A plumber is sizing the cold water distribution piping for a small commercial building with a peak demand of 3.8 L/s. The available water pressure from the municipal supply is 480 kPa, and the highest fixture is located 12 metres above the main service entrance. The piping will be Type 'L' copper, and a maximum friction loss of 25 kPa per 30 metres of pipe is acceptable. What minimum nominal pipe size (NPS) is required for the main cold water supply line?

Question: A plumber is sizing the cold water distribution piping for a small commercial building with a peak demand of 3.8 L/s. The available water pressure from the municipal supply is 480 kPa, and the highest fixture is located 12 metres above the main service entrance. The piping will be Type 'L' copper, and a maximum friction loss of 25 kPa per 30 metres of pipe is acceptable. What minimum nominal pipe size (NPS) is required for the main cold water supply line?

Answer options:

• 1 1/4 inch
• 1 1/2 inch ✅ 2 inch
• 2 1/2 inch

Correct answer: 2 inch

Explanation: First, calculate the pressure loss due to elevation: 12 m * 9.81 kPa/m = 117.72 kPa. The minimum required residual pressure for most commercial fixtures is about 140 kPa, but even if we aim for 200 kPa for safety, the total pressure drop allowed for friction is 480 kPa - 117.72 kPa (elevation) - 200 kPa (residual) = 162.28 kPa over the entire run. Assuming a run of 60m for calculation, the allowable friction loss per 30m is 162.28 kPa / 2 = 81.14 kPa/30m. However, the question states a maximum acceptable friction loss of 25 kPa per 30 metres. Using a pipe sizing chart for Type 'L' copper and a flow rate of 3.8 L/s with a friction loss of 25 kPa/30m, a 2-inch pipe (often 50 DN) is generally the smallest size that can accommodate this flow and pressure drop. Smaller sizes would result in excessive friction loss or inadequate flow.