[chrismorse]

There are a couple of variables worth considering. First is the amperage rating of the 240 volt item. You don’t want to melt the wire and start a fire, or constantly trip the breaker trying to pull 40 or 50 Amos through a 10 gage, ( common dryer supply wiring), so size the wire to handle anything you might want to add.

Second, is the possible need of a tool or appliance for a neutral, or ground. It is better to have it and not need it than find some aspect of your equipment won’t work, or lack the safety of a ground.

Another possibility is to size the feed wire so that you could install a small sub panel, allowing you to pull 120 off for lights or a grinder, or beer refrigerator.

Barns have a way of evolving.

To recoup, if you need at least 30 amps, 240, I’d recommend going with the next size up wire, from #10 to #8, minimum, or to a #6 or #4, so you have a little upgrade room, and have a ground brought in as well.
chris