Battery disconnect wires

4 gauge is more than capable of handling the current to your 12V loads . The wiring to the electric panel is going to be 8 gauge as there are 40 amps fuses in that line. You are half right on using the same wire size. You can always go to larger conductors, never to smaller. In this case you are going larger so no worries.

Per the electric code you should never switch or disconnect the neutral (grounded) conductor, which is the negative. If you do then the whole electrical system will be floating with no chassis ground reference. So place your battery disconnect in the positive conductor. Be careful not to short out your battery positive terminal to the trailer frame while working on it, it’s best to disconnect the negative conductor temporarily first to minimize this.

Like most things the real answer is complicated.

First, the National Electrical Code, (National Fire Protection Association or NFPA 70) covers both AC and DC circuit design. I’ve been designing solar power systems (which are of course DC) for 40 years and participated in writing the NEC Article on solar installations. So for sure the NEC applies to DC circuits. Just ask any solar installers who have failed their electrical inspection. So if you design a DC electrical system for your house (regardless of voltage) you need the follow the currently approved version in your city/county (not all are on the same version) and have the Authority Having Jurisdiction (AHJ- typically your county building department) approve it.

Where the confusion comes in is that an RV is not a house nor is it a motor vehicle. There is a section in the NEC specifically for RV electrical installations (Article 551). But an RV moves around so who is the AHJ? To solve that jurisdictional problem the RV industry association (RVIA) worked with the NFPA to produce NFPA 1192 specifically for RV safety. So it covers propane, ingress/egress, etc. It also covers electrical by referencing NEC Article 551 for all electrical systems. So again, the NEC applies to RV’s.

However, it also references another standard, ANSI/RVIA LV specifically for low voltage electrical systems. I’m too cheap so far to drop the $45 to buy that one (all the current NFPA docs including the NEC are available on the NFPA website for free viewing if you want) so I don’t know what’s in it.

But I can tell you that a basic rule in electrical system design is to maintain a solidly grounded neutral at all times and place all fuses, breakers, and switches in the ungrounded line so that when you open circuits they are dead and you can’t accidentally reenergize them by creating a connection to ground somewhere else. There are rare exceptions mostly for industrial applications but we don’t need to get into that here.

So the summary is tha the NEC applies to both AC and DC systems and to RV’s. There is another standard I don’t have access to for RV low voltage electrical systems that also applies. And good electrical design tells us to install disconnects, switches, and fuses in the ungrounded (positive) conductor. Sorry it’s so complicated.

Thanks for the background.  Lots of information out there and some gray areas dont make it easy for newbs.  I will connect the switch on POSITIVE.

Not sure if this helps but here is the thread where I installed my switch.

I connected the line from battery thru circuit breaker to one side of switch terminals.  I took all the wires that were formally on the battery + and put them on the other terminal on the switch. If you turn the switch off it kills all the DC to the trailer.  I have noticed if you plug the trailer into A/C power the convertor seems to supply 12 volts to some circuits not sure which but I know the refrig fan comes on even if the battery switch is off.