I realize I will be in the minority, but I believe too many people commit cable overkill when designing their vehicle power systems. In many cases, it turns out to be a case of “penny-wise, pound-foolish” as they fail to take into consideration the total system performance and the impact of relatively high-impedance interconnections and components. A little practical knowledge regarding basic electronics will help immensely.
You are definitely on the right track by using online voltage drop calculators, as suggested by Question #2. The key is to understand precisely what you are trying to achieve and/or avoid. Are you concerned that undersized cabling will heat up under load and lead to a fire? Are you attempting to minimize power loss because you want to maximize your power system’s efficiency?
Question 1 - “What size cable?” See Question 2 - use a voltage drop calculator to appropriately size the wire for the intended amperage and the length of the run, based upon the minimum acceptable criteria you establish… plus a little bit extra for margin.
Question 2 - “What is an acceptable voltage drop?” You’ve already calculated your maximum load to be 100 Amperes. At 13.6 Vdc, a 1.5% voltage drop would be 0.204 Vdc. Would less than a quarter volt decrease likely affect any of your appliances? No. Your power (P=I*V) loss would be approximately 20 watts. Is that enough to heat up a cable to the point that nearby objects would combust or plastic melt? Unlikely, especially if the 20W are distributed over a substantial length of wire.
Question 3 - “Copper or tin?” This is almost a “plastic or paper” question… there is no “right” answer. However, unless everything else to which you’re connecting your cables are exclusively copper, I’d recommend tinned copper for corrosion resistance and dielectric compatibility.
Additional Consideration #1: The cost of higher capacity cables and/or connections increases disproportionately to the derived benefits. Despite some of the other comments on this thread, my experience has been that there is a HUGE difference in the cost of 4 GA and 4/0 GA cable and lugs.
Additional Consideration #2: Availability of higher capacity cables and components. Guess what? That 4/0 GA lug only comes with a 1/2” hole and you want to connect it to a 1/4” lug on your battery separator. Yeah, you can probably make it fit by grinding down the edges, but…
Additional Consideration #3: Heavier cables and connectors are more difficult to handle. Not only are the cables themselves difficult to bend, route through firewalls and such, they are also more difficult to terminate. Prepare to invest in several different flavors of heavy-duty connector crimpers and/or heavy-duty soldering equipment to ensure you aren’t creating high-impedance connections that will totally overwhelm any power savings from the heavier cable.
Additional Consideration #4: Take a look at all of the components in your proposed design and make certain that they are properly matched for your requirements. Keep in mind that every time you route power through a set of relay contacts or circuit breaker, there is a set of metal contacts with a small, yet perceptible resistance, that is going to rob a finite percent of your electrical power. Quite frankly, if maximizing your overall power efficiency is your goal, I’d be more worried about the 1.5 A of power that is typically consumed by the energizing coil of your typical battery separator relay than the relatively insignificant line loss of your cable - scrap your SurePower 1315-200 for a Blue Sea 7622 magnetic switch.
Anyway, these are just my thoughts on your topic. I thought that at the very least, I would attempt to respond to each of your three questions.
