Re: wiring gauge question not really boat related
Picking wire size is based on 2 factors: how much current can the wire carry without overheating (safety) and how much voltage drop can you accept. You need to consider both.
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For safety you have minimum wire sizes based only on the current. As Fishing Dude too said for 25 amps you need 10ga. (rated for 30 amps)
For voltage drop you need to consider both the current and the voltage. You want to measure the voltage drop as a percentage of the voltage. For 120v systems voltage drop is typically kept under 2%. For 12 volt systems up to 10%.
The simplest way to find out what the voltage drop would be is to use one of the many calculators you can find on the web. Just google voltage drop calculator. Here is one
http://www.supercircuits.com/resources/tools/voltageDrop you can try.
For a 12v system carrying 25A on 10ga. wire for 15 ft. it comes to a voltage drop of .83v, or about 7%. This is below the industry standard of 10% so you are OK.
Note: voltage drop means power lost. You are running 12v lights and only providing 11.17v to the bulbs. Not much dimmer, and probably helps the life of the bulbs a little bit. Not a big deal here.
