Condensate Drain Size Calculator

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Every unit draining to this line, added together (1 ton = 12,000 Btu/h ≈ 3.5 kW). A single unit uses its own capacity.
tons
Developed horizontal length of the drain from the pan connection to the termination — used only for the slope-fall figure.
ft

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Minimum drain size(ID)
1in
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1 in nominal — the band runs to 40 tons, leaving 14 tons of headroom before the next size.

Minimum internal diameter — the code floor is 3/4 in no matter how small the unit.

Also computed

Size band ceiling40tons

Required fall over run(Δh)5in

At the code-minimum 1/8 in/ft; confirm the route can actually drop this much before it terminates.

Total elevation drop at the 1/8 in/ft (≈1%) code-minimum slope — steeper is better.

Method notes 4 notes
  • Bands are the model-code minimums (IMC 2021 Table 307.2.2 / UPC Table 814.3): 3/4 in to 20 tons, 1 in to 40, 1-1/4 in to 90, 1-1/2 in to 125, 2 in to 250 — the locally adopted code governs.
  • Condensate drains never shrink: 3/4 in minimum everywhere, and no size decrease from the drain pan to the point of disposal.
  • Slope at least 1/8 in/ft (IMC 307.2.1 calls this 1-percent), support the pipe so it cannot sag into traps of its own, and trap the unit per the manufacturer.
  • Fuel-fired appliance (condensing furnace/boiler) condensate is acidic and separately regulated — neutralization and material rules differ from cooling-coil condensate.

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