Shop Talk: Small Boat Electrical Systemby Terry McIntyre First, we need to do some design work - what do we want the electrical system to do? My little boat is an inboard, so it must have a bilge blower to meet the USCG ventilation requirements. Although not a formal requirement, I think an electrical bilge pump is something that every boat should have. One thing I really enjoy is a cruise at twilight, so running lights were added to my list. I also wanted a set of instruments (speedometer, fuel, volts) and finally, the engine ignition/starting circuit. That's my "load list". Now, I added up the total electrical current required to run these devices - 5 amps for the lights, 7.5 for the bilge pump, 7.5 for the bilge blower, less than 1 amp for each of the gauges, about 2 for the starter solenoid. Adding these all up says that I might need as much as 25 amps capacity in the (unlikely) situation that everything is running at once. For some margin, I decided to design my system for 40 amps. Once the loads are known, and the approximate lengths of the wires, the required wire gauge can be found. From a voltage drop table (there is one in Caulder's book, but they are available in many places, including the West Marine catalog) I determined that I needed 12 AWG wire for my main circuits, and 18 AWG wire for the individual loads. Being the conservative engineer that I am, I went one size larger for each - the main supply wire is #10 AWG and the individual circuits are #16 AWG. I also decided to have four circuits - the lights, the blower, the bilge pump, and ignition, which supplies power to the gauges and starter solenoid.
Figure 1 shows a block diagram of how the boat is wired. There is a single positive feed from the battery to the main fuse panel. In accordance with ABYC standards, this feed is protected by an in-line fuse no more than 12 inches from the battery. This fuse should be less than the amperage capacity of the main feed wiring, but more than the maximum expected load; 30 amps
The wiring is all marine grade stranded wire, color-coded red for
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