Hi Daniel,
If you want to use the constant loss option to model wake losses, choose "Constant Loss" from the Wake model list. You can then set the value to zero. If you choose one of the other wake loss model options, SAM ignores the constant loss value and instead calculates wake losses as part of the simulation based on the spacing between turbines, wind direction, and other parameters as described in SAM's Help system.
Best regards,
Paul.