Installing Your Own DC Accessories Once you deal with all of the above, you’re ready to install your own DC accessories. Some common additions follow, with my personal step-by-step approach to installing each accessory outlined. After reviewing these examples, you should be ready for just about any circuit you may wish to add to your boat.
Installing a New Cabin Light If you’re installing a conventional incandescent light or one of the newer halogen lights, don’t worry about polarity. These light bulbs really don’t care which way the electricity flows through them. If you’re adding a low-voltage fluorescent fixture, however, you’ll have to observe any positive or negative wiring indicated by the manufacturer. Circuits with dimmer switches (rheostats) and light-emitting diodes (LEDs), which are commonly used in instrument lighting and are even available now as replacements for conventional low-voltage light bulbs, are also sensitive to polarity. Some of the companies that make light fixtures recommend the correct fuse to use and the appropriate wire gauge based on the length of the wire run. If you don’t have these specifications, you must figure these numbers out for yourself. The packaging should at least tell you the operating voltage of the bulb in the fixture, so you’ll have at least one of the needed values. Also, the wattage will be given with most light fixtures. By using the wattage equation from chapter 1, you can easily calculate the amperage by dividing the wattage by the voltage. The result will be the amperage the fixture will need. For example, a 20-watt bulb on a 12-volt circuit will draw 1.7 amps (20 ÷ 12 = 1.7). Once the amperage is known, measure the distance of the run from the distribution panel to the new fixture and back again. A quick reference to the table will give you the wire size to use for the job. Since this is a cabin light, you should use a dark blue wire to the light and a black or yellow (preferably yellow) wire for the return to the panel. One precaution regarding fixtures of this type: Often a manufacturer provides a short length of wire lead from the fixture to facilitate attachment to your
wiring harness or new circuit. Typically these leads will be 16 AWG. Regardless of which gauge wire you select to run from your distribution panel to the new fixture, you must always rate the circuit overcurrent protection at the ampacity of the smallest wire in the circuit. In this example it would be the 16 AWG wire supplied with the fixture, assuming you use a 14 AWG feeder and ground return wire for a long run.
Step-by-Step Instructions So, with the basics above in mind, my step-by-step approach to adding a new cabin light looks like this: 1. After selecting a light fixture you like and making sure that the voltage is the same as it is on your boat, find the wattage of the bulb. 2. Once the wattage is known, determine the amperage requirements for the fixture. In the above example, a 12-volt, 20-watt bulb would require 1.7 amps. Always round up, so in this case the amperage requirements would be 2 amps. 3. Use the table of wire gauges in figure 4-3 on page 44 for acceptable voltage drop and length of wire run. If the amperage requirements are less than the minimum specified on the table, use the gauge indicated for the minimum value (5 amps). In any event, never use wire smaller than 16 AWG when adding anything to your boat, no matter how little current is drawn by the equipment you’re installing. Wire smaller than 16 AWG is only used for electronic control circuits and small connecting links on the back of instrument panels and the like. 4. Select the switch or circuit breaker on the distribution panel you’ll be using to feed this new circuit. 5. Determine that the amperage rating of the fuse or circuit breaker is appropriate for the total of all circuits or loads being fed by that breaker. For example: In this case you may already be servicing several cabin lights in addition to the one you’re adding. Total circuit protection at the panel must cover the amperage of all the lights on any given circuit. The fuse or circuit breaker must not be rated at more than 150 percent of the ampacity of the smallest wire used, which 139
