Do you have an electrical system diagram for your boat? Do you know where the wires in your switch panel go? If not, you could be in for a nasty surprise should a switch or other component fail.
That surprise can take several forms. The first is the scorch marks left as the wire shorts out (It may or may not burn your boat to the waterline!). The second is the yard bill for fixing a simple switch problem without a wiring diagram. Without a wiring diagram to tell the electrician where the wires go, the pro might have to trace the route each wire takes to find the right one. This job could take several hours for a complicated circuit. Hours that you are paying for.
Making an Electrical Diagram
To map your system, first you need to find out what equipment is installed on your boat and where it is connected. Armed with a simple multimeter available from Radio Shack, you can figure out your own wiring diagram. First, disconnect the electrical power and access the back of the electrical panel. Set your meter to resistance, and attach a long wire to one side of the meter. Disconnect each wire one at a time on your wiring panel and check the other end to see where the wire goes. If you disconnect a wire at both ends there is no possibility that stray current can backfeed to your meter or that a circuit can be established by wires back feeding through the grounded side of the system. When you have found both ends of each wire and they check out, draw the lines on your wiring diagram. (If you really want to make future work simple get number tags from your marine electrical supplier and install numbers on each wire as well as drawing your wiring diagram.) Eventually, you will come up with a complete diagram. You might also find some wires connected to nothing at all.
Now let’s suppose you want to add a depth sounder to your boat’s electrical system. First check that you have enough space behind your navigation panel to flush mount the electronics. If you do not have space, you will have to surface-mount the unit. Check also to make sure that you have space behind or below the instruments to install plugs and wires. Next check to see that you can get wires from your breaker panel to the unit. (Remember to check the other side of a bulkhead before you drill or screw through it!)
Figure out where your easiest cable run is while still keeping the cables hidden. If your boat has wires installed in conduit, your work will be much easier. You may find that you have to go into the overhead and down inside a hollow handrail to get the wires to the unit. Check at the breaker panel that you have space for a new breaker or can attach the wires to an existing breaker easily. Make sure you have a suitable breaker, cable ties, terminals, tools, and anything else you need before you start.
When feeding wires, be aware of how wires can chafe. Boats tend to bounce around and fiberglass is hard stuff which can chafe electrical wires. Thus you will need to protect the edges of the hole or install conduit.
Feed your wires through the hull; make the terminal ends and connect them up. Make a note of where each wire is connected. Ideally, you should adhere to the ABYC recommendations for cable colors and terminals, but it is your boat and you can do what you prefer.
Go back to the breaker panel and attach the wires to the breaker. Use your multimeter to check out the circuits. Install the breaker in the breaker panel and turn it on. If you haven’t hooked it up right, you will notice sparks traveling down the wire and a burning smell coming from the electronics. If nothing happens, turn on the unit and set it up.
CALCULATING VOLTAGE DROP
Scenario: A light with a 30 foot wire run from the breaker panel. The wire from the switch and back. (30’ x 2 = 60’)
Using #10 copper wire, its resistance is 1.02 Ω per 1000 feet (60/1000 x 1.02 = .0612 Ω)
If the current flowing along the wire is 5 amps (5 x .0612 = 0.3 Volts)
For a 12-volt system a #10 wire is fairly heavy and might only be used for a battery or electric stove. If the wire size were to be #16 wire the resistance per foot is 4.014 Ω. (60/1000 x 4.014 = 1.2 Volts)
With a 12 volt system, this might be enough to cause a sensitive instrument to stop working properly.