CATAMARAN DC WIRING TECHNIQUES
Those sailor’s building a new multihull or renovating an existing one are eventually faced with the task of wiring from scratch or rewiring the DC side of the boat’s electrical system. This process can be less daunting if you understand what DC circuits are all about, and spend time planning for a proper installation.
Think of a circuit as a path that, if followed completely, leads back to the starting place. In this column we are concerned with electrical circuits that provide a reliable path for direct current (DC) on a sailing multihull. While there are definite rules to follow when wiring a new boat or rewiring an existing one, the DC portion of the electrical system can be set up in a variety of ways. With some creative thinking, the right circuit components, and good wiring techniques, you can take the best advantage of battery-supplied electricity on board.
In DC circuits an electrical potential, or voltage, drives the electrical current. Charging sources such as solar, wind and water generators, alternators, and AC battery chargers create the necessary voltage to drive current into the batteries, and in turn the batteries serve as a voltage source for operating all of the DC loads on board, including inverters that take battery power and invert it into AC power. A boat has both charging circuits (upstream of the battery) and load circuits (downstream of the battery), yet all DC circuits on a multihull must include the batteries. Individual batteries can be connected in a variety of ways to give you the system voltage and total battery capacity you require for house loads and engine starting. As we’ve discussed in previous columns, one of the simplest battery configurations is to have one large house bank and separate batteries for engine starting. The house bank can be made up of 12 volt batteries wired in parallel (positive to positive to create the needed capacity), or 6 volt batteries wired in series/parallel (positive to negative to create 12 volts, then positive to positive to increase capacity if needed).
In a DC system there is a positive and negative side to each circuit. The positive side originates at the positive terminal of the voltage source and typically goes to a battery switch, either a standard 1-2-Both-Off switch (used mainly for dual house banks), or a simple ON/OFF disconnect switch. Both starting and house battery banks should be equipped with a means of disconnect.
Bilge pump circuits are usually placed on the battery side of a disconnect switch so they won’t be accidentally turned off.
From the switch the house positive circuits lead to a main positive distribution point (the switch itself can serve as a distribution point for simple systems), then on to the branch circuits and individual loads. Good examples of marine DC distribution points are Blue Sea’s “Power Post”, a plastic mounting block with a heavy duty threaded stud for accepting large wire or cable with ring terminal connectors, the “Power Post Plus”, essentially a Power Post with additional mounting screws for smaller gauge wires, and the “Heavy Duty Busbar”, a single block with a row of large distribution posts.
Heavy duty busbars come in handy when there are multiple large branch circuits to consider. For example, a four-position busbar might be used to distribute power to the main DC load panel, a large inverter, the refrigeration system, and a windlass. Keep in mind that according to the ABYC, each circuit must be provided with circuit protection devices (CPDs, fuses or circuit breakers), with the exception of starting motor circuits.
The negative side of the circuit provides a return path back to the voltage source. The flow of electricity through a circuit is determined by the electrical resistance of the loads. For example, engine starters, freezers, and large inverters have relatively little electrical resistance compared to lights and navigation instruments, and therefore allow much more current to flow.
Wire conductors carry the current from the voltage source to the loads. Conductors in a DC system must be sized to safely handle the expected current. We’ll discuss wire selection in detail in a later column. Other circuit components to consider include connectors to join conductors to electrical equipment or to join two or more wires together, distribution posts or busbars to conveniently join groups of positive or negative wires, manual or automatic switches to control current flow, CPDs, and meters and monitors to help make electricity visible.
In the next issue we’ll take a look at creating a schematic diagram for the DC circuits on board.
Part II Schematic Wiring Diagrams For DC Circuits
Whether you are in the process of wiring a new electrical system on your boat or simply trying to make sense out of the system you currently have, it’s a good idea to get comfortable reading and working with electrical schematic wiring diagrams. They help bring order to the seeming chaos of wires, connections and “black boxes” found behind the scenes on a typical sailboat. Schematic diagrams are useful for understanding your electrical system, for planning future updates or renovations, and above all for trouble-shooting problems in your system down the road.
A schematic diagram is essentially an outline or explanation of a system. It’s not intended to show things exactly as they appear in real life, but rather serve as a summary of what the system components are, an indication of what their function is, and how they are interconnected. An electrical schematic shows the pathways of electricity flow, as well as ways of controlling the flow (switches), monitoring the flow (meters), and using the electricity in your appliances. A well conceived schematic serves as a street map for negotiating your way through the maze of electrical wires and components on board.
In standard schematic diagrams certain conventions are followed so that anyone, anywhere should be able to interpret the information. Standardized symbols are used to represent the electrical components, and wiring paths are organized in such a way to avoid confusion. This doesn’t mean that you have to be an engineer to be able to read one. In fact, these days schematic diagrams intended for end users of electrical equipment tend to be more user friendly, with more life-like graphic images for many of the system components such as batteries, renewable and engine-driven charging sources, electric panels, control boxes, electrical loads, and distribution posts and bus-bars. You’ll still find standardized symbols for electronic components such as diodes (electrical check valves), but most of those items are contained within specific pieces of gear and don’t have to be shown on your schematic. You can adopt your own graphic style, as long as enough of the conventions are used to allow others (fellow sailors or marina mechanics, for instance) to easily follow what’s going on. While most schematics are initially drawn in black and white, adding colors to identify individual circuits or wiring paths can help tremendously with visualizing electrical flow.
When working with electrical systems your approach can vary, and be as simple or sophisticated as needed for your situation. You can create a diagram for a specific circuit, or pull it all together in an overall drawing of the system. I prefer to see the big picture, even when troubleshooting individual circuits, so I like to see everything at a glance on a single page.
Planning a new wiring job is quite different from just creating a drawing of your existing system, but in either case I suggest you take it at a comfortable pace in the beginning and let the diagram evolve. Remember that in a DC wiring system there has to be a positive and a negative side of the circuit. On a DC schematic it’s not unusual to show only the positive wiring in it’s entirety, especially if the drawing is done in black and white. The negative wires can be abbreviated to avoid confusion.
I start by sketching the major system components on a blank sheet of paper, then I incorporate minor components and wiring paths as I get a feel for the intricacies of the system. My first attempts at putting an electrical layout in schematic form are usually rough sketches, with simple circles or boxes for all equipment, loosely drawn lines for wire paths, and dots for connection points. As the system takes shape, the symbols become more refined and the lines more accurately and systematically drawn. When I have what I feel is a good representation of the system, I make one final drawing. I archive the original and, if there’s room, mount a laminated copy somewhere near the electrical panel or in the engine room for quick reference.
In the next issue we’ll examine several common electrical layouts for multihulls. For each standard layout I’ll suggest refinements that make system operation safer, more efficient and easier to monitor and control.