Why 0.8 gallons per hour per ton?

Roughly 0.8 GPH of condensate per ton of cooling is a widely used field estimate for moderately humid conditions; in very humid climates it can reach 1.2 GPH per ton or more. It gives a conservative flow figure to confirm the drain has ample capacity, even though code sizing is by equipment tonnage, not the computed flow.

How does IMC Table 307.2.2 size the drain?

The International Mechanical Code sizes condensate drains by equipment cooling capacity: up to 20 tons uses a minimum 3/4 inch line, over 20 to 40 tons uses 1 inch, over 40 to 90 tons uses 1-1/4 inch, over 90 to 125 tons uses 1-1/2 inch, and over 125 to 250 tons uses 2 inch nominal pipe.

Do I need a secondary drain line?

Most codes require a secondary drain or an overflow safety (auxiliary pan with drain, or a water-level switch that shuts the unit down) wherever condensate overflow would cause damage. The secondary line is sized the same as the primary per the same table.

Does pipe slope matter?

Yes. Condensate drains must slope at least 1/8 inch per foot toward the discharge so they drain by gravity and do not hold standing water that grows algae and clogs. Sizing sets the diameter; slope ensures it actually flows.

Draw-through air handlers (most residential units) need a properly sized P-trap on the primary drain so the negative pressure at the coil does not hold water in the pan or pull air through the line. The trap depth depends on the unit's static pressure, set per the manufacturer's instructions.

Condensate Drain Line Sizing Calculator

The condensate drain sizing calculator estimates how much water an HVAC coil produces and returns the minimum drain pipe diameter required by the mechanical code. It keeps installs both code-compliant and free of the overflow callbacks that come from undersized or clogged drains.

How it works

Two steps: estimate flow, then size by code:

condensate flow (GPH) = tonnage x 0.8 GPH/ton   (humid-condition estimate)

IMC Table 307.2.2 minimum drain diameter, by equipment capacity:
  up to 20 tons        -> 3/4 in
  over 20 to 40 tons   -> 1 in
  over 40 to 90 tons   -> 1-1/4 in
  over 90 to 125 tons  -> 1-1/2 in
  over 125 to 250 tons -> 2 in

Note that the code sizes the drain by equipment tonnage, not by the computed GPH; the flow estimate is shown so you can sanity-check capacity and plan the discharge point. The secondary or auxiliary drain is sized identically to the primary.

Example and tips

A 5-ton residential air handler produces roughly 5 x 0.8 = 4 GPH of condensate and, being under 20 tons, needs a 3/4 inch minimum primary drain and a matching 3/4 inch secondary or overflow safety. A 50-ton rooftop unit needs a 1-1/4 inch drain per the table. Always slope the line at least 1/8 inch per foot, fit a manufacturer-sized P-trap on draw-through units, and add a cleanout near the unit so the line can be cleared without cutting pipe.