Trying to power 100 Arduino's from single power line
You are already using a differential signalling system that doesn't care about ground levels, so you don't need to worry about that side of things.

As for power distribution - a strategy known as point of load regulation is what you want. That is where you supply a higher voltage (but lower current) power feed to one, or in this scenario more likely a group of, boards and regulate that power down using a switching regulator to a lower voltage (5V) and higher current. 

This has the advantage that:


You are only worrying about small groups of Arduinos, not the whole lot
You can use thinner wires to feed in the power to a group
You get less losses from wire resistance at high currents


For 100 Ardunos you may want to have maybe 10 to a group with a 3A switching regulator for each group, giving an average of 300mA per board (depending on your current needs, of course - you may want less per group and more groups to give more average current per board).

In that example you'd have a total output current limit of 30A in 10 sections at 5V. That equates to (P=V×I) 150W. So a 200W 24V power supply would be adequate.

At 24V, 150W would demand 6.25A (plus some more to account for the efficiency of the regulators, so call it 8A - hence the desire for a 200W supply).

For 3A you need at least 24AWG wire - but preferably considerably thicker - maybe 18AWG.  The 8A for the 24V would need minimum 15AWG, preferably bigger - although you don't need that size all the way to the end of your chain of regulators. If you had each regulator wired directly back to the power supply individually that would only need thin wires (absolute minimum 30AWG, which is tiny - though to keep losses to a minimum you'd want more like 20AWG).

So for simplicity and to reduce your wire costs, just stick to one size all the way through so you're buying only one gauge of wire (so you get a bigger discount for buying more).  You could quite easily use:


18AWG direct from the PSU to each POL regulator (24V, 0.8A)
18AWG from each POL regulator to all Arduinos in a group (5V, 3A)


Of course, this is all using numbers plucked out of thin air, and you'd need to adjust it to fit your specific project, but you get the idea - higher voltage to a group, regulate to 5V, then 5V to each Arduino in the group. You don't need to care about grounding etc because you're using RS-485.