Galvanized steel potable water supply and distribution pipes are commonly found with nominal pipe sizes from 3⁄8 inch (9.5 mm) to 2 inches (51 mm). It is rarely used today for new construction residential plumbing. Steel pipe has National Pipe Thread (NPT) standard tapered male threads, which connect with female tapered threads on elbows, tees, couplers, valves, and other fittings. Galvanized steel (often known simply as "galv" or "iron" in the plumbing trade) is relatively expensive, and difficult to work with due to weight and requirement of a pipe threader. It remains in common use for repair of existing "galv" systems and to satisfy building code non-combustibility requirements typically found in hotels, apartment buildings and other commercial applications. It is also extremely durable and resistant to mechanical abuse. Black lacquered steel pipe is the most widely used pipe material for fire sprinklers and natural gas.
Apprentice plumber: Apprenticeship programs generally provide the most comprehensive training for novice plumbers. Either union locals and their affiliated companies or nonunion contractor organizations can administer this training. Apprenticeships typically consist of at least three to four years of paid, on-the-job training and some hours of related classroom instruction.
After completing an apprenticeship program, plumbers, pipefitters, and steamfitters are considered to be journey workers, qualifying them to perform duties on their own. Plumbers with several years of plumbing experience may earn master status by passing an exam. Some states require a business to employ a master plumber in order to obtain a plumbing contractor’s license.