Tubing made of copper was introduced in about 1900, but didn't become popular until approximately 1950, depending on local building code adoption. Common wall-thicknesses of copper tubing are "Type K", "Type L" and "Type M"; Type "M" are relatively thin-walled and generally suitable for condensate and other drains, but sometimes illegal for pressure applications, depending on local codes. Type "L" has a thicker pipe wall section, and is used in residential and commercial water supply and pressure applications, Type "K" has the thickest wall section of the three types of pressure rated tubing and is commonly used for deep underground burial such as under sidewalks and streets, with a suitable corrosion protection coating or continuous polyethylene sleeve as required by code. Types "K" and "L" are generally available in both hard drawn "sticks" and in rolls of soft annealed tubing, Type "M" is usually only available in hard drawn "sticks". Thin-walled types used to be relatively inexpensive, but since 2002 copper prices have risen considerably due to rising global demand and a stagnant supply.
In the plumbing trade the size of copper tubing is measured by its nominal diameter (average inside diameter). Some trades, heating and cooling technicians for instance, use the outside diameter (OD) to designate copper tube sizes. The OD of copper tube is always 1/8th inch larger than its nominal size. Therefore, 1" nominal copper tube and 1-1/8th" inch ACR tube are exactly the same tube with different size designations. The wall thickness of the tube, as mentioned above, never affects the sizing of the tube. Type K 1/2" nominal tube, is the same size as Type L 1/2" nominal tube (5/8" ACR).
Generally, copper tubes are soldered directly into copper or brass fittings, although compression, crimp, or flare fittings are also used. Formerly, concerns with copper supply tubes included the lead used in the solder at joints (50% tin and 50% lead). Some studies have shown significant "leaching" of the lead into the potable water stream, particularly after long periods of low usage, followed by peak demand periods. In hard water applications, shortly after installation, the interior of the pipes will be coated with the deposited minerals that had been dissolved in the water, and therefore the vast majority of exposed lead is prevented from entering the potable water. Building codes now require lead-free solder. Building Codes throughout the U.S. require the use of virtually "lead-free" (<.2% lead) solder or filler metals in plumbing fittings and appliances as well.

Copper Pipe Types

Nominal
size
Outside diameter (OD)
(inches)
Inside diameter (ID)
(inches)




Type K
Type L
Type M
3/8
1/2
0.402
.430
.450
1/2
5/8
.528
.545
.569
5/8
3/4
.652
.668
.690
3/4
7/8
.745
.785
.811
1
1 1/8
.995
1.025
1.055
1 1/4
1 3/8
1.245
1.265
1.291
1 1/2
1 5/8
1.481
1.505
1.527
2
2 1/8
1.959
1.985
2.009
2 21/2
2 5/8
2.435
2.465
2.495
3
3 1/8
2.907
2.945
2.981