I have a subpanel to an outdoor patio 75 yards from the house main breaker box and don't think it is wired properly. There are two large hot lines from the main controlled by a 50 amp breaker that go to the subpanel. There is a large ground connected directly from a grounding rod next to the subpanel. Only limited power comes out of individual 15 amp breakers at the subpanel. Is there supposed to be a neutral coming from the main breaker box to the subpanel? If so, what is the minimum size I need due to the distance?
Phone quickie answer:
Tester101‘s method is the conventional one. Her are some potentially cost saving alternatives.
If you need 120/240 and don't want to run new cable, you could obtain a (perhaps used) 12KVA or larger outdoor rated transformer and install this as a new main service. 5KVA would suffice if you changed the supply breaker from 50A to 20A. You'd redesignate the panel as a main panel, add a main breaker (it could be back fed) and ground rods just as you would have for a house.
Your 2 hots would deliver 240V to the primary of the transformer, and the secondary would supply split 120/240 with neutral. This would be isolated from house neutral, and would bond to the local ground rods. This might not require running a ground wire from your main panel. Might.
Or, if you only need 120V service at this location, you could keep this as a sub-panel but re-designate one of the two "hots" as a neutral - but this is only allowed if the wire is 4 AWG or larger, which it may well be. In this scenario you definitely need to run a ground wire.
If you want to bring a new neutral from the house, it cannot be routed any way you want. It must be bundled or in the same conduit with the other "hot" wires. Ground wires have no such restriction and can take any viable route, provided they are not being misused as a neutral.
Anytime you are working in wire 6 AWG or larger, seriously consider the new AA-8000 series alloy aluminum wire. It is in wide use for this "big stuff". Kill on sight any 1350 alloys as they are now outlawed.
Perhaps this thing was originally wired for a 240V-only hot tub. And when that was removed, some enterprising handyman tried to provide local 120 service via the ground rod, which assumes the earth conducts electricity well, which it does not. In fact every time you plug a 120V appliance into that panel, you are energizing ground and creating a shock hazard.
Sounds like they're trying to use the earth as a neutral, which in the US is not allowed.
If the wires are through conduit, and the conduit is large enough, you can simply pull two more conductors.
If the wires are in a cable, or the conduit is too small. You'll want to pull a new four wire cable, or four conductors through a larger conduit.
To compensate for the voltage drop across the 225' long conductors, you're going to have to use larger conductors. In this case, you'll want 4 AWG copper conductors. If you're going to install a new four wire cable, you can use 4-4-4-6 type USE copper cable.
*Not an electrician, just my layman's understanding:
1) Yes you should have two hots and a neutral, unless (and this is highly unlikely) your sub panel is on a single phase, in which case you could have 1 hot 1 neutral but that would be wrong wiring of a sub-panel to begin with. You actually should probably also have a ground so 4 wires to the panel.
2) Technically you could go 6ga but I would go 3ga here.
3) PLEASE DO NOT CHANGE THIS YOURSELF. HIRE AN ELECTRICIAN TO CONFIRM YOUR THINKING!