Some of the most common ways to use a copper pipe nowadays is for construction industry for water supply lines and refrigerant line in heating, ventilation, and air-conditioning (HVAC) systems. Whilst this can mean AC, many don't realize that this copper tube system is what keeps our refrigerators and our freezers alive.While the majority of the copper tube applications lie in the residential sector, where they are used to convey potable water in homes and offices, refrigerator industries, firefighting systems, and medical and residential gas suppliers are also catching up.

How to Choosing Copper Pipe
Copper plumbing pipe (often known as tubing) comes in several types based on the thickness of the walls in the pipe relative to the diameter of the pipe. As outlined above, selecting the right copper pipe is really just a matter of matching the thickness of the pipe walls to the needs of the application. Type K has the largest wall-thickness to pipe diameter ratio and is usually used for underground applications where extreme strength is needed. At the other end, DWV grade has the lowest wall-thickness to pipe diameter ratio, making it suitable only for drain and vent lines that are under no pressure at all. In between, Types L and M are the pipes most often used for standard pressurized water supply pipes. In summary:2

DWV-grade: With a wall thickness of 0.42 in. for 1 1/2-inch pipe, this grade of copper pipe is recommended for above and below-ground drain, waste, and vent lines. However, given the expense of copper, it is rare to choose copper DWV pipe when PVC and ABS plastic is so much cheaper and easier to work with. An exception might be in exposed locations, especially vintage homes where there is a need to be consistent with historical practices.
Type M: With a wall thickness of 0.28 in for 1/2-inch pipe, this grade of copper pipe is commonly used for residential water supply pipes. (It is also used in fire protection, fuel oil, HVAC, and vacuum system piping, though such uses are not common for DIY homeowners.) It is less expensive than thicker grades of copper, so where code allows for its use, it is the most popular choice for DIYers and professional plumbers making repairs or replacements to existing copper water supply systems in homes.
Type L: With a wall thickness of. 0.40, this grade of copper pipe is also used for the same uses as Type M, but its extra thickness gives it the strength needed for natural gas lines. It can also be used in exposed above-ground outdoor locations, such as swimming pool or spa pipes. Where local codes have limitations on the use of Type M, this is the grade used for common repairs and replacements to existing indoor water supply systems.
Type K: With a wall thickness of 0.65, this grade of copper pipe can be used for all the applications appropriate for types M and L, but because of its high cost, it is usually reserved for underground applications where great strength is required. It is not the usual choice for indoor plumbing repairs and replacements.
When purchasing pipe, be aware that the actual outside diameter (OD) of rigid copper pipe is always 1/8 inch larger than the nominal size—how the pipe is labeled. For example, a 1/2-inch copper pipe has an outside diameter of 5/8 inch. It is true with all three common types pipe, K, L, and M.