Articles, Catamaran Electrical Systems

Beyond the glamour of high-output alternators, solar and wind chargers, battery banks and system monitors are the hidden essentials of any respectable marine electrical power system, those heavy-duty DC circuit components that provide safe, efficient wire termination points and switching ability. Long ignored by boat owners, boat manufacturers and electrical system suppliers, these important pieces of gear are now standard equipment on most high quality marine installations.

I use the term “heavy duty” to describe this class of electrical devices, not to imply that they all have the same degree of robustness, but to focus on devices that are designed and built to perform under high amperage conditions in rugged marine environments.

Wire Termination Devices

The first heavy-duty DC circuit components to plan for are wire termination devices, which include heavy duty electrical busbars, distribution posts, and terminal blocks. In the past battery posts, battery switch terminals, and other ad hoc connection points were used for attaching high-amperage positive and negative wires in an electrical system. Now specially made wire termination gear provides convenient, safe and orderly methods of connecting wires and cables carrying high levels of DC current.

Heavy Duty Busbars

Heavy Duty DC electrical busbars are solid blocks of conducting material (typically tin-plated brass) with multiple wire connection points. These connection points can be some combination of individual studs and/or screw terminals for attaching positive or negative wires in a DC circuit. All termination points on a busbar are electrically connected, which means the busbar itself acts as a large jumper from one connection point to the next. Therefore, busbars are best suited for connecting common groups of positive or negative wires from either power sources or loads. Special busbars for appliance ground wires are also available.

The conducting material on a busbar is fastened to a non-conducting base block made of ABS or other plastic, which in turn is used for mounting to a bulkhead or other convenient surface.

Heavy duty busbars are available in a wide range of styles and have current ratings from 100 to 600 amps. Some models have only stud termination points (typically four). Wires and cables must have ring lugs or terminals correctly sized to fit over the stud without being oversized and risking poor electrical contact. Some heavy duty busbars have two studs (one at each end) with smaller screw terminal connection points in between, and some busbars with lower amperage ratings have only screw terminals.

Dual busbars with screw terminals are also available for terminating both positive and negative wires on a single mounting block. Dual buses have one common bus for terminating positive wires and a second common bus, electrically isolated from the first, for terminating negative wires.

Heavy duty busbars are rated according to their continuous current capacity and their maximum DC voltage. Since busbars can also be used for AC systems, a rating for AC maximum voltage is also given. When selecting busbars, make sure to select models with a sufficient current and voltage rating for your needs.

Busbars have some advantages over single-stud distribution posts. One is that they can handle more wire terminations on a single piece of gear. The potential for poor electrical contact increases with the number of wires connected to a single post or stud. To address this point, ABYC standards state, “No more than four conductors shall be secured to any one terminal.” Another plus for busbars is that they allow wires of varying size to be connected on the same piece of gear, and allow wires to be arranged in a more orderly pattern.

The connections on a busbar should always be protected from accidental contact by a non-conducting cover of some type. Some busbars on the market have specially made plastic covers for this purpose.

Distribution Posts

Distribution posts provide a safe, secure connection point for up to four high amperage cables on a single post. Distribution posts are not typically rated for amperage since current flow is between the various conductors and their terminals, not the post itself. They are, however, rated for maximum DC voltage. Distribution posts for the marine market are typically rated up to 48 volts DC.

Some distribution posts also provide low-amperage screw terminals around the base for connecting conductors from controls, displays, monitor shunts and appliances with small electrical loads. In certain applications this piece of gear can eliminate the need for an additional post or busbar.

As with busbars, distribution posts typically have a mounting base made of non-conducting material, with recessed mounting screw holes to avoid contact with the conductors on the post. Some models come with a rubber or plastic insulating boot color coded for positive (red) or negative (black) wires. The boot is only good, however, for one or two wires. Terminal protection for the other wires on the post is often left to the customer.

Terminal Blocks

Terminal blocks provide a convenient common connection point for wires in multiple DC circuits. Using a terminal block, individual circuits in a wire harness can be terminated and rerouted to either an appliance, a power source, or a monitoring or control device.

In a typical terminal block there are individual pairs of screws serving as circuit connection points for incoming and outgoing conductors. These isolated screw pairs-isolated by the mounting base itself and by raised plastic separators between circuits- can easily be joined by a properly rated metal jumper to give the required number of connection points for each circuit.
Terminal blocks typically have a closed back design, which means the electrical power at the screw terminals is completely insulated from the mounting surface.

In the next installment of this column we’ll review the heavy duty battery switching devices currently on the market, and how best to use them.

In the last issue we discussed heavy duty wire termination devices such as busbars, distribution posts and terminal strips. In this issue we’ll take a look at the various heavy duty battery switches currently available and how best to use them.

Heavy Duty Battery Switches

For many years the ubiquitous 1-2-BOTH-OFF battery switch was the only heavy duty switch on a boat. Now there are several varieties to chose from allowing boaters to perform a wide range of tasks.

