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ACR Copper Tube & Line Set Sizing Explained

How ACR copper tube is sized by actual outside diameter, how that compares with plumbing copper, and what to confirm before selling a line set. Line sizes for a given system come from the equipment manufacturer.

Ref HV-0001ASTM B280ASTM B88ASTM B743

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

ACR tube (air conditioning and refrigeration) is seamless copper tube made to ASTM B280 for refrigerant circuits. Its size is its actual outside diameter (OD): 3/8 in ACR measures 0.375 in OD. ACR tube is cleaned, dehydrated and capped or plugged (and often nitrogen-charged) at the mill so moisture and debris stay out of the system. A line set is a matched pair of tubes, a smaller liquid line and a larger insulated suction (vapor) line, sold in fixed lengths with the ends capped.

How to identify

  • Measure the OD with a caliper or OD gauge. The measured OD is the ACR size.
  • Markings: hard straight lengths are marked "ACR" along with the size and manufacturer. Soft coils are boxed or labeled as ACR/B280.
  • End caps: ACR is shipped capped. Open-ended copper in a plumbing bin is not ACR, even if the OD matches.
  • Plumbing tube (ASTM B88) is named by nominal size, which is 1/8 in smaller than its OD. That is why the same tube can be called "1/2 nominal" by a plumber and "5/8 ACR" by an HVAC technician.

ACR OD vs plumbing nominal

ACR size (actual OD, in)1/43/81/25/83/47/81-1/81-3/81-5/82-1/8
Plumbing nominal with the same OD1/81/43/81/25/83/411-1/41-1/22

Key points

  • Line size is set by the equipment manufacturer, not by a rule of thumb. It depends on the refrigerant, capacity, line length, lift and oil-return needs. Use the installation manual's line-set table and its long-line instructions.
  • Service-valve sizes on the outdoor unit show the factory connection size. They do not always equal the correct line size for long runs.
  • Wall and pressure: higher-pressure refrigerants (R-410A, R-32, R-454B and especially R-744 CO₂) need tube rated for the system's design pressure. Check the tube manufacturer's pressure data and the equipment requirements. Some CO₂ systems use special high-pressure tube or copper-iron alloy tube.
  • Insulation: the suction line is normally insulated. Insulation thickness and UV protection may be set by the energy code and the manufacturer.
  • A2L systems: some jurisdictions and manufacturers add rules on joints in occupied spaces. Check UL 60335-2-40 and the installation manual.

Common mistakes

  • Selling plumbing Type L in a "5/8" request when the customer needs capped, clean ACR.
  • Reusing an existing line set that is undersized, or of unknown cleanliness, after a refrigerant change without following the manufacturer's guidance.
  • Leaving coils uncapped in the warehouse. Moisture and dirt get in.
  • Treating the outdoor unit's stub size as the line size for every length.

Governing standards: ASTM B280 (ACR tube); ASTM B743 (seamless copper tube in coils); ASTM B88 (water tube, for comparison); UL 60335-2-40; ASHRAE 15; equipment manufacturer installation instructions.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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