Stereo Question: Dual amp power lead wire gauges

Quick question that is the result of a half-assed, part hangover, part mild laziness situation today...

Here's the question:

I'm running a 4 gauge power lead from battery to boot, into a t-style distribution block that feeds two amps. Should I use 8 gauge wire for each of the distro block to amp connections, or 4 gauge?

My reason for confusion here is unsure if the 2 x amp power feeds from the distro block should be halved to equal the single power draw of the main 4 gauge, or if all wires should be equal gauge throughout the circuit.

What gear do you plan on using?? Depends what the current draw is like. for example. sub amp could have 4 gauge and speaker amp could have 8 gauge.

You really dont need to halve the gauge.... like mitsi said above if 1 is powering a sub and the other speakers. Your going to have more draw on the sub amp. Having all the same gauge is not going to have a negative effect. Having smaller gauge after the split can have a negative effect

As long as you're not trying to pull more current than the wire can deliver, wire diameter doesn't really matter. Undersized is a problem but overkill won't hurt, but it is more expensive. Fat overkill explanation + math below.

If you've got two amps you'll need three calculations; both combined, amp 1, and amp 2. The power P in watts (W) is equal to the voltage V in volts (V), multiplied by the current I in amps (A).
P(W) = V(V) × I(A)

Current is what we're interested in to determine cable size soa quick rearrange gets us...
P(W) / V(V) = I(A)


So start with the RMS rating for your amps, roughly double it because amplifiers are not very efficient (if you know the efficiency of your specific amp you can use that instead, I've just assumed 50% here to be safe), then divide by 13.8 (nominally your alternator runs at 14~14.2V but I like to assume 13.8 is what you'll actually see at the amp) and you should be able to work out your current draw. W RMS x 2 / V = A. Again, do this for each individually and combined.

Next up measure how long each cable will be and plug your results into the table below.




So lets assume you've got a 1000W RMS amp for the sub and an 800 W RMS amp for the speakers. 8' from the battery to your distribution block and then another 2' to each amp.

First, combined.
W x 2 / V = A
1800 x 2 / 13.8 = A
2000 / 13.8 = A
~261A

At 8' in length you'd need at least 2GA cable to supply enough power.


Sub amp
W x 2 / V = A
1000 x 2 / 13.8 = A
~145A @ 2' length
Would need at least 8GA cable


Speaker amp
W x 2 / V = A
800 x 2 / 13.8 = A
~116A @ 2' length
Would need at least 8GA cable


Would I bother stepping down to 8GA from 2GA? Depends what cable you had. Maybe the 2GA cable you bought came in a 10' length and you could make do just with that? Maybe you bought it by the metre at which point there is no benefit to going 2GA all the way so yes, step down. Maybe you couldn't find a distribution block that would nicely step from 2 to 8 so you went from 2 to 4? Whatever works. Ignoring the increased cost, it really doesn't matter what cable you run as long as you are above the minimum spec size.