Found a solution

ThisWorkbook.Activate
Application.Calculation = xlCalculationManual

because:
You need to have a workbook active before you can set the Calculation
property and although it seems counterintuitive, you can "activate"
your
add-in workbook to serve this purpose even though it's not visible.
You need to have a workbook active before you can set the Calculation
property and although it seems counterintuitive, you can "activate"
your
add-in workbook to serve this purpose even though it's not visible.

Source:http://www.excelforum.com/archive/in.../t-260962.html

Thanks
Lars