I hate to confuse you more but 2800 watts divided by 5 equals 560 watts. That’s roughly the heat from 5 light bulbs. A typical plug in space heater is 1300-1500 watts. Are you sure it’s not 2800 watts each? But if it is then 2800 divided by 208 equals about 13.5 amps. That’s a bit over the recommended constant load for a 15a circuit. I would use 12 gauge wire but I would use the 15A breakers. Breakers are rated for overload before they trip. BTW I use 208 for less than optimal conditions.