It appears that the OP is burning propane. Propane != NG. Maybe the OP means natural gas, but I am thinking the OP is on a propane tank. Propane tends to be quite a bit more expensive than natural gas. The reason for the change might also be that the OP changed the other items that used gas over to electric and perhaps the WH is the last thing on the tank. If that is the case, it would be good to switch to electric so the tank could be eliminated. We don't know the details.
Around here, a natural gas WH would be only slightly cheaper to run than electric (and propane would be more expensive). However, if the WH was the only gas device, the basic sevice charge for just having natural gas available would kill any potential savings. Many people assume that gas will always be less expensive than electric (often is), but you have to run the your numbers to know for sure.
For the actual question:
- hardwired, 220V, 30A circuit, 10-2 Romex wire is typically what is needed for an electric unit