I have a hot tub I would like to set up for 220V/30A. I have a J-box with 3 wires, 2 black and 1 green, that is wired to a 30A circuit breaker. Measuring either black to green gives me 110, measuring black to black yields 220. There is a solid copper for earth ground as well. The connections for the tub are Brown, White and Black.

Hope it's a double 30. Black to black, black to brown, ground to ground. Usually see red or blue but brown is acceptable. Doesn't the tub have a wiring diagram?

BTW, usually see 120 and then 240. Are you in the US?

Yes. Tub Schematic shows L1 to Black/Blue stripe. L2 to Black. Neutral as White.

Yes, my bad. Using an analog meter.

If it shows a neutral are you sure it's straight 240VAC? There is no neutral in 240. Make sure it isn't 120/240VAC. If it is, you will need to use a 10/3 not a 10/2.

Most 220v spas ALSO use a white neutral wire. You're going to have to pull an additional white wire.

The bare copper wire is a BONDING wire.