What size cable wire should I use for an amp that is 400 watts RMS, 700 peak, and a 1.5 farad capacitor?

You will be fine with 8 gauge wireing. You do not need the cap at all though, that was a total waste of money. I am running a 900.4 amp 450 watts rms and a 5500/1 3200 watts rms, I have no cap what so ever. I did however have to do the big 3 upgrade, but no cap. Besides for running 400 watts rms the biggest cap that you would realisticly really have is a 0.5 farad cap, nothing more. Rule of thumb is 1.0 farad for every 1000 watts rms, or 0.5 farad for every 500 watts rms. Which is why you should only have a 0.5 farad cap. But in all reality you do not need a cap, just a waste of money. A cap is not going to stop your headlights from dimming ever, that you need to do the big 3 upgrade. The cap is not a magical device. If your having your lights dim on you adding a cap is going to add another strain on your already over taxed electrical system. Can not provide enough electrical power for everything your running now, throw a cap in there. How is the electrical system going to charge the cap? By not sending proper electrical power to other devices that you may need to have running.

Go with 4gauge for ground and power that is what you need and keep your rca wires away from power wire to prvent noise.

I have 8 gauge wire. That's power and ground. But im pushing a few more watts than that. No matter what u decided be sure that your amps are getting enough power.

as big as you can run threw the car,the bigger the better.