I have a Square D 100 amp main panel with plenty of capacity left and I am wanting to add a 50 amp outdoor sub panel that is 215 feet from the main panel. The sub panel would have only two 120 volt 25 amp breakers. I only plan to run a 1500 watt livestock tank heater, a possible overhead light and have one circuit open for additional small load on a 25 amp breaker. I was going to run three wires of #4 stranded wire in conduit buried in the ground. Using one for hot, one neutral and one ground. I was going to use a single pole 50 amp breaker in my main panel, but I am not finding a single pole 50 amp breaker to purchase from Square D. Can I use a double pole 50 amp breaker and only hook to one side of it or what other options do I have to feed this sub panel out of the main panel? Thanks, Kevin
Given your actual use case: 20A double (240V) GFCI breaker feeding a 4-wire MWBC is by far the most cost effective way to serve the described loads. Given that one of those loads is "future expansion" already, overkill that makes things cost a lot more is a dubious choice unless you really want to burn money. I would say to go ahead and put the lighting on the other side than the heater, though, in case "future expansion" never happens, at least until it does. The load does not need a 25A breaker, and a 25A breaker can't be connected to an outlet, so it's a mostly useless size anyway.
A 25A double (240V) GFCI breaker would give the same capacity (50A at 120V) as your 50A single, but that might as well be 30A (same required wire size) but either of those lose the special exception for a 20A GFCI protected circuit only needing to be buried 12", so you have to go with full depth depending on type of conduit or direct burial cable.
Of course, if you're willing to buy 210 feet of rigid or intermediate metallic conduit (RMC or IMC) you get to have only 3 wires (the metal conduit is ground) and you only need to bury it 6" to top of conduit (unless going under a driveway.) But it will generally cost more than other options with 4 wires.
Use #6 aluminum off a 50A breaker, or #2 aluminum off a 60A breaker.
The distance is very punishing to 120V power, requiring several wire size bumps. The only way #6 is going to work is if you run 4 wires (so a proper 120/240V subpanel).
#2 could work as a 120V-only subpanel with 3 wires.
Unfortunately there is no way to downsize the ground wires beyond the sizes found in cable.