how to size ground cable from house battery to frame

Do you have a battery isolator (switch) to disconnect the house batteries from the van battery(ies)? I.e., will the starter/other vehicle loads draw current from the house batteries?

Here is a simplified chart from the NEC regarding wire AWG and current:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

For off grid solar power systems, I suggest that you should use the NEC derating of wire and fuses for maximum continuous current flow. of 1.25x upsizing of cable. For example, if you have a 150 Amp alternator/charging source, then:

• 150 Amps * 1.25 NEC derating = 187.5 Amp minimum rated branch circuit rated wiring and fuses/circuit breaker.

Note: In reality, most automotive type Alternators, even if they are rated at 150 Amps don't really supply that amount of current when charging your batteries... As the alternators heat up, their output current falls quite a bit--Plus the extra wiring length (increased voltage drop) also reduces average maximum current flow.

Regarding voltage drop, for wiring between your charge controller and battery bank, a 12 volt bank should be around 0.05 to 0.10 maximum drop ideally. And for your inverter and DC circuit wiring, around 0.5 volt maximum. You can use a simple voltage drop calculator to estimate the voltage drop (and play with the AWG numbers to find the right size/length of wiring):

https://www.calculator.net/voltage-drop-calculator.html

-Bill

You are in an "awkward zone". Vehicles typically use the frame/chassis metal as the "return path" (negative) for battery bus power. And solar power systems and our "standards" use wiring for +/- battery bus and only use the greenwire/chassis ground for "safety" (someplace for short circuit current to flow and trip a fuse/breaker if a "hot wire" gets shorted to metal).

Your starter and alternator use the chassis for current flow--And the starter may hit 100-200 amps (depending on your engine type/size/etc.).

For "safety ground" chassis grounding... A 6 AWG cable between negative house battery bus and chassis ground is usually "good enough" for up to a 200 Amp battery bus (and fuse/breaker).

So, now you have to look at the 12 volt loads... The AC inverter, solar and house battery charger are "easy"--They have Hot and Return wires directly from the devices back to the battery bus... No normal current flow in chassis.

The diesel heater and various DC vehicle devices may (for example) use the chassis ground for their return current path. Using the NEC link and derating, you would be looking at a 55 amp design:

• 55 amps * 1.25 NEC derating = 68.75 Amp minimium rated fuse wiring (round up to next "standard" size fuse/breaker) Say 75 Amps
• NEC Table: 75 Amp rated branch circuit = 6 AWG to 2 AWG (depending on wiring insulation rating)

And then looking at voltage drop... Say 55 Amps with 10 foot one way wiring run and 0.5 volt maximum drop (note, most of the time, chassis ground resistance is less than the return copper cable designed for the branch circuit):

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=1.296&voltage=12&phase=dc&noofconductor=1&distance=10&distanceunit=feet&amperes=55&x=69&y=23

6 AWG:
Voltage drop: 0.43
Voltage drop percentage: 3.62%
Voltage at the end: 11.57

And a similar set of calculations for other loads and your charging sources connections (Voltage Drop 0.5 volts for loads, voltage drop of 0.05 to 0.10 volts for charger to battery bus drop). Note that the Alternator voltage drop to house battery may never be that low--Vehicle charging systems are just not designed for optimal deep cycle power system charging.

Other issues with using chassis ground for the return leg... The engine, frame, and chassis tend to be separated by rubber mounts. And these mounts should have a heavy cable (or braided cable) that "jumps" the mount to electrically connect the motor/frame/chassis to battery ground.

For a vehicle, they have relatively small loads (lights, radio, etc.). And if you add a bunch of larger DC loads, then it is possible that you will exceed the current capacity of the chassis ground bonding cables. Probably not an issue, but it is something to understand.

Also, if you do something "strange", you may have to add bonding wires. For example, you put your diesel heater on the back door (yea, you probably would never do it, just an example), you would need to add a ground bonding strap from the door back to the door frame/chassis. Otherwise, the return current flowing through the door hinges/bearing will spot weld/ruin the bearings.

Again, you probably will not have "rebuild" your chassis ground system. But there are issues and, in some cases, extensive ground bonding is required (for example, HAM radio folks find that they may need to add ground bond wires "everywhere" like doors, gates, etc. to reduce RF electrical interference from poorly ground sheet metal/electronics.

Is this even helping, or just confusing you more? Grounding is a complex subject and easy to make mistakes.

-Bill

I (and everyone here) are very happy to help.

Regarding:
I just want to be clear here... The battery side of the shunt goes to all of the batteries (if parallel) negative posts/bus.

The load side of the bus, is where all of your load returns, and the chassis ground cable, are connected.

If you put any return wires on the battery side of the shunt, those currents will not be "seen" by the shunt and the Trimetric will not log those currents (and time).

-Bill