Boat wiring should be neat and orderly. Not a jumbles of wires running in every direction. Yet, the wiring on many older boats looks like it was done by a bird building a nest. If the previous owner of you’re new to you boat thought he could do his own wiring then chances are you know what I’m talking about.
The secret to great boat wiring is keeping things simple. Run as few wires as you can the shortest distance you can. Keep the wires dry, cool and secure with as few connections as possible.
It takes planning, but it’s the best way to keep your wiring trouble-free.
Easy Underwater Light Wiring
When it comes to adding more than two underwater lights there’s an easy way to keep your wiring simple, secure, and trouble-free. The Shadow-Caster SCM-PD Power Distribution Box.
The SCM-PD is a watertight box that allows you to securely power and control up to six underwater lights. Each light has its own color-coded connector and its own fuse. Cable glands tighten around the lights cable to help keep water out. A single wire from the switch supplies power to all of the lights. A single ground wire connects the SCM-PD to a negative bus bar to complete the circuit.
Mounted the SCM-PD in a dry location below deck in the bilge area, or inside the console. To prevent water from seeping into the box, mount it so that the cable glands and wire connector do not face up. Also, it’s a good idea to leave a droop in the wire to let water drip away from the box.
Correctly size the fuse, switch, and power wire for the type of light and the number of lights connected to the box. The more powerful the light the more amps it draws. Below is a list of Shadow-Caster underwater lights and how many amps they draw at 12 volts.
- SCM-4: 1.4 amps @ 12 volts
- SCM-6: 2.1 amps @ 12 volts
- SCM-10: 3.5 amps @ 12 volts
- SCM-CC: 3.5 amps @ 12 volts
- SCR-16: 5 amps @ 12 volts
- SCR-24: 7.8 amps @ 12 volts
Every light added to the SCM-PD adds more amps. For example, an SCR-16 draws 5 amps at 12 volts. When connecting 4 SCR-16s to an SCM-PD box you need a wire rated to at least 20 amps. (4 lights X 5 amps each = 20 amps)
To handle inrush current, the switch and fuse must handle at least twice the amperage being drawn. In the case above you would need a switch and a fuse rated to at least 40 amps.
Each wire gauge size carries a specified number of amps. But, as the distance those amps travel increases, the number of amps drops due to resistance in the wire. When this happens, a larger gauge wire is needed. Keeping the distance between the switch and box as short as possible allows you to use smaller gauge wire.
The table below will help you calculate the wire gauge based on total amperage and wire length.
Connecting the Lights
Now its time to connect the lights. Strip back around 1.5″ of the outside sheathing and 1/4″ of the insulation from each wire. Feed the wires through the cable glands. The connectors are color-coded to match the wire insulation. Insert the end of the wire into its connector and snap it down. The blue wire allows you to control the lights if you decide to add an SCM-WC wireless controller.
Tighten all cable glands. Insert the provided nylon dowels into any unused cable glands and tighten them down.
Insert fuses into the fuse holder for each light installed. Fuses must be rated to at least twice the rated amperage of the light used. The 7.5 Amp fuses provided are for SCM-10 lights. See the chart below for the correct fuse for your lights.
- SCM-4: 3 amps @ 12 volts
- SCM-6: 5 amps @ 12 volts
- SCM-10: 7.5 amps @ 12 volts
- SCR-16: 10 amps @ 12 volts
- SCR-24: 20 amps @ 12 volts
Replace the cover and tighten down the 6 stainless steel screws provided.
Now you have several underwater lights installed without wires running all over your boat. Your lights wires will remain neat, secure, and trouble-free for years to come.
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