Running wire from alternator directly to rear bank

[snafu]

Fuses have been covered.  Do it.

So, how many runs do you need for 600A of alternator?  For 900A?  That's simple to determine using some common sense, a carbon pile load, and a few quality DMMs.  Voltage drop is what we want to know.  Less than 3% and we can typically move on.

1/0 AWG Copper Cable is rated for 300A over a length like this with minimal voltage drop across it.  So, one could say (2) runs for 600A and (3) runs for 900A and have an excellent starting point.  Then, comes the validation with the carbon pile.  Keep in mind the cable in and of itself is not our circuit - our circuit includes the complete path from the alternator and then returning to the alternator, including every single termination along the way.  When we measure voltage drop greater than 3% of the entire circuit, this doesn't necessarily mean we don't have enough cable.  It means that our circuit has higher resistance than desired and work is being wasted as heat.  Then, we use our DMM across terminations, junctions, components, in an effort to determine if we have excessive voltage drop at various places in our circuit.  Repairs are made and then we retest.  At that point, our voltage drop is 3% or less or it isn't.

So, why can't we just use amplifiers or SPL readings in lieu of a carbon pile?  Bueller?

[Smoove]

20 hours ago, ChevyBoy95 said:

I have a 270a alt and a 320a alt and have 4 runs of 2/0 positive alone, with 7 rear frame grounds and 5 up front (steel frame truck so no full length).

I had a similar setup

250 and 370 alt. 7 rear grounds and 7 or 8 front (steel frame truck so no full length)
4 runs of + 1/0 70/30 soundquest ( Stinger ).
zero voltage drop at the amp. 14.89 at the front battery and 14.89 at the amp.
after PROPERLY fusing the path to the amp I saw 14.85-14.86 at the amp.

Fun fact to remember especially on music. You will not see a sustained amp draw it will be bursty. Also, you only burp on a meter, so it's just a short burst.

After my rewire and 3rd upgrade I will have the following
350, 250 and 370 alt. 7 rear grounds and 7 or 8 front (steel frame truck so no full length)
4 runs of + 1/0 welding .

6runs for 900amps is a bit much.
4runs for 600amp is a bit much.

Remember current draw is bursty.

Updated
LOL, Tony C beat me to the info.
 

[ChevyBoy95]

32 minutes ago, Smoove said:

I had a similar setup

250 and 370 alt. 7 rear grounds and 7 or 8 front (steel frame truck so no full length)
4 runs of + 1/0 70/30 soundquest ( Stinger ).
zero voltage drop at the amp. 14.89 at the front battery and 14.89 at the amp.
after PROPERLY fusing the path to the amp I saw 14.85-14.86 at the amp.

Fun fact to remember especially on music. You will not see a sustained amp draw it will be bursty. Also, you only burp on a meter, so it's just a short burst.

After my rewire and 3rd upgrade I will have the following
350, 250 and 370 alt. 7 rear grounds and 7 or 8 front (steel frame truck so no full length)
4 runs of + 1/0 welding .

6runs for 900amps is a bit much.
4runs for 600amp is a bit much.

Remember current draw is bursty.

Updated
LOL, Tony C beat me to the info.
 

Overkill is the name of the game lol. Plus wire is cheap compared to most things in audio. But there is diminishing returns of continually adding more and more wire after a certain point. 

Under load which is where the percentage change comes in, I have a little under 2% loss front to rear. 13.1v front, 12.85v rear under full load. With no load, there will be no drop in voltage (no voltage drop if there is no current through the cable) which is why you should check it under load.

[snafu]

Indeed.  Measurements under load are what we're after.  All voltage measurements need to be made over time and MIN readings analyzed.