Up until the mid-1970s, sprinkler pipe was predominantly Schedule 40 black steel pipe.
Then an upstart company named Allied Tube and Conduit decided to shake up the piping business. At the time, Allied only produced electrical conduit and fence pipe. The owner, looking to expand his business, figured the same pipe used for fences could be used for sprinkler systems. The pipe was strong and durable, just thinner than the standard Schedule 40 pipe. Since the water just sat there waiting for a fire, why not use a thinner wall material? This started the long debate over what quality is really required for a sprinkler piping system.