49 CFR §192.373

1. (a)Cast or ductile iron pipe less than 6 inches (152 millimeters) in diameter may not be installed for service lines.
2. (b)If cast iron pipe or ductile iron pipe is installed for use as a service line, the part of the service line which extends through the building wall must be of steel pipe.
3. (c)A cast iron or ductile iron service line may not be installed in unstable soil or under a building.