Huh? That's how you know what size cable to use. Figure your load and run length and calculate the volt drop based on your voltage. Go up in cable size until you have an acceptable drop.
Using a welding lead calculator is going to have you spending a lot more on copper than you need to. 40' of cable is going to give you a lot more drop than 6'.
Nah, you know what size cable (and fuse) to use when you know what the max amps are that it will need to carry. Voltage drop doesn't really matter. I know - everyone says it matters - especially the "solar experts" - but it pretty much doesn't.
As soon as he hits the power switch on that winch under load, it's going to draw down the voltage big time anyway. When the battery is at 12.8v, and you hit the switch and dump 150a of load on it, and the voltage of the circuit (battery, wire, winch motor) drops to 11v...how is a couple of percentage points either way actually going to matter? It won't. What matters is that the wire can carry however many amps it needs to.
But how about a different sort of load - like an inverter? Okay, let's do that. To make the math easy, I'll say an inverter rated 1200w continuous, 1800w surge. At 12v that'd be 100a and 150a. Let's say it's 5' from the battery, so a 10' loop And I'll use the "common wisdom" of 3% max voltage drop as "acceptable".
I'll use this voltage drop calculator, because it does DC:
http://www.calculator.net/voltage-d...ce=10&distanceunit=feet&eres=100&x=55&y=10
And I'll use this AWG wire gauge table from some university because it's handy:
https://www.eol.ucar.edu/rtf/facilities/isff/LOCAL_access_only/Wire_Size.htm
Okay, so we need to carry a max of 150a at 12v. According to the AWG chart, if we were doing chassis wiring (we're not) we could get away with #3, but since we're doing power transmission, we need 1/0 (one-ought) to carry 150a.
So let's plug that into the voltage drop calc - 150a, 12vdc, 10' loop. We get 1.25% voltage drop.
Oh...but that's way too low - we can use 3%...so let's make the wire smaller until we get to 3%. Well, #4 gives us 3.08%, but exceeds our "3% maximum". #3 gives us 2.50%, which is below the 3% max. Now we go back to the AWG wire chart and find that #3 is only rated for 75a for power transmission.
So which wire do we choose? The one that can carry 150a, or the one that gives us 3% or less voltage drop...but can only carry half the amps we need?
We choose the one that can carry the amps we need - of course.
But that's all deceptive anyway. Say the inverter has a cutoff of 10.5v. So now we need to calculate at 1800w / 10.5v = 171.42a. So let's redo the voltage drop calc with the true max amp number of 172a and the true voltage of 10.5v and we get 3.24% drop with #3. Woops, that's exceeds the "3% max". So we move up to a bigger wire. With #2 we get a 2.57% drop. That's under 3%. Now we check and see that #2 is rated to carry 94a for power transmission.
But we need to carry 172a...
So instead, let's just size the wire for the expected amp load and see what we get. AWG chart says we need 2/0. So 2/0, 10' loop, 10.5v, 172a - 1.24% drop.
Well...that's certainly less than the "3% max" everyone prattles on about, and it will also carry the 172a load.
Now, just for ******ts & giggles, let's crank up the distance to see how far we can get before we exceed 3% with those numbers. At 24' we'll hit 3.05%.
So if we size the wire to carry the expected amp load, we can put the inverter up to 12' from the battery before we have to worry about voltage drop exceeding 3%. But what if we go 20' away from the battery. That'd be a 40' loop. According to the calculator, that's going to give us a .54v drop.
But all that means in the real world, is that the inverter is going to shut down with the battery at 11v instead of 10.5v.
And if we're maxing out that inverter, and dragging the battery down to 11v - is that extra half a volt really going to matter? Nah - if the inverter shuts down with the battery at 11v instead of 10.5v...so what? Who cares? Besides...the only time it's actually going to happen is when the inverter is running at max overdrive and the battery has been dragged down to damned near dead.
Now if we're speccing out a solar rig for a house and talking about one-way distances of 50' or 100', then we have to give a crap about voltage drop and we need to dial everything in to where we might save a couple grand on copper.
But we could instantly cut our copper need in half just by doubling the voltage of the system. Double it again, and cut it in half again. And, at higher system voltage, voltage drop is less of an issue anyway.
Just because everyone says, "voltage drop is soooo important" doesn't mean it's actually true in every situation.
It just depends.
