What is the ruling on working as an electrical engineer in the establishment of a restaurant that includes a section dedicated to serving alcoholic beverages, and is expiation obligatory after 80% of the work has been completed?

If your work in designing or implementing the electrical system of a restaurant cannot be separated from the electrical system of the place where alcohol is consumed (the bar), then this work is impermissible for you, because it is aiding in what is forbidden. As for the work you have already completed, seek Allah's forgiveness for it. The money earned from it does not entail any obligation on your part, because the act of electrical work itself is permissible, and the prohibition lies in the forbidden use. It is preferable to donate a percentage of the money equivalent to the bar's proportion of the total work.