If there is any doubt go with the heavier gauge. The wire isn't a big cost item and there isn't any downside in having heavier wire than you need. I think the wire gauge depends on the material used as well as on the length of the run.
If you're charging at 240v at home you may not even need 240v charging at work. Charging needs are based on the miles you need to drive not on the size of the battery pack. If you're only driving 30 or 40 miles a day, packs with a capacity of 30 kWh or 60 kWh or 90 kWh would all charge in the same time, and eight hours charging at 120v will give you 40 miles. You might never need to charge at work, and if you did, charging at 120v might be just fine. Just a suggestion that might save you a few bucks.