The calculator sizes a copper DC conductor for voltage drop: the smallest cable that keeps the drop at the load within 3% or 10% of the system voltage. Enter the current, the distance from the source to the load and the system voltage. The result comes in AWG, as US boats are wired, and in mm², as CE-marked boats are.
The formula
From Ancor's voltage drop tables:
CM = 10.75 × I × L ÷ E
- CM is the circular mil area the conductor needs.
- 10.75 is the mil-foot resistance of copper.
- I is the current in amps, the total on the circuit.
- L is the length in feet from the positive connection at the battery, panel or switchboard to the device and back to the negative.
- E is the voltage drop allowed at the load, in volts: 0.36 V for 3% at 12 V, 1.2 V for 10%.
When the result falls between two sizes, use the larger one. Ancor notes that the NFPA and Coast Guard require it.
A worked example
Ancor's example: a bilge pump draws 10 A, the positive run including the float switch is 11 ft and the negative 10 ft. That's a 21 ft circuit. At 12 V and 3%, CM = 10.75 × 10 × 21 ÷ 0.36 = 6,271. 12 AWG, at about 6,500 circular mils, is the answer. Automotive 12 SAE has only 5,833, so a boat wired in SAE cable needs 10 SAE for the same pump.
What it doesn't check
Voltage drop is one test. The conductor also has to carry the current without overheating, which depends on its insulation temperature rating and whether it runs inside or outside an engine space, and it needs overcurrent protection sized to it. Check both against ABYC E-11 or ISO 13297 for the boat in front of you.
Using it during a survey
Long runs on small cable show up as dim navigation lights and electronics that reset when the windlass runs. Measure the voltage at the device with the load running and compare it with the battery: if the drop is more than the circuit allows, the calculator gives the size the run should have been.
"Bilge pump circuit run in 16 AWG over a 36 ft circuit, pump drawing 6 A; 0.9 V drop (7.5%) measured at the pump. Replace with 12 AWG or larger, sized for 3% voltage drop." Name the circuit, the cable fitted, the reading and the size required.