A 1.5-inch drain can handle up to 20 gallons per minute:
• Maximum flow rate of 15 GPM for smooth interior pipes.
• Maximum flow rate of 20 GPM for corrugated or ribbed pipes.
• Flow rates may vary depending on the diameter, length and slope of the pipe.
This is a significant amount of water that needs to be handled carefully in order to avoid flooding or other issues with drainage systems.