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DC Electrical System Design: Wiring Your Boat

April 6, 2018

There are plenty of articles out there on installing new electronic gizmos and gadgets in our boats, but very few about the infrastructure those toys attach to. The fact is that many of our boats, especially the older ones, are a patchwork of wiring ill suited for new additions.
I quickly discovered that was the case on my current project boat, a 1965 Columbia 26. The wiring was old, the insulation brittle and frayed and there were a host of wires—all red and black—that I found both connected and disconnected. After looking at the mess, I decided that it would be both easier and less exasperating to remove all the old and questionable wiring, as well as the electrical panels, and start from scratch.
While you may not want to be as drastic when it comes to your own boat, there is much to recommend taking stock of your boat’s electrical system and making diagrams of what exists. The exercise will help you get better acquainted with the boat, identify weak points in the system and help plan for upgrades. The diagrams themselves can also add value to the boat, by increasing the onboard documentation.
By going through this exercise and drawing these diagrams yourself, you’ll be much better prepared to track down problems if something goes wrong. You’ll also be aware of your system’s shortcomings, and you can fix them yourself or hire a licensed marine electrician to do the job. And, of course, the next time you add gizmos and gadgets, installation will be far simpler.
There is only one caveat here: Be very careful around your boat’s electrical system. Getting a sense of your boat’s wiring will require some crawling around. Disconnect your boat’s shore power cable and turn everything off at the breaker panel. Do your crawling in daytime and use a flashlight. Electrical shocks can be fatal, so take all the necessary precautions.  

Functional Diagram
I like to start with what I call a “functional diagram.” To create the diagram, sketch out a rough approximation of the boat on a piece of graph paper, noting the location of major interior items like bulkheads, berths, settees and the navigation station. Then, plot the location of outlets, electrical panels, lights, batteries, instruments, fans and any other DC appliances.  
Creating the diagram familiarizes you with the boat’s systems and gives you a chance to consider wiring options and locations. For example: Instead of stringing several sets of smaller wires to v-berth lights, fans and outlets, you might consider running one set of heavy conductors to power bus bars, which would in turn feed those separate electrical items.
A functional diagram will help you visualize just how many wires you need and where they are going to run. Is there an accessible path or will you have to drill holes? Is a wiring chase or conduit a possibility?
Alter the diagram until the basic wiring runs and your boat’s complement of electrical items are established. If you use a book or binder, you can track successive versions of the drawing and make close-up diagrams of particularly busy areas
You’ll use this diagram to estimate the length of the circuits and the size of the wire you will need, so be as realistic as you can be. When in doubt, estimate a longer run.

Circuit Diagram
Once I have this information in hand, I begin developing a circuit diagram. This is what is commonly found in maintenance manuals. The circuit diagram will roughly mimic the layout of your functional diagram but without the structural details of your boat.
The electrical trade has a set of symbols that are used to make such diagrams, so that all electricians can “speak a common language.” For a listing of these symbols, see the link below. If you familiarize yourself with these symbols and use them, any electrician or mechanic should be able to look at the diagram and understand the system on your boat. But don’t get too caught up in the exercise. What really matters is that you can recognize and understand your drawing.
I like to take this part of the drill one step further. Most circuit diagrams don’t show a circuit’s ground legs, which are usually depicted with a symbol that looks like an upside down Christmas tree. I show the full ground circuit back to the actual place where the ground is connected. This alerts me that an actual wire has to be run, and reminds me to leave room in wire chases or conduits.
At this point you’ll begin adding wiring aids like terminal strips and bus bars, which connect multiple wires to a single power supply. These are particularly useful when it comes time to do some disconnecting, such as when you’re “pulling” the mast. Be prepared to make multiple alterations to your diagram as changes are noted, items added or subtracted and strategies to reduce wire runs are worked out.

