Installing the new class of USB power ports

There might be some confusion here on how to properly size and fuse wires and loads.

The fundamental principle to remember is that a fuse or circuit breaker in your trailer or house is there to protect the wires not the load. Of course, the wires have to also be heavy enough to pass sufficient current to operate the load.

If a specific load device requires additíonal fusing that should be provided for internally in the device itself. In some cases the installation instructions might require you to add additíonal fusing. If so do that but that doesn't change the need to protect the permanent wiring in the trailer.


So the proper design procedure is:

Determine the total maximum current that will be needed at the point of use. Add 25% to avoid nuisance fuse blows. Select the next higher standard fuse size.

Select a wire gauge with ampacity of that fuse size or higher. For normally used copper wiring that is

gauge amps
14 15
12 20
10 30
8 40
6 55
4 70

Don't run anything smaller than 14 gauge in your walls.

To do it's job the fuse or circuit breaker has to be upstream from the wire it is protecting (at the end closest to the source of current, in this case the battery). So it does no good for the wire to put a fuse at the far end of the wire next to the load. If that wire shorts it will burn up and the fuse won't protect it because the short circuit is upstream from it. And, if you tap current from a wire using a smaller gauge wire then that smaller wire will need to be separately protected at the point where you tap the larger wire.

Fuses and cbs need to be placed in the ungrounded conductor, in our 12V systems that is the positive. The negative wires don't need and in general should not be fused, but they do need to be the same gauge as the positive they are paired with.

One other consideration that cam be important for 12V systems (although usually not for small loads like USB devices) is voltage drops caused by wire resistance. It is generally not good to let the voltage drop be higher that 5%, or about 0.6V in a 12V system. That can require selection of a wire larger that you would need otherwise. There are numerous calculatiors online that you can use to determine voltage drop if you know how long the wire run is and your current demand. The important thing to remember is that you have to select a wire that meets both the ampacity and voltage drop requirements. If you wind up needing a heavier wire to reduce voltage drop you don't need to also increase the fuse size.

Hope that helps