Why does ductile iron pipe come in 21 foot lengths? I know that 21' is a standardized Irish "perch", but I don't know why they use that for lengths of ductile iron pipe. I am just curious...

I don't know what a " Irish Perch" is, (would that be a fish with a Irish brogue?) but since Galvanized and Black Iron pipe all come in 21 feet lengths I guess they figured 21 feet would be OK for ductile iron pipe as well. Regards, Tom