It depends on the size of your inverter. Typical draw on the 12V side of the inverter is at least 10X the draw on the 120V output. For example, If you have a 1000 watt inverter, the maximum output draw will be 1000 watts / 120V or 8.3 amps. Since there are losses in the inverter & you might not have a calculator handy, an easy way to do the calculation is pretending the output voltage is 100, adding some space for inverter losses. Using that, you can expect to draw 10 amps out which ends up being 100 amps in.
Even if you don't plan to "ever" load the inverter to its maximum, the wiring should be planned for it. You don't know what the future will bring & rewiring later is a pain. A wire size table such as Powerstream's
is a starting point. Using the the table values for chassis wiring, it appears that a #6 wire would be adequate for 100 amp load (table quotes 101 amps), however you also have to take in the voltage drop caused by that current going through the wire. There is a calculator further down the page that lets you determine voltage drop.
For example, using the calculator for a 10' run of #6 wire, the voltage drop would be 0.813V so the input of the inverter would see 11.187V. Most inverters would shut down at this low voltage, thinking the battery was going dead. (Note - there is actually some leeway since a fully charged 12V battery is going to be at more than 12V.)
Anyway, the wire size is going to have to be increased to keep the voltage drop within reason. Xantrax recommends a minimum wire size for its 1000W inverter of #0. While that is pretty conservative, even with that size you will lose .2V heating the wiring. The important thing is to consider voltage drop in the wiring since you are working with low voltages. While 3% loss is used to calculate house wiring; that doesn't work for 12V wiring since you are starting with a lower voltage.
Sorry to go on so much about inverter wiring, but as I said, it is a pain to redo it. As to the wiring itself, I had cables made for me by GenuineDealz
. While it would have been less expensive to do it myself, they have a good range of colors, can put what ever ends you want on the cable (and cover them with heat shrink) etc. I enlarged the hole in the floor the original #10 wiring ran through and resealed around the new wiring. If you have a 6V system, don't forget the jumper between batteries.
Be sure to add a catastrophic fuse as close to the battery as possible. I used a 150 amp fuse. I also wanted the ability to shut off the high power wiring into the inverter so I added a 250 amp switch.
Hope I answered all your questions.