We live in a rural area and I am running a circuit to to our gate for a couple of post lights and maybe a receptacle. The run is 485 feet from the closest panel. I am installing THHN wire in a 1" diameter PVC conduit. Due to the low amperage draw of the lights (<5 amps) I can calculated I can get by with #6 THHN. I will install a 15 amp breaker on this dedicated circuit. My question is can I get by with a #8 ground or should I run a #6 THHN ground as well?
Where the grounding conductors are increased in size (from the minimum) the grounding conductor also needs to be increased in size proportionately per 2014 NEC 250.122. I used the southwire voltage drop calculator. The calculated value needed for a 5 amp draw is #8 copper for 5 amp load at this distance 2.8% voltage drop. The problem with down sizing the conductor in this case would come if the lighting was changed at a later date using more of the available ampacity of the circuit then the conductor would be under sized. 120V @ 12 amps would require a #4 conductor and the #8 would be two small to limit the voltage drop for a short circuit.
Your numbers are valid, but please do not buy 1/4 mile of 6 AWG wire. Instead, let's rethink this.
I'm assuming you want what you say - 2 post lights in the range of 800 lumens each (like the old 60W bulbs.) Incandescent bulbs are not a "sacred cow" for you and you'll use new tech if it saves money. The convenience outlet is expendable.
You don't need to worry about the harsh colors of LEDs, that problem has been solved.
Core strategy: higher voltage.
What happens if you double voltage? For the same useful power, amps are cut by half. Voltage drop (in volts) is cut to 1/4. However this is only half as important, since voltage has doubled. All in all, double the voltage means 1/8 the power loss, which means 6-8 wire sizes smaller to do the same thing.
How do you get 240V lighting? Several ways. First, many lamps are made multi-voltage for the world market (100V Japan to 240V England). You can't use Edison screw bases over 120V in the USA, but you can use lamp sockets like GU24 or Mogul. An LED "bulb" will outlast the fixture, so many fixtures just have non-removeable LEDs. Those are fine too, as long as they are rated for 240V.
Also, some fixtures/bulbs are made for industrial American voltages, including 120V, 208V, 240V and 277V. An example is metal halide lighting commonly used in parking lots; these use jumpers on transformers.
Stay 120V, drop the outlet, use LED bulbs
In this strategy we stay at 120V, but cut the convenience outlet and power only two GE 88373 800-lumen LEDs, 10.5 watt, same brightness as a 60W incandescent. 21 watts is 0.175 amps, call it 0.2 amps and plug it into the voltage drop calculator.
It says.... 14 AWG wire! At 0.2 amps draw, voltage drop is only 0.52 volts.
That's not bad. What if we kept the convenience outlet? We could actually draw 12 times this, or 2.4 amps, or 264 watts, before voltage drop went over 6 volts or 5%. So there's headroom for light loads like (LED) Christmas tree lights.
Run it as a 240V circuit. Keep the convenience outlet.
Obtain a multi-voltage LED lamp which is, say, 15 watts each and works at 240V. Total 30 watts at 240V = 0.125 amps. I'm not even going to run the numbers on that. Obviously, that's going to work.
Let's see what else we can do. Two "175W" metal-halide lights (this is A LOT of light!) These are 225W each actual, so 450W, 1.875 amps. Voltage drop calculator says this will result in 2.04% actual voltage drop using 14 AWG wire still.
Now, what about that convenience outlet? It'll be a NEMA 6-15 type, looks like this. Now, buy a portable step-down transformer for about $100 which plugs into this and gives 120V. How much could you draw? Most 120V appliances max out at 80% of a 15A circuit, or 12 amps. That's 6 amps at 240V. Voltage drop calculator says 6.5% voltage drop, which is an 8 volt drop for a 120V appliance. Higher than we'd prefer for fixed wiring... but common when using extension cords. And remember, we're still on 14 AWG wire. Not bad.
Run 4 wires for both 120V and 240V
In this scenario, run 120/240V with neutral - 4 wires. Combine the first two methods: use 120V (hot to neutral) for tiny LED lighting loads and an amp or two on 120V outlets (think LED Christmas lights). Use 240v (hot to hot) to run more serious lighting and plug-in loads via a step-down transformer. All of the above math applies.
Go with the 240V or 240/120V option. I would price 14, 12 and 10 AWG wire, as the cost difference is not large. As I show, with 240V, 14 AWG will do what you need, and 12 AWG will do it quite comfortably.
You must use THWN wire, the "W" being "Wet". Most places sell THHN/THWN, which is fine.