Capacity and are rated for up to 32VDC.

MaxiTM Fuses

These fuses are the most economical choice for between 30 and 80 ampere circuit protection. They have a1000 Ampere DC Interrupt Capacity and a 32VDC Voltage Rating. They are inexpensive and widely available through automotive stores.

AGC & MDL Glass Fuses

Available in current ratings from less than one up to 30 amperes, these inexpensive fuses are used for branch circuits in a variety of applications. AGC models are fast-acting fuses, while MDL models are time-delay fuses for high inrush motor type loads.

Thermal Circuit Breakers

The T-1 Series CPDs from Blue Sea are thermally responsive bi-metal breakers combining switching and breaker function in one unit. They are available with ampere ratings from 25 to150 amperes, a Voltage Rating of 48VDC, and a 5000 Amperes @24VDC Interrupt Rating. Blue Sea’s standard Thermal Circuit Breakers are similar to the T-1 Series but have a 3000 Ampere DC Interrupt Rating, a 30VDC Voltage Rating.

Magnetic Circuit Breakers

Magnetic circuit breakers are available in a wide range of styles and ratings. There are standard DC and AC single pole circuit breakers used for protecting branch circuits in electrical distribution panels. Some low ampere models rated as “quick trip” are designed specifically for electronics. Double pole AC breakers are available to switch both hot and neutral legs of a 120VAC circuit or two hot legs of a 240VAC circuit. Standard magnetic circuit breakers typically have a 2000-3000 Ampere Interrupt Rating, although some models are available with a 5000 Ampere Interrupt Rating.

It used to be that only fuses could handle heavy DC loads, but high load circuits can now be protected with single, double or triple pole breakers such as the DC C-Series models from Blue Sea. In this series single C-Series breakers rated up to 100 amperes each are ganged to provide various levels of protection. Sizes range from 50 ampere single pole to 300 ampere triple pole models.


CPD Ratings

CPDs have ratings that help describe their intended purpose and how they function, as well as help customers make good decisions where and when to use them.

Amperage Rating

The amperage rating of a CPD refers to the amperage that is used to calculate the opening speed of the device, not the actual amperage at which the CPD will trip or “blow”. It usually takes an additional 20 percent or so of amperage above the rated value for the CPD to trip or “blow”.

Opening Speed or Delay

Opening speed, or delay, defines the relationship between the percentage a CPD is operating over its amperage rating and the length of time that will be required for it to open. The higher the percent of current flow to amperage rating, the faster the circuit protector will open.

Interrupt Rating

The Interrupt rating specifies how much current the fuse can safely handle in short circuit situations. Interrupt Ratings are related to total connected battery capacity for DC circuits, and system voltage and maximum amperage for AC circuits. See the ABYC tables I and II to determine what minimum interrupt rating is required. Some sample Circuit Interrupt Ratings from these tables are given below:
*for 12 and 24 volt DC systems:

Total Connected Battery CCA (cold cranking amps)
650 CCA or less 651-1100 CCA over 1100 CCA
Main Circuit Breaker     
1500A 3000A 5000A
Branch Circuit Breakers    
750A 1500A 2500A
Voltage Rating

The voltage rating specifies the maximum voltage for the circuit in which the fuse is used.

Sizing and Selecting CPDs

When choosing CPDs for your electrical power system, take it one circuit at a time. First choose whether you want a fuse or circuit breaker for each circuit. Circuits in an explosive area-gasoline engine rooms, battery compartments, and propane lockers-must be protected by a vapor-proof circuit breaker. Then check to see what Ampere Interrupt Rating is required by ABYC for the application (see ABYC Standards, section E-9). Next make sure the CPD is rated to open at an amperage greater than the maximum circuit load and less than the rated amperage capacity of the smallest wire in the circuit. It is also useful to know the maximum momentary or surge current that will be experienced in the circuit. Choosing CPDs that can withstand this surge and still offer the required protection means you’ll avoid nuisance tripping of the CPD. As a final check make sure the CPD’s voltage rating meets or exceeds the circuit voltage.

Locating CPDs on a boat

Before locating CPDs in your electrical system on board, it is important to understand the concept that CPDs are installed to protect individual wires in a circuit, and that they should be sized specifically for those wires. As secondary circuits get further from the batteries or other power sources, smaller gage wires are typically used and, therefore, smaller CPDs should be used.

Ultimately every wire on board would be protected, but that is impractical. In the DC side of an electrical power system ABYC has taken a reasonable approach to circuit protection by exempting mandatory CPDs from wires between batteries, battery switch and engine starter motor. A diagram illustrating CPD placement according to ABYC Standards is given in ABYC Standards, section E-9. You’ll notice in the diagram that CPDs are required within 7 inches of the battery switch and starter motor on wires leading to various loads. This dimension can be extended to 40 inches if the wires are enclosed in a sheath or other enclosure in addition to the wire insulation.

In the AC side of an electrical power system on a boat there should be a main circuit breaker at each AC power source-shore power inlet(s), generator output, and inverter output. In addition, there should be branch circuit breakers on every branch circuit on the boat. On boats with multiple sources of AC power, the main circuit breakers can be conveniently located in a single AC source selector panel such as those available from Blue Sea. This type of panel allows only one AC power source to be capable of supplying power at any given time. In this case the main circuit breakers also are serving as manual circuit disconnect switches.


About the Author

Kevin Jeffrey is a long-time multihull sailor, independent energy consultant, author and book publisher.  He is the author of Independent Energy Guide, a valuable resource for cruising mutihull sailors, and is the publisher of Adventuring With Children by Nan Jeffrey and the first three editions of the Sailor’s Multihull Guide.