1-2-BOTH-OFF Battery Switches

1-2-BOTH-OFF battery switches, supplied with most standard production boats, are primarily intended for electrical systems which have two battery banks, with each bank used for both house loads and engine starting. The switch directs which battery bank accepts the charging current (from alternator(s), renewable chargers, etc.) and which battery bank supplies current to the loads, and for disconnecting the batteries completely from the rest of the electrical system.

This type of battery switch has three main cable connection points in the form of heavy duty threaded studs. One stud is for a positive cable from positive side of battery bank 1, one stud is for a positive cable from the positive side of bank 2, and one stud is a common terminal. Typically the electrical charging sources and loads on board are connected to the common terminal. When the switch is in Position 1, everything that is connected to the common terminal is linked to battery bank 1. When the switch is in Position 2, everything that is connected to the common terminal is linked to battery bank 2, and in the BOTH position the common terminal is linked to both battery banks, which are temporarily connected together in parallel through the switch. The OFF position allows you to disconnect the battery banks from all charging sources and loads.

While mainly for a 2-house battery bank system, a 1-2-BOTH-OFF switch can also be used when setting up a system with one house bank and a designated starting battery. Simply do the following:

put the house bank on position 1 and the start battery on position 2 (if required multiple start batteries can be connected to position

2)connect all charging sources and all loads to the house bank, leave the switch in position 1 for normal operation• charge the start battery(s) either with an echo-charge device (this charges a start battery from a house bank) or by switching to position 2 occasionally to direct alternator charge to the start battery(s).
ON-OFF Battery Switches

1-2-BOTH-OFF battery switches still have their place, but many boaters are setting up their electrical systems with one large house bank and designated engine starting batteries for auxiliaries and gen-sets. For these systems, a simple ON-OFF battery switch to isolate or disconnect the house bank makes more sense.

Lighter duty ON-OFF switches, often called Mini Battery Switches, can be used to disconnect small starting batteries, but they have other uses in an electrical power system. They also serve as convenient circuit disconnects for larger branch DC circuits on board, including circuits for high amperage appliances and charging sources.

When setting up a system with a single house bank, run a heavy duty cable from the positive terminal of the house battery bank to the input terminal of the ON-OFF switch. As a first line of defense against short circuits I recommend that a class T fuse be placed in that cable as close to the battery bank as possible. Direct the output of the battery switch to a heavy duty positive busbar, where all charging sources and loads are connected. Of course, all charging sources and loads should have their own means of disconnect and circuit protection.

Use a Mini Battery Switch in the cable between the positive terminal of the start battery and the engine starter. The start battery can also be connected to the house bank through another Mini Battery Switch used as an emergency parallel switch to temporarily join the start and house banks.

Switch Construction & Ratings

Battery switches for the house battery bank and engine starting battery(s) should have an amperage rating to handle the maximum current expected in the system. A house bank battery switch with a continuous rating of 300 amps and intermittent amperage rating of 400 amps is a good choice for most marine systems. Blue Sea Main Battery Switches are well made, reasonably priced, and meet these amperage ratings. Mini Battery Switches from Blue Sea are rated at 250 amps continuous and 375 amps intermittent. Some models have a removable key to avoid accidental switching of critical loads or charging sources.

Many battery switches on the market come equipped with an Alternator Field Disconnect (AFD) switch that protects the diodes in an engine-driven alternator in the event someone inadvertently changes the position of the main switch while the engine is running. An AFD is a secondary switch inside the main switch, connected to the field wire of the voltage regulator. When the position of the main switch is changed, the AFD opens (which stops the alternator output) before the main switch contacts open, and the AFD closes again before the main switch contacts close.

Battery switches can be direct mounted to a bulkhead or other flat surface, or they can be flush mounted to standard or custom electrical panels. The electrical contacts are on the rear side of the switch, protected from accidental human contact.

In the next issue we’ll continue on with heavy duty DC circuit components by reviewing the heavy duty circuit breakers currently available.

Part III

In part three of our treatment of heavy duty DC circuit components we will review the heavy duty circuit protection devices on the market, including both fuses and heavy duty circuit breakers.


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, especially for high amperage circuits. 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. Multihull sailor’s can now choose either fuses or circuit breakers for high-amperage applications.

Heavy Duty Fuses

Heavy duty fuses are strictly thermal devices that melt at a predetermined amperage. They are reliable and relatively inexpensive, although total cost includes 1) the purchase of a fuse mounting block and a protective cover of some type, 2) spares since fuses must be replaced after each overcurrent condition, and 3) some form of circuit disconnect.

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.

Heavy Duty Circuit Breakers

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 in the long run they can actually be less expensive since they also serve as circuit disconnects and since they are resettable the need to carry spares is not as critical.

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.


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.