Sizing copper line sets for air conditioning and refrigeration comes down to a tight balance between pressure drop and gas velocity. Run a suction pipe that is too large, and refrigerant vapour slows to a crawl, trapping compressor oil in the indoor coil and starving the compressor crankcase. Run it too small, and line friction chokes cooling capacity while driving up power bills.
For standard back-to-back split systems, pre-insulated pair coils make sizing straightforward. But once you tackle multi-storey townhouses, commercial ducted systems, or runs exceeding 15 metres, guessing pipe diameters causes noisy gas hiss through walls, low suction pressure, and premature compressor burnouts.
This guide covers standard copper tube sizing under AS 1571, outlines the minimum velocity rules (4.5 to 15 m/s) needed for reliable oil return, and details the AS/NZS 5149.2 requirements for vertical riser P-traps.
Working on unit sizing or line trim charge? Calculate your kilowatt thermal capacity with our Air Conditioner Size Calculator, or determine the extra gas to weigh in for extended pipe runs using our Refrigerant Charge Calculator.
Australian Pipe Sizing Chart (AS 1571)
Residential split systems and commercial line sets in Australia use seamless copper tube certified to AS 1571. The table below lists standard Australian pair coil line set pairings matched to unit cooling capacity in kilowatts ($kW$):
| Capacity Tier (kW) | Typical Unit Type | Suction Line (OD) | Liquid Line (OD) | Suction ID (mm) | Wall Thickness |
|---|---|---|---|---|---|
| 2.0 kW – 3.5 kW | Small Bedroom / Study | 3/8" (9.52 mm) | 1/4" (6.35 mm) | 7.90 mm | 0.81 mm |
| 5.0 kW | Master Bedroom / Living | 1/2" (12.70 mm) | 1/4" (6.35 mm) | 11.08 mm | 0.81 mm |
| 7.1 kW – 8.0 kW | Open Plan Living / Office | 5/8" (15.88 mm) | 3/8" (9.52 mm) | 14.06 mm | 0.91 mm |
| 10.0 kW – 12.5 kW | Mid Ducted / Multi-Head | 5/8" or 3/4" | 3/8" (9.52 mm) | 17.01 mm | 1.02 mm |
| 14.0 kW – 16.0 kW | Large Residential Ducted | 3/4" (19.05 mm) | 3/8" (9.52 mm) | 17.01 mm | 1.02 mm |
| 18.0 kW – 22.0 kW | Commercial Ducted / Multi | 7/8" (22.22 mm) | 1/2" (12.70 mm) | 19.94 mm | 1.14 mm |
| 25.0 kW+ | VRV / Package Condenser | 1-1/8" (28.58 mm) | 1/2" or 5/8" | 26.14 mm | 1.22 mm |
Suction Line Sizing & Oil Return Velocity
Compressor lubricating oil mixes continuously with circulating refrigerant. In the suction vapour line, where the refrigerant travels as a low-pressure gas, the oil travels along the pipe walls strictly by gas velocity momentum:
- Horizontal Runs: Maintain a minimum gas velocity of 4.5 m/s (optimally 6.0 to 10.0 m/s). Pitch horizontal lines at 1:200 toward the outdoor unit so gravity assists oil movement.
- Vertical Risers: Require a higher velocity of at least 6.0 to 7.0 m/s to pull oil droplets upward against gravity.
- Upper Velocity Cap (15 m/s): Velocity above 15 m/s causes audible gas rushing noise through walls and creates excessive friction.
- Allowable Pressure Drop: Design suction lines so total saturation temperature drop ($\Delta T$) stays below 1.0°C to 1.5°C (roughly 15 to 25 kPa on R32 systems). Every additional 1°C of suction loss robs roughly 3% to 4% of system cooling efficiency.
Liquid Line Sizing & Flashing Prevention
Liquid lines carry high-pressure subcooled liquid from the condenser to the expansion valve. Sizing is governed by friction pressure drop rather than oil return, because lubricating oil dissolves fully in liquid refrigerant:
- Target Velocity (0.5 to 1.5 m/s): Keep liquid velocity below 1.8 m/s to prevent hydraulic liquid hammer when solenoid valves slam shut.
- Preventing Flash Gas: If an undersized liquid line causes pressure to drop below saturation before reaching the indoor unit, the liquid boils into gas bubbles. This starves the expansion valve (TXV/EEV) and causes severe valve hunting and hiss.
- Subcooling Buffer: Modern reverse cycle units provide 3°C to 5°C of subcooling at the condenser exit, which protects against reasonable pipe friction on standard runs.
AS/NZS 5149.2 Vertical Riser P-Trap Rules
When an outdoor condenser sits on a roof or upper-level bracket above the indoor unit, oil draining backward down vertical suction pipework during system off cycles can flood the indoor coil:
- Base of Riser P-Trap: Mandatory under AS/NZS 5149.2 whenever a vertical suction riser exceeds 3 metres. The trap collects oil at shutdown. On compressor startup, the trapped oil temporarily reduces the pipe cross-section, boosting local gas velocity to blast the oil plug up the riser.
- Intermediate P-Traps: For tall risers exceeding 6 metres of vertical rise, install an additional intermediate trap every 6 vertical metres.
- Inverted Trap at Coil Outlet: Install an inverted trap rising slightly above the top of the indoor evaporator coil before heading down to the main run. This stops liquid refrigerant pooling into the coil during off cycles.
Equivalent Length & Fitting Friction Losses
Pipe sizing must account for total equivalent length, not just measured linear metres. Every bend, elbow, and valve adds friction resistance equivalent to straight pipe:
- Standard 90° Long-Radius Bend: Add 0.4 m to 0.8 m of equivalent pipe.
- Tight 90° Elbow: Add 0.7 m to 1.2 m of equivalent pipe.
- 45° Elbow: Add 0.2 m to 0.4 m of equivalent pipe.
- Service Isolator Valve: Add 0.3 m to 0.6 m of equivalent pipe.
For example, an installed 14-metre run with four 90° bends and two service valves has an equivalent friction length of roughly 17.5 metres. Always size your pipe and verify pressure drops against this total equivalent length.