Is it:
// current capacity in amps
#define DEFAULT_CURRENT_CAPACITY_L1 12
#define DEFAULT_CURRENT_CAPACITY_L2 24
At line 419 of open_evse.h?
Yes that is it.
Depending on the firmware version on your station you can also send a serial command $SC 12 to set the current to 12A. You should receive a $OK response.
Bryan White