I know that normally a receptacle or device in the gang box counts as 2 conductor volumes. Does a line voltage thermostat just count the same as any other device? I'm replacing an old mechanical 240V thermostat with a programmable digit one (Honeywell TL8230A1003) and I've found that this new thermostat takes up significantly more space in the gang box than the old one and it is a very tight fit. Does the box fill calculation account for a device like this differently?
The thermostat is on a 20A breaker and 12/2 wire is coming into the box. There is also 12/2 wire going to heater and 12/2 wire going to another thermostat/heater for a total of 6 conductors. The 3 bare grounds count as another conductor and then the thermostat as 2 for a total of 9 conductors. This would indicate that I need a 9 * 2.25 = 20.25 cu. in. gang box unless the thermostat somehow counts as more.