How to use this calculator
- Add up the connected capacity. Every unit draining to the line, in tons of refrigeration (1 ton = 12,000 Btu/h). A single unit uses its own capacity.
- Read the band. The calculator returns the minimum internal diameter and how much capacity headroom the band still has.
- Check the fall. Multiply the horizontal run by 1/8 in/ft — the route must be able to drop that much, continuously, with no sags.
- Verify against the adopted code. IMC and UPC agree on the bands, but the locally adopted edition and any amendments govern.
How it works
Condensate drain sizing is a band lookup, not a formula. The two model codes publish the same minimum internal diameters by combined cooling capacity, and they agree exactly:
≤20 tons → 3/4 in · ≤40 → 1 in · ≤90 → 1-1/4 in · ≤125 → 1-1/2 in · ≤250 → 2 in
Two rules ride along with the table: the drain is never smaller than 3/4 in anywhere, and it never decreases in size between the pan and the point of disposal. The third constraint is slope — at least 1/8 in per foot of horizontal run — which this page converts into the total fall the route has to find. Cooling capacity itself comes from the airflow & BTU load calculator, and what the coil actually condenses ties back to the coil bypass factor.
Worked example
Verified against the live calculator
Three rooftop units — 8, 8 and
10 tons — manifold into one condensate main with
40 ft of horizontal run:
26 tons → over 20, within 40 → 1 in minimum · fall = 40 ft × 1/8 in/ft = 5 in
The 1 in band carries to 40 tons, so there are 14 tons of headroom before a fourth unit would push the main to 1-1/4 in. The run needs a continuous 5 inches of drop — set the high point at the farthest pan and confirm the termination sits low enough before pipe goes in.
Frequently asked questions
What size condensate drain for a 5 ton AC unit?
3/4 inch. The first code band (IMC Table 307.2.2 / UPC Table 814.3) runs all the way to 20 tons of refrigeration, so every residential unit — 1.5, 3, 5 tons — lands on the same 3/4 in minimum, which is also the absolute code floor.
When does a condensate drain need to be 1 inch or larger?
When the combined capacity draining to the line passes 20 tons: 1 in carries to 40 tons, 1-1/4 in to 90, 1-1/2 in to 125 and 2 in to 250. In practice that means manifolded equipment — rooftop units sharing a main — not single residential systems.
What slope does a condensate drain need?
At least 1/8 inch per foot of horizontal run (IMC 307.2.1 labels it a 1-percent slope). Over 40 ft that is 5 inches of fall — worth checking the route can actually drop that much before the termination point.
Can a condensate drain reduce in size along the run?
No. The code is explicit: the drain shall not decrease in size from the drain pan connection to the place of disposal, and it is never smaller than 3/4 in internal diameter anywhere.
Method & assumptions
- Bands are the model-code minimums — IMC 2021 Table 307.2.2 and UPC Table 814.3 publish identical values (verified against two independently published adoptions). The locally adopted code and its amendments govern.
- Gravity drains from cooling-coil pans; pumped condensate, combination waste connections and indirect-waste rules are separate code territory.
- Fuel-fired appliance (condensing furnace/boiler) condensate is acidic and separately regulated — neutralizer and material requirements differ.
- Beyond 250 tons the published table ends; large plants size condensate hydraulically as an engineered design.
- Traps, cleanouts, air gaps at termination and approved pipe materials come from the code text and the equipment manufacturer, not this screen.