Under IS 1239, fire hydrant pipes must be Mild Steel Heavy Class (C Class) pipes. The wall thickness for a standard 100mm (4-inch) pipe should be 5.4mm, and for a 150mm (6-inch) pipe, it should be 5.4mm to 6.0mm to withstand hydrostatic testing pressures (15 kg/cm²) and prevent external soil corrosion over the years.