Rule of thumb only. For more detailed design, consider coil load condensate or psychrometric chart
Minimum Condensate Drain Size
0-20 tons 0-8,000 cfm 1″
21-40 tons 8,001-16,000 cfm 1-1/4″
41-60 tons 16,001-24,000 cfm 1-1/2″
61-100 tons 24,001-40,000 cfm 2″
101-250 tons 40,001-100,000 cfm 3″
251- 100,001 cfm 4″
AC Condensate flow…
Range: 0.02-0.08 GPM/ton
Average 0.04 GPM/ton
Unitary Packaged AC equipment: 0.006 Gpm/ton
Air Handling Units (100% outside air): 0.100 GPM/1,000 CFM
Air Handling Units (50% outside air): 0.065 GPM/1,000 CFM
Air Handling Units (25% outside air): 0.048 GPM/1,000 CFM
Air Handling Units (15% outside air): 0.041 GPM/1,000 CFM
Pipe size shall not be smaller than drain pan outlet
Minimum size below grade and below ground floor shall be 2-1/2″. Drain shall have a minimum slope of 1/8″/ft
Verify pipe size and discharge requirements with local authorities and codes
Depth of trap must exceed by one pipe diameter the total static pressure of the fan