Rep Power: 200

well u made it sound like we need to measure and check all welding joints then get a calculator and go crazy with the wires.
if ur running under 1000 watts amp and most of all its not even 1000 watts
with at least 700 watts rms and most of the wires you buy at best buy or any audio mobile store will give you the correct recommendation of what ratings and length should be at. and plus who wants to go crazy with car speakers,

 
Rep Power: 211

Not sure I picked up half of what you said here, but who honestly is going to want to run TWO fat wires through the firewall, nevermind one? (Not to mention you have to buy the extra wire)...It's way easier to just bolt down the wire to a bolt on the chassis near the location of your amp. Of all the stuff I've read I've never heard of this "high/low ground resistance" stuff..not to say it doesn't exist, it just sounds like something I should be concerned about if I spent $5000+ on a sound system. If you really wanted to, you could sand the bolt or clean it with adhesive remover to get rid of all the gunk that usually builds up on them, to lower the, uhhh "ground resistance" Besides, how would you even measure that? Oh, and, not talking down to me like im 5 years old would be appreciated.

 
Rep Power: 195

I never made it sound like anything even remotely like that. His statement was the ground should NEVER be longer than 3 foot. That is inaccurate. You do want to keep the ground short if possible for "reduced resistance" not for "Protecting the amp from a short".
There are cars where you may not find a suitable ground within 3 feet and in those cases it is best to run the ground to the negative side of the battery. There are many reasons why you will not find a good ground point near the amp. Why do you guys do the "Big 3" rewiring under the hood. If the manufacturer did such a great job, why is everyone replacing their ground straps under the hood???
If you stay within the "MAX" 3 foot rule and cannot find a low resistance point, you cause the amp to have to work harder as it is not receiving enough power and it will eventually damage the amp.
The only thing you need to measure is the resistance of the spot you are wanting to ground to.
As far as going crazy with car speakers, have you ever been in a car with a bad ground loop. The alternator whine will drive you crazy. And all it is is a simple grounding issue.

All I wanted to do is correct some mis-information. Too much of it is spread around and taken as gospel. Your ground wire can be longer than 3 feet for proper grounding and you will not reduce the "short protection" that the ground wire supposedly provides for the amp.

 
Rep Power: 195

Glad to hear the average 5 year old can understand algebra. I am trying to explain why and how, and contradict bad information.
You can easily measure resistance with a cheap DMM (Digital Multimeter)
Sure it's easier to just connect to a bolt near the amp. That's what most people do (me included) and it's fine most of the time.
If you don't understand resistance then how do you know what size wire to get for your amps? Can you power a 700w mono sub amp on 20 feet of 10 gauge wire that's mounted in a trunk? Not for very long, at least not at a higher volume. The resistance of the cable will drop the current that reaches the amp below it's operating level.
Your amp requires a certain amount of power to operate. Generally you need 14.4 volts to reach optimum output. For my sub amp the specs are "Power Requirements: 14.4V DC (11V to 16V) " This means that it will operate on power of between 11 and 16 volts, with 14.4 being optimum.
Resistance is power loss traveling over the surface to get to the destination. The larger the cable the less resistance. Car parts do not always provide a good path for electrical current. Paint, glue, poor tack welds all create resistance. In these cases, it is best to go directly to the battery.
Where I think the confusion may lie is in the term grounding. When you ground your amp, you are completing a circuit, not protecting it. DC travels in a linear path. It starts at the battery, continues to the amp via a power wire, then back to the battery. Usually through a body panel which is connected to the negative battery terminal.

And a good ground is just as important in a $300 system as it is in a $5000 system.