Use 3% voltage drop for any "critical
application" affecting the safety of the vessel or its passengers:
bilge pumps, navigation lights, electronics, etc....
Determined by measuring the length of the conductor from the positive
(+) power source connection to the electrical device and back to the
negative (-) power source connection. Note that the power source
connection may be either the battery, panelboard or switchboard.
Determined by adding the total amps on a circuit. Conductor sizes not
covered in Table B or Table C may be calculated by using the following
After calculating the Circular Mil Area (CM), use Table E to determine
the proper conductor size (National Fire Protection Agency and Coast
Guard require that the next larger conductor be used when the
calculated CM area falls between the two conductor sizes).
CM = Circular Mil Area of Conductors
K = 10.75 (Constant representing the mil-foot
resistance of copper)
I = Current - amps / L = Length - feet
E = Voltage drop at load (in volts)
Q: A bilge pump draws 10 amps. The positive run is 11
feet from the power panel, including the float switch. The negative run
is only 10 feet. What size is the wire?
A: Use the formula to reach the correct answer:
Table D shows that 12 AWG wire has a CM area of 6,500 and is the
correct choice. However, SAE wire has a CM area of only 5,833. Under
NFPA and USCG regulations, 10 SAE wire must be used.