How to Calculate DC Wire Size for Boats

Written by james stevens
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How to Calculate DC Wire Size for Boats
Knowing correct wire size is important when wiring a boat. (Goodshoot/Goodshoot/Getty Images)

Boats use direct current (DC) electricity, produced by a battery and generator to power the electrical items on board. Some electrical devices use more energy than others, such as the starter motor to get the engine running. Calculating the correct wire size is important; if you use a wire that's too thin and connect it to an electrical device that uses a lot of energy, the wire will get hot. It may melt the external plastic and possibly cause a fire on board the boat.

Skill level:

Things you need

  • Pen
  • Paper
  • Calculator
  • Tape measure

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  1. 1

    Use a pen and write the headings "Amperes," "Wire Length" and "Wire Size" starting left to right across the top of a sheet of paper. You need amperes and wire length to calculate the wire size.

  2. 2

    Look on the label of the electrical device or devices you intend to wire on your boat. You must know the amperes each device needs to operate. For example, the label will say "20 Ah," meaning 20 amperes.

  3. 3

    Make a note of the amperes for each device. Write the details on the sheet of paper under the heading "Amperes."

  4. 4

    Measure the distance between the battery and the device it powers using a tape measure. Wire creates resistance, so the longer the distance, the thicker the wire size needs to be. Double the distance you measured using a calculator. For example, if the distance is 10 feet, then double it to make 20 feet. Write down the figure you have calculated on the sheet of paper under the heading "Wire Length." Repeat the process for other electrical devices you intend to wire in your boat.

  5. 5

    Consult the amps and wire gauge table at: The top yellow line is amperes and the green column on the left is wire length. The central white boxes relate to the wire sizes.

  6. 6

    Refer to your sheet of paper. Use the details you wrote down for the first device on the sheet of paper. For example, you may have written 10 amperes and 20 feet, so look at the table and find the corresponding numbers on the top row and side column.

  7. 7

    Follow the line down from the top and the row across from the side until they meet. The box where they meet is the wire size you need. Write down the wire size under the heading "Wire Size" on your sheet of paper. Using the same example as in the previous Step, the wire size you would need is 12. If you can't find an exact match always round up the number. A thicker wire is better than a thinner wire, as it's safer.

  8. 8

    Repeat the process using figures on the sheet of paper and the table on the Web page. Write down all the wire sizes.

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