If you are placing an MPA on a gantry, using the 'MPA on Gantry' support type, you will be unable to directly adjust the messenger wire or system height.
The messenger wire height is fixed for an 'MPA on Gantry' as it must attach to the supporting equipment at a specific point relative to the gantry. Changes to the system height would also move the messenger wire, therefore Lineardraft will not allow you to make any adjustments to this value either.
Instead, you should either adjust the contact wire or gantry height to achieve the required system height.
