Need of a License : Master plumbers need a license to work in Delaware. Prepare to Pass the Actual Plumbing Exam:Ħ00 Plumber Exam Questions, Flash Cards, and Testing Tips Whether you live in Wilmington or Dover, or another city and county, the resources below will help you get started on your path to receive your master plumber license. To become a master plumber in Delaware, you will need a license and may need to take an exam.