When to use chassis and when to wire to battery neutral?

I would use the chassis for the ground and the added 4AWG (or larger) wire with a suitable fuse (with a switch if necessary) for the supply to the inverter or other high-load device.

If you can find an easy spot to mount on the chassis, this is the best place to run the ground to for three reasons.

• First, it's usually a lot more convenient than running your ground to the battery.
• Second, it's a really good grounding place (as in solid and works well).
• Thirdly, you run a lot less wire, which helps in cost and clutter.

According to this guide, you'll want to use 3AWG or larger wire. 4AWG wire is only rated to 60A for power transmission. 750 watts at 12vdc is 62.5A, which is more than what the 4AWG is rated for. You may actually want to run 2AWG wire for both if the length of run is longer than you're comfortable with. This is just an added measure of protection.

750 watts is slightly less than 63 amperes. So, the question is whether the car chassis can supply 63 amperes.

Let's pick a 4 meters long steel object. If it weighs 500 kg (I assume a car chassis weighs 500 kg, correct me if this is incorrect), its volume for density 8000 kg/m3 (different types of steel have different density, but 8000 kg/m3 is a rough approximation) is is 0.0625 m3 and its cross sectional area is then 0.015625 m2.

The electrical resistivity of carbon steel (NOT stainless steel) is 1.43e-7 ohm*m. This means the resistance of 0.015625 m2 cross section is 9.1520e-6 ohm/m, and for 4 meter long object it's 3.6608e-5 ohm. For a current of 63 amperes, it's 0.0023063 volt drop. In other words, 2.3 millivolts. Do you think this is a problem? I don't think it is a problem.

The loss of power in watts is 0.145 watts, compared to 750 watts. Quite minimal.

Now, how much copper do you need to achieve the same resistance? For 4 meters long wire, you need 1.59e-8 ohm*m / A * 4 m = 3.6608e-5 ohm, solving the equation gives A = 0.0017373 m2. In other words, 1737 mm2. Such a 4 meter long copper wire would weigh 62.3 kg! My 5 meter long jumper cables are just 50 mm2.

I don't know if you consider 62.3 kg copper wire a problem, but I do. There's no way you can match the resistivity of the chassis with copper!

In summary: almost always you want to use the amount of metal in the car chassis instead of installing separate wires. Even though steel does not have optimal resistivity, the sheer amount of metal in the car chassis overcomes the slightly less than optimal resistivity.

Edit: what is the current carrying capacity of the car's chassis? If you consider 0.1 volt drop ok, it's 0.1 V / 3.6608e-5 ohm = 2732 amperes. This would make the car chassis dispose of heat at 273 watts. I'm pretty sure you could perhaps even double the voltage drop to 0.2 V, meaning 5463 amperes and 1.1 kilowatt heat disposal. I'm certain the car chassis can dispose of that amount of heat.