Voltage drop is the killer
The problem is, voltage drop is extremely acute at very low voltages, because currents are inherently higher, and voltage drop is a function of currents (E=IR). It's a square function against voltage. That means that distributing 5 V instead of 12 V is worse by a factor of 5.76. As a quick example, for a 100' (30 m) run with 3.6% voltage drop:
5 volts @ 12 amperes (60 W) needs 212 kcmil (4/0 AWG)
12 volts @ 5 amperes (60 W) needs 26 kcmil (6 AWG)
Don't "bomp the wah"; actually do the voltage drop calculation
One time I looked at a solar project. 15 A @12-19 V, 14 AWG wire would be fine. Except he was going 200' (60 m). So he gave it a wire size bump, and an extra one or good measure, clear up to 10 AWG wire. Of course, the system was a complete failure and didn't recharge his batteries as calculated. He couldn't understand. "Bat ah bomped the wah!"†
† Translation: "But I bumped up (increased the size of) the wire"
And you armwaved that too. You gave it a perfunctory bump to #10 and went "oughta be enough!" You actually have to go into the voltage-drop calculator and check. This should be based on actual current drawn by the loads on the wires, not breaker or supply rating. Rating a long branch for 20 A when it only needs to carry 2 A is a waste.
I've done bump calculations where it turned out I didn't need the bump.
Comply with Code ... But it's not too bad
You still need to follow the Electrical Codes for installation that is part of the building. Fortunately, the electrical codes are very relaxed for low voltage installations, and further relaxed for <55 W installations.
Neighbor site diy.stackexchange.com is a good one for Code questions.
12 V is so much better that you ought to do it.
As you saw from the 12/5 voltage drop calculation above, 12 V works worlds better. It has voltage drop considerations, still; but they are vastly reduced. Meanwhile, it is highly versatile, and the system can even be extended to be a whole-home backup system.
- LED 12 V lighting has huge variety and is more dimmable and hackable than mains lighting
- Most Internet routers and modems can run on it
- Some TVs can run on it
- Batteries are readily available for it cheap or in a number of chemistries
- Most solar charge controllers can supply it
- which means in a power outage, you have lights and Netflix
- Some smart doorbells can even run on it.
- That with a gas-only furnace like an Empire, gas water heater and stove can make a house perfectly livable with the grid down or absent
What's more, getting from 12 V to 5 V is really really not a problem. It is possibly the most commonly sold power converter anywhere!