Ryan, As I have posted this link on your tow bar electrics thread, why don't you open it, read it, better really, to print the 12 pages off.
It explains what cable size, voltage drop and all the formulas you will need to calculate voltage drop. You will only need a simple calculator, as seen on your phone, to do the calculations.
http://www.caravantalk.co.uk/library/files/Beginners-Guide-To-Electrics.pdf
It also explains % charge available in a battery when it has been resting some time, so if you have a second battery, you will be able to understand what the voltmeter is actually telling you.
Remember voltage drop on a cable is on both the feed AND on the return, if you are using a return cable like in boat and caravan wiring.
If you are using the car body as earth return, then the voltage drop is minimal, on the return, but make sure you use star washers to cut through the paint, and some grease, vaseline, or waxoyl over the connection to reduce corrosion.
If you are using a return wire, and you are using it for several things, then you need to ensure that this cable is big enough for the return current for ALL items assuming that they could all be on at the same time. Otherwise the voltage drop could be considerable, not only that, the cable will get warm due to resistance of the wire. The wattage or heat output of the cable will be given by the formula IxIxR ( pronounced, I squared R) Current x Current x resistance in ohms.
An example where the return cable isn't big enough, is on trailer and caravan wiring. Watch the tail lights go dim, when the indicators are on, on the trailer or caravan, they dim in unison with the indcators.
Also seen on many ford cars, that use the galvanised metal circuit tracks on the rear light cluster. BAD EARTH, clean the contacts on the plug, bit of vaseline or waxoyl... sorted.
For Vaseline... Ryan is your man.:thumb2
For Waxoyl... I am your man.:thumb2
With my posts you always get more advice than you asked for.
Others say I go on a bit...:lol:lol
Rustic
