No matter what wire you run outside the engine compartment, it MUST be fused. In a crash, a 4 gauge wire will carry 1000 amps for quite a while before it burns off. Just buy a good quality fuse holder. I have had inverters on my fleet vehicles for 20 years, and rarely need more than 1500 watts, and that only intermittenly to start a motor. Read your appliance requirements, and multiply by 2 for safety.