Saved me the trouble CK.
I have 165' of CAT-6e POE (power over ethernet) going to a Ubiquiti NS2 that requires at least 11.8V. POE injection voltage is 32V to get 12.8v at the other end.
I have several Icom IC-735's that are aligned and tuned at 12v. Apply 14v to them and they act like they have a turbocharger and NOS hung on them. Tune them up at 14v and supply them with 12v and they barely run.
I too have a FT-857D. What a power hog. I not only upgraded the power wires but did away with the fuse holders and use a fast voltage/current trip normally seen in aircraft use. Just changing the DC feed to 10ga got a significant increase. The fuses may be good protection but are a choke point.
CK used published and well known wire tables for resistance and voltage drop. These calculators can be found all over the interwebz. It's not rocket surgery, just math.
Now a bit of a gotcha. The boost converters easily available and all over China-bay are in a word "noisy". For marginal DC runs and limited current drain they may work. Leave them on in receive and watch the noise floor . The limited input filtering (usually just a small choke/cap input) of the radio can't filter out the hash on the DC input and it shows up on the transmitted signal as well. Not only does the hash from the converter end up on the signal but some digital radios have a fit over the dirty power and exhibit it in what can only be called "gremlins" .
Practical solution:
Make the battery bank at least 18v (24v may be better) and use a linear regulator at the source (the battery bank) that produces 14v at the sink (radio or other device). You still have the resistance and voltage drop but you have what you need in (at) the end, and you don't have to suffer the square wave hash of a switching regulator.
Not sold in stores
YMMV
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Only your electrical engineer knows for sure
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