Copper tubing is a hollow, cylindrical pipe made from copper or copper alloys, used primarily to carry water, refrigerant, and fuel gas through buildings. It’s one of the most common materials in residential and commercial plumbing, and it shows up in HVAC systems, medical gas lines, and underground water service connections. What makes copper tubing distinct from other piping materials is its combination of natural corrosion resistance, ability to conduct heat efficiently, and antimicrobial properties that actively reduce bacterial growth inside the pipe.
Types K, L, and M
Copper tubing for plumbing comes in three standard grades, each defined by its wall thickness. The letter stamped on the tube tells you how thick the walls are, which directly determines how much pressure the tube can handle and where it’s safe to install.
- Type K has the thickest wall of the three. It’s the go-to choice for underground water service lines running from the meter to the house, direct burial installations, high-pressure water systems, and commercial plumbing. When maximum strength and durability are required, Type K is the standard.
- Type L has a medium wall thickness. This is the most commonly used grade for interior residential plumbing, covering hot and cold water supply lines, main distribution lines inside the home, and fire sprinkler systems where code permits. Most residential and interior plumbing projects call for Type L.
- Type M has the thinnest wall and a lower pressure rating. It’s used for branch water supply lines, repair work, and low-pressure residential systems. Not every local building code allows Type M, so it’s worth checking before using it.
Beyond these three, there’s also ACR (air-conditioning and refrigeration) tubing, which is sized by its outside diameter rather than nominal pipe size. ACR tubing is cleaned and capped at the factory to prevent moisture and debris from contaminating the refrigerant system, making it the required choice for HVAC and refrigeration work.
Hard-Drawn vs. Soft Copper
Copper tubing comes in two tempers that behave very differently. Hard-drawn (rigid) copper is sold in straight lengths, typically 10 or 20 feet. It holds its shape, resists the stresses of expansion and contraction, and is the standard for exposed runs along walls or through ceilings where you want clean, straight lines.
Soft-temper (annealed) copper is extremely malleable. It’s sold in coils and can be bent by hand around corners and through tight spaces without cracking. This makes it the better option for underground burial, running through walls, and any situation requiring curves or intricate routing. The tradeoff is that soft copper has less structural rigidity, so it needs support more frequently in exposed installations.
Why Copper Works for Plumbing
Copper naturally resists corrosion in most water conditions. Over time, it develops a thin protective layer (the green patina you may have noticed on old copper roofs) that shields the metal underneath from further degradation. This is why copper pipes in older homes can last 50 years or more without failing.
Copper also has strong natural antimicrobial properties. When bacteria come into contact with copper surfaces, the copper ions damage cell membranes, generate reactive oxygen species that destroy the cell’s internal structures, and degrade RNA. This process works against bacteria, viruses, and fungi alike. For drinking water systems, this means copper tubing actively discourages biofilm buildup inside the pipe, the slimy bacterial colonies that are notoriously difficult to remove and can harbor pathogens.
Thermal conductivity is another advantage. Copper transfers heat rapidly, which is why it’s the standard material in heat exchangers and hydronic heating systems. In hot water supply lines, this means the pipe reaches temperature quickly, though it also means uninsulated copper pipes lose heat to surrounding air faster than plastic alternatives.
Gas Line Applications
Copper tubing is approved for natural gas and propane distribution in many areas, though not universally. Gas-rated copper tubing is coated in a yellow polyethylene (PE) jacket to protect against external corrosion, especially when buried or run through concrete. You’ll see it sold as Type L coated or Type R (refrigeration grade) coated, in sizes ranging from 3/8 inch to 5/8 inch for residential gas lines. Local codes vary on whether copper is permitted for gas, so this is one application where checking your jurisdiction’s requirements matters before purchasing materials.
How Copper Tubing Is Joined
There are several ways to connect copper tubing, and the right method depends on the pressure and application.
Soldering is the most familiar method for residential plumbing. A torch heats the joint and filler metal (solder) flows into the gap between the tube and fitting by capillary action, creating a permanent, watertight seal. Soldering works well for low-pressure water lines and is the technique most DIY plumbers encounter first.
Brazing uses the same basic concept but at higher temperatures, producing a much stronger joint. It’s required for high-pressure systems like HVAC refrigerant lines, where the pressures involved would compromise a soldered connection. Both methods demand clean, properly fluxed surfaces and adequate ventilation when working with a torch.
Flaring involves expanding the end of a soft copper tube into a cone shape that mates with a flare fitting. This is common on low-pressure refrigerant lines and gas connections. Swaging expands one tube end to accept another tube of the same diameter, and the joint is then brazed or soldered for a permanent seal.
Compression fittings use a brass ferrule and nut that tighten around the outside of the tube, creating a mechanical seal without heat. These are especially useful where you may need to disassemble the connection later, such as at fixture shutoff valves or appliance hookups. Press-fit fittings, which use a specialized tool to crimp a fitting onto the tube with an internal O-ring, have become increasingly popular for commercial and residential work because they eliminate the need for an open flame entirely.
Common Sizes and Availability
Copper tubing for residential plumbing typically comes in 1/2-inch, 3/4-inch, and 1-inch nominal sizes. The nominal size refers to the approximate inside diameter, while the actual outside diameter is always 1/8 inch larger. So a 1/2-inch nominal copper tube actually measures 5/8 inch on the outside. This standardized sizing means Type K, L, and M tubing of the same nominal size all share the same outside diameter. The difference is entirely in wall thickness, which changes the inside diameter and the weight per foot.
ACR tubing follows a different convention. Its size designation refers to the actual outside diameter, so a 3/8-inch ACR tube truly measures 3/8 inch on the outside. This distinction trips people up when shopping for fittings, because a 3/8-inch ACR tube and a 3/8-inch plumbing tube are not the same size.
Hard-drawn tubing is sold in straight sticks at hardware stores and plumbing supply houses. Soft-temper tubing comes in coils, commonly 50 or 100 feet, which makes it practical for long, continuous runs with minimal joints. Fewer joints mean fewer potential leak points, one reason soft copper coils are preferred for underground water service lines.