When sizing pipes, the following criteria are applied in determining compliance with UK and USA recommendations. Non-corrosive water is assumed. For large pipes, UK recommendations are a maximum flow velocity of 10 ft/sec and in the USA 15 ft/sec is recommended. UK Recommendations Pipe diameter <= 2 in All pipes flow velocity >= 2.5 ft/sec and <= 5 ft/sec All pipes pressure drop >= 0.5 ft/100ft and <= 3.5 ft/100ft Pipe diameter > 2 in All pipes flow velocity >= 4 ft/sec and <= 10 ft/sec All pipes pressure drop >= 0.5 ft/100ft and <= 3.5 ft/100ft USA Recommendations Pipe diameter <= 2 in All pipes flow velocity >= 2 ft/sec and <= 4 ft/sec All pipes pressure drop >= 0.75 ft/100ft and <= 4 ft/100ft Pipe diameter > 2 in Plastic pipes flow velocity >= 4 ft/sec and <= 5 ft/sec Steel, copper pipes flow velocity >= 4 ft/sec and <= 10 ft/sec All pipes pressure drop >= 0.75 ft/100ft and <= 4 ft/100ft