Determining Wire Size    
Once I have this basic circuit diagram laid out, I develop a spreadsheet showing the electrical load on each circuit. Assume a worst case: If you have several branch circuits fed by a common line, it will need to carry enough amperage to safely ferry the load demanded by all the devices on each of those branches. This spreadsheet will be used to determine the size of the wire and the circuit protection—either fuses or breakers—required.
The size, or gauge, of the wire you need will depend on the current carried and the length of the circuit. The length of the circuit is the round trip from the positive power supply, to the device and then back to ground (you can see why adding the ground wire to the circuit diagram is useful). The longer the circuit and the more power it carries, the larger the wire.  
With the current and the length established, you can go to a wire sizing chart to determine the size of wire you need. These tables can be found in catalogs such as West Marine (see link below) or Ancor Wire and in books like Nigel Calder’s Boatowner’s Mechanical and Electrical Manual.
Two tables are usually presented to show 3- and 10-percent voltage drops. A 10 percent drop can be tolerated by devices like lights and DC motors. Electronics, on the other hand, should hew to 3-percent wire sizing. To simplify matters I usually stick to the 3-percent scale all around. As I determine the wire sizes, I enter them on my spreadsheet. I structure the spreadsheet so I can total the various wire gauges at the bottom and determine the amount of each kind of wire I’ll need for the job.    
This is the time to decide if you want to use duplex wire (two insulated wires enclosed in an outer sheath) for any of your circuits. Using duplex means pulling one wire instead of two and offers a further level of protective insulation.
Whatever you choose, plan to use marine-grade wire only. Your boat’s electrical system is not the place to save money by buying inferior products designed for home or commercial use on land. Plan to buy marine-grade wire and captive marine-grade connectors, which are shaped like an “O” rather than a “U.” Solder your connections and wrap them in heat-shrink waterproof tubing. This is covered in the second article. I don’t like to blindly tell folks to solder without explaining the possible problems. ABYC will not accept solder as the only means of securing a connector.
Older boats are usually wired with red for DC positive and black for DC negative, meaning the positive and negative wires coming from the battery bank. The problem arises when a boat is also wired for AC power, such as that coming from shore power, a generator or an inverter. AC wiring utilizes white, black and green wires. It is potentially deadly to confuse a black DC wire with an AC black wire. For that reason, the American Boat and Yacht Council (ABYC) has established yellow as the new color for DC negative wires.
The ABYC has also developed a further set of color codes for boat wires (see link below). While manufacturers use these color schemes, most boat owners end up using just red and yellow wires when adding circuits.

Circuit Protection
With the amperage of each circuit determined, you can now plan the circuit protection you will need. That protection can be a fuse or a circuit breaker and may depend on the type of electrical panel or panels you choose.
Fused panels can easily be changed by swapping-out the fuse for a higher or lower value. Be sure and read the instructions for your electrical panel to make sure you don’t overload the capacity of the panel wiring with too high a fuse or too high a circuit load.
You may have to divide up loads across more circuits, or even add additional panels. In the case of my Columbia project boat, I ended up with a main DC panel with several fused circuits, a battery test meter and a 12-volt accessory socket. I also added a secondary panel with eight additional fused circuits.
One important thing to remember about circuit protection is that the protective device is there to protect the wire, not the device the wire is connected to. In other words, the job of the fuse is to blow before the wire overheats and damages the wire insulation or starts a fire. For that reason, most electronic devices also have an inline fuse holder to protect the device itself.

Wire Identification
Whether you use the ABYC color codes or not, you should identify each wire with a unique code. This code should go on each end of the wire and several places in the middle of the wire—especially in areas where wires are gathered. Searching through a bundle of unidentified red and yellow wires is a sure cause for frustration.

I use a code that I found in an old issue of DIY Boat Owner magazine. It looks like this:

100: Pumps, Fans, Ventilators
200: Internal Lighting Circuits, Ventilators
300: External Lighting, Mast Wiring, Nav Lights
400: Communication Equipment
500: Electronics, Instrumentation, Stereo
600: Safety, Security, Tank Monitoring
700: Heavy Current Equipment
800: Charging Circuits
900: Bonding Systems
X1000: All AC Circuits

For example, a bilge pump wire might be labeled 100.1, a blower wire 100.2 and so on. Enter these wire IDs on the spreadsheet you use to set up the circuits. Do it for yourself and for the next owner of the boat. He’ll thank you and it will help sell the boat.
When this is complete, you will have accomplished a great deal. You will have a very good understanding of the wiring that drives your boat and its systems. You will have wiring diagrams, a spreadsheet of the wires and wire sizes required, and a general idea of the needed supplies and materials. If your list is long, you can take that list and fax it around to several marine stories in order to get quotes. For a shorter list, just walk in.
But before you buy, do yourself a favor: Take some time and just sit in the boat with your circuit diagram. Visualize where the wires will run, where the conduits or wire chases will go and verify your wire lengths. You will almost certainly spot some trouble areas and add a few missing items to that growing list.

 

 

 

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