For an official PhD dinner following your disputation, you can deduct up to NOK 24,450, subject to a maximum of NOK 1,227 per head (rates applicable for the 2016 income year) if you are unable to document higher expenses.
You will only be entitled to a deduction for the dinner for tutors, disputation opponents, etc. No deduction will be given for food expenses relating to family and friends.
Deductions for PhD expenses are in addition to the minimum standard deduction.
You enter the deduction under item 3.3.7 Other deductions. You must be able to document the deduction using receipts and present a list of participants' names if the tax office asks you to.