NA systems differ in several ways from British Standard (BS) rated pipe and fittings:
Diameter of the pipe
The outside diameter of the plastic pipe (which is always the measurement we are concerned with,as this fits into the socketed fittings) is the same as the UK specification with one exception. 2 1/2" pipe in the UK is 75mm outside diameter. In NA it is 73mm, and so it is NOT compatible. Apart from this size, North American pipe and fittings are fully compatible with UK pipe and fittings.
Pressure rating of pipe
The pressure rating system for imperial pipe in the UK is done in "Classes". For example every size of Class C pipe is rated to 9 bar, Class D to 12 bar etc. North American pipe follows a different set of standards. In plastic pipe, the ones we are concerned about are schedule 40 and schedule 80 (sometimes this is abbreviated to sched 40 or sch40):
Schedule 40 and 80 pipe pressure ratings are based on the wall thickness of the pipe. This is taken from set tables, originally specified by the American Standards Association (ASA) and since by the American National Standards Industry (ANSI) which were originally designed for wrought iron pipe. In this system, Each size of pipe is allocated a wall thickness for each different schedule . This, combined with the bore of the pipe (called the Nominal Pipe Size [NPS] in North America), results in a pipe that withstand a certain amount of pressure. The resulting pressure table is somewhat different to the Class system as each pipe size has a different pressure rating.
So what does all this mean for us in the UK? The main points are that the lowest pressure rating of any plastic pipe with a schedule 40 specification is 9 bar, and as the pipe diameter gets smaller, this rating increases. For example 6" pipe is 12.4 bar and 1" pipe is 31 bar. Sch80 pipe also follows the same pattern, with a minimum of 15 bar and in general has a rating 1.5 times that of sch40 pipe.
The North American system does not have a set standard that specifies the pressure rating of fittings. In general, fittings are manufactured to have a burst pressure the same as the pipe; so schedule 40, 6" fittings will have a pressure rating of 12.4 bar and a 1" 31 bar( see paragraph above).
So what does this mean for us in the UK in terms of fittings? It means that if we use sch40 fittings, they will of equivalent or higher pressure rating to Class C and sch80 fittings will be of equivalent or higher pressure to Class E.