backup-by-copying-when-privileged-mismatch is a variable defined in `files.el'.
Its value is

Non-nil means create backups by copying to preserve a privileged owner.
Renaming may still be used (subject to control of other variables)
when it would not result in changing the owner of the file or if the owner
has a user id greater than the value of this variable. This is useful
when low-numbered uid's are used for special system users (such as root)
that must maintain ownership of certain files.
This variable is relevant only if `backup-by-copying' and
`backup-by-copying-when-mismatch' are nil.

You can customize this variable.