Just a note, it would be a HUGE waste of money and very short sighted buying and installing a cable that is too small for the job, just because it is cheaper. The extra cost of a bigger cable is insignificant when you account for how long it will be there, and the spare capacity will allow future extensions or additional equipment to be used. Having to dig another trench and install another cable next year when you need to run more than one thing at a time, is vastly more expensive!