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SvWhat should I consider when sizing cables?
From the Basics to Practice
If you already understand the basics of cable sizing, you can continue to the Cable Sizing Guide or go directly to the calculator to calculate the thickness of the cables for your new electrical system.
Otherwise, start dimensioning by answering these three questions:
How long is the cable?
How long is the cable from the power source to the load and back again? That is the question you must have an answer to. Examples of such circuits are alternator → battery → alternator, or fuse panel → lamp → fuse panel.
The shorter the distance, the thinner the cable you can choose.
When calculating the cable cross-sectional area, you must remember to include both the positive and negative runs to get the correct dimension. For example, if there are 2.5 meters between your alternator and the battery, you calculate based on 5 meters.
How much power?
How much current will flow through the cable? For a lamp or a TV, you can find this out by looking at how much power it draws and dividing by the voltage, which is normally 12 volts.
That means a lamp that draws 24 watts will require 24 / 12 = 2 amperes. For a charging circuit, i.e. from alternator to battery and back again, the alternator’s rated current is used as the reference. A common alternator size is 55 amperes.
How much voltage drop?
How much voltage drop can you tolerate in the circuit in question? The answer depends on what is connected to the circuit. For the charging circuit, i.e. between the alternator and the batteries, our measurements have shown that you should stay below 0.1 volts to maintain good charging.
For a normal consumer such as a navigation light or a TV, about 0.5 volts of voltage drop from the fuse panel is a good level. A bow thruster, anchor windlass, or starter motor can handle 1 volt of voltage drop without losing power, but more than that should be avoided.
