Looking back, I realize I didn't plan this very well. In my trailer, I have a couple shelves behind the flat-screen TV. I wanted to install some components to make watching TV/Movies easier without any network connection. The specific components don't matter for this story, just that the combined set was going to draw a little under 15 Amps of 12vdc power.
I decided that the single 12v cigarette lighter-style power outlet in the area was not going to provide enough current to run the whole set, so I decided to make a dedicated 12v power run from the power panel - utilizing a spare fuse location in the panel.
For the wire run, I thought I would be just fine with 14awg, 2-conductor wire. Why did I think this? For the life of me I can't remember.
After running the new power line and setting up all the equipment, including a dedicated fuse block, everything seemed to work - I thought I was done.
Later, it was having problems, like the TV turning off when I tried to change inputs, or just turning off once the screen tried to light up. I checked the voltage at the dedicated fuse block (in the entertainment area) and found that when the TV was on (and the screen was lit), the voltage dropped to about 10 volts. Also, the power wires were getting pretty hot.
I checked a wire gauge chart like this one:
https://www.bluesea.com/resources/1437
and found two mistakes I had made:
1. When considering the length of my wire run (from the trailers power panel to the entertainment closet's fuse block), I didn't consider the return trip. When my run was 15 feet, that's actually a 30 feet circuit.
2. The 14 gauge wire was totally inadequate for the 15amp load - by a lot.
I installed a new 8 gauge power cable with a 15 foot run. Based on the chart (above) this provides enough capacity for a 15amp load. Everything is working fine. The voltage at the entertainment center is a lot better (12.3 volts while the voltage at the battery is 12.6 volts) and the wires are no longer getting hot.
I really found that it takes a lot heavier wire than I thought to provide the necessary current over a given length from the power source.
Lesson learned: Use the published chart and size the wire as needed for the load.