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.
PAUL ESTERLE
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."
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.
