Custom formatting of Numbers

Hi,
I am trying to store and display a number with a scale of 10. The number that am pasting to the cell is : 9999999999999.9999999999. The custom format that I've chosen is " #,###,###,###,###.########## " The thousand's seperator gets displayed correctly but only 2 digits after the decimal point are displayed.
So the display changes to 9,999,999,999,999.99 thereby losing scale. Is there a limitation to the number of digits that can be displayed ? Or is it that the format I am using is inappropriate ?

I would be grateful for any pointers.

Thanks !

There is a limit of 15 digits in a number in excel.