Read this, written by our very own "Hosspuller"..! 

http://www.mrtrailer.com/hosspuller.htm

Then this..

http://todd.redwrench.com/Electrical.htm

http://bart.ccis.com/home/mnemeth/12volt/12volt.htm

I don't know about the 12 volt world, but in the 110v where you may run off a generator or camp ground plug in. use the following.

The gauge of the wire should also correlate to the amperage that the breaker will support.

14 gauge wire = 15 amp

12 gauge wire = 20 amp

10 gauge wire = 30 amp

8 gauge wire = 40 amp

6 gauge wire = 50 amp

Example.  If you intend to plug the main plug into a 50 amp service at a camp ground,  that should be a 6 gauge wire from the plug to the breaker panel. ( most RV camp grounds offer 50 amp service to the bigger RV's and 30 amp plugins in general, Most of the portable generators we haul around have 30 amp plugs) If you are running a 20 amp service from the panel to outlet in the bathroom that might be used for a hair dryer. It needs to be a 12 gauge wire.  Usually the same for Microwave.  Wiring for lights can usually be 14 gauge and come off a 15 amp breaker.

Look at the wattage your appliances will consume and figure the amps used. A quick and dirty calculation that can be done in your head is  Wattage/100 x 80% = amps  So a 1800 watt coffee maker/100 = 18 x 80% = 14.4 amps That just about maxes out a 15 amp circuit. It will work until you plug in a second appliance on the same circuit and try to run it. ( the true formula is Wattage / Voltage = amps.)

The distance of the run is not as critical in a horse trailer, Since even the longest run in a horse trailer is much shorter than the length of runs in the average home.

If you search some of the older postings, you will find complaints about dim running light conditions on various trailers. Many of these problems were caused by the manufacturers saving money, by installing 16 awg wiring for these circuits. As the wiring aged and the connections corroded, the minimal wiring capacity became a liability, and an overloaded circuit and dim lights resulted.

On any new construction, when you're not sure about future expansion, it is wise to install wiring that allows provisions for additional load factors. The difference in cost between 14 and 12 awg wiring is minimal. As Willie has done, install the 12 awg and you won't be disappointed later.

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