- Calculate the maximum sustained amperage of the circuit. Measure the length of the circuit from the power source to the load and back.
- Decide whether the circuit runs in an engine space or non engine space. Engine spaces are assumed to be at 50 degrees C, non engine spaces are assumed to be at 30 degrees C.
- Multiply the maximum current times the length of the circuit to calculate Famps (Feet x amps).
- Base the wire on either the 3% or 10% voltage drop. In general, items which affect the safe operation of the boat and its passengers (running lights, bilge blowers, electronics and distribution panel supply circuits) use 3%; all other loads use 10% (cabin lights, bait pumps).
- Starting in the column which has the right voltage and voltage drop shown at the top, run down the list of numbers until arriving at a value which is greater than the calculated Famps. Move left to the Ampacity column to verify that the total amperage of the circuit does not exceed the maximum allowable amperage of the wire size for that row. If it does, move down until the wire ampacity exceeds the circuit amperage. Finally, move left to the wire size column to select the wire size.
a. A 12 volt system at 10% drop with a 40’ circuit x 45 amps = 1800 Famps. A wire size of 8 is required.
b. A 24 volt system at 3% drop with a 10’ circuit x 100 amps = 1000 Famps. A wire size of 6 is required.