Originally posted by Jack L:

I would like to get some opinions on this. I am doing maintenance/upgrades on my boat, some of which is electrical. I have been trying to study up on all the marine standards in order to avoid mistakes. I have found tons of rules and standards for safety, and most of the new ones for me concern the need to prevent any kind of spark that would ignite gas fumes.

But with all the strict safety precautions I was completly suprised when I found out the specs for wire gauge and current capacity.

I found a site listing ABYC standards (

http://www.pkys.com/Reference.htm ) which tells me that I can run 20 amps through an 18ga. wire.

This was very suprising in that electrical codes (for dry land) limit you to 7.5 amps to prevent overheating! Even worse is the voltage drop you get if you actually try to run 20 amps through 18ga.

He comes the math. I looked up the standard resistance for 18ga copper wire and got 6.6 ohms / 1000 feet...

Suppose I have something that draws 20 amps that has 10 feet of wire from the fuse block, and it is 15 feet of wire from the fuse block to the battery. (Not hard on a 20 foot boat).

Total of 25 feet of wire, but you have to consider the path from the battery to the device and back, for a total of 50 feet of wire. Total resistance is .33 ohms (doesn't sound like much yet, does it?)

Voltage drop = resistance x current.

Voltage drop = .3 x 20 amps

Voltage drop = 6.6 volts

Voltage to device = 12.6 volts from battery - 6.6 volt drop

Voltage to device = 6 volts

Wattage dissapated in wire = 6.6 volts x 20 amps

Wattage dissapated in wire = 132 watts.