United will open the defence of their trophy against a team they have beaten in their previous three domestic cup campaigns.
Peter Houston's men defeated County 3-0 at Hampden in May and clinched a 2-1 victory after extra-time at Dingwall in the Co-operative Insurance Cup in September.
Rangers have been handed a potentially difficult tie against in-form Kilmarnock at Ibrox.
Rangers needed Kenny Miller to complete a hat-trick after beating 10-man Killie 3-2 with the help of two penalties in the Clydesdale Bank Premier League on Saturday.
Celtic will face Berwick Rangers with the tie scheduled to be held at Shielfield Park.
Source: PA
Source: PA