ACCA Manual D • 0.1 in. wg Friction1200 CFM • 3 TonsStandard Residential (<900 FPM)
1,200 CFM Duct Size: 3.0 Ton HVAC Main Supply Trunk
Calculated duct sizing specifications for 1200 CFM of conditioned airflow. Sized via the Equal Friction Method and Huebscher equivalent rectangular equation to maintain static pressure and eliminate air rush noise.
Round Duct Size14" RoundSpiral / Snaplock Pipe
Air Velocity1123 FPMFeet Per Minute
Cooling Tonnage3 Tons@ 400 CFM / Ton
Register / Grille12x20Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 14" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 10" Deep | 24" Wide | 240 sq in | 720 FPM | 2.4:1 | Optimal (<2:1) |
| 12" Deep | 20" Wide | 240 sq in | 720 FPM | 1.7:1 | Optimal (<2:1) |
| 8" Deep | 30" Wide | 240 sq in | 720 FPM | 3.8:1 | Acceptable (<3:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:12x20 Supply Plaque / 16x25 or 20x25 Filter Grille
Maximum Target Velocity:500–700 FPM at register face
Diffuser Throw:Spread toward outside walls
Return Air Intake Rules
Return Velocity Target:<450 FPM through filter face
Filter Surface Requirement:Minimum 2 sq ft per Ton
Return Friction Target:0.05 – 0.08 in. wg / 100 ft
HVAC Sheet Metal & Installation Directives
- 3-ton systems are the most common residential installation. Use a 10x24 or 12x20 rectangular trunk leaving the plenum.
- If using round spiral pipe, step up to 16-inch to keep air velocity below 900 FPM.
Frequently Asked Duct Sizing Questions
What size duct does a 3-ton AC need?
A 3-ton AC requires 1,200 CFM of airflow, requiring a 10x24 or 12x20 rectangular supply trunk, or a 16-inch round duct.