Confused about DC ground and fuses

[Minor Project]

I am getting ready to wire the trailer for 12v DC and I am feeling confused about the grounding and fusing. I have been thinking of the trailer lights (brake, turn signal, etc.) and the cabin electric as two separate systems, where the trailer lights get power from the proper terminal in my 7-port plug and the cabin electric gets power from a battery, that is charged from the hot terminal on my 7-port plug.

Here is what I am having trouble understanding about grounding. Do I bring all of the ground wires for the fixtures on either part of the system to the negative terminal on the trailer battery and then wire the battery to a ground on the frame or do the fixtures get grounded without going through the battery?

For fuses, I think I should have a main fuses on the feed to the battery and a fuse to each of the cabin electric fixtures. Does that sound right? Do I need a fuse for the trailer lights?
Thanks, Tim

[MtnDon]

The trailer running lights are fused by the vehicle system. No extra fuses needed for that. If it helps to keep things straight, I=in my mind the trailer running lights are part of the tow vehicle system. The trailer interior lights and DC power are a separate system, but the ground is common. And I prefer to think of the ground as the negative wire. A circuit needs a positive (hot) wire and a negative (ground) wire.

Trailer running lights tend to use the frame as the negative or ground wire. Everything you add inside is best served by running two physical wires, IMO. Run the positives to a fuse block. Run the negatives to a common ground point such as a buss bar. Some of the better fusing systems will have a negative buss built in. If not make life easier and install a ground buss near the fuse block, run two wires, one from each.

The battery in the trailer should have a main fuse or breaker as close to the battery positive as possible. You don't need a fuse for every trailer device, but more fuses do make troubleshooting easier. It can also be nice to have lights on more than one fuse in case one fuse blows. You still have somelight then.

FYI, fuse sizes are determined by the wire size,not the load.

Hope that helps.

[Dale M.]

...

FYI, fuse sizes are determined by the wire size,not the load.

Hope that helps.

Not totally true... It is a combination of wire size, its ability to carry current and not over heat when shorted PLUS load of the device the circuit powers....

You can no put a 5 amp load on a 2.5 amp fuse no matter what the wire size with out fuse blowing.....

Dale

[H.A.]

FYI, fuse sizes are determined by the wire size,not the load.

Hope that helps.

This is correct to a point but not best practice. Especially so as we read about folks going serious overkill with conductor size (presumably fearing Voltage Drop Boogeeman ?) to carry a small load.

Anyway, Better practice is sizing a fuse to carry its expected load. Of course some margin of overage is desirable for most application. As well, slo-blow fuses may be desired.. Its sort of a case by case engineering decision.
Obviously this commentary takes into account never fusing above a conductors ampacity. But simply selecting fuses based on what load a conductor can carry is bad practice.