Copper Tube Type Table

Wall thickness only - the outside diameter is identical within a nominal size, so one fitting fits all three. Type K has the thickest wall of the pressure grades, which makes it the underground-burial tube (under sidewalks and streets, with corrosion protection or a polyethylene sleeve where code demands) and the refrigeration industry choice; in the US its printing is green. Type L steps down to the everyday wall for residential and commercial water supply and pressure applications, printed blue. Type M goes thinner still for residential and low-pressure heating work, printed red. Type DWV is the thinnest of all and is generally legal only in unpressurized drain, waste and vent lines - the tube that must never meet supply pressure. The practical selection rule: K survives burial and abuse, L is the do-everything supply tube, M saves money where pressure and code are lenient, and the colors exist so a helper can sort a job site at a glance.

Because the nominal size is an inherited name, not a dimension. Copper tube follows the CTS (copper tube size) system, which carried over the nominal labels of the older iron pipe era: nominal 3/4 corresponds to an actual OD of 7/8 inch (22.2 mm), nominal 1/2 to 5/8 inch, nominal 1 to a full 1 1/8 inch. Nothing about the label is a measurement - it is a category name that keeps the trade ordering the right fittings. What the system does guarantee is consistency: every type (K, L, M) of a given nominal size shares the same OD, which is why one solder fitting or press fitting covers all three types, and why the inside diameter - the number that decides flow - is the variable the type letter controls. Anyone converting drawings should work from the table's OD and ID columns, never from the nominal name.

Type K, and the code reasoning is worth knowing rather than just following. A buried line under a driveway or street sees loads, ground movement and - the silent killer - corrosion from outside; Type K is the thickest-wall pressure grade, and plumbing codes require it (or equivalent protection) for burial, typically with a corrosion-resistant coating or a continuous polyethylene sleeve. The extra wall thickness is not primarily about burst pressure - a soldered K line easily handles domestic pressures - it is about how many years of outside-in corrosion the wall can absorb before it weeps. The same logic makes K the refrigeration trade's tube: it gets brazed into sealed systems where a pinhole is a system-wide failure. Above ground, the calculus flips back to L or M, because corrosion protection and abuse loads are gone and the larger bore of a thinner wall is often a hydraulic bonus.

It matters more than most homeowners expect, and the type letters are secretly a flow decision. Within one nominal size, the ID grows as the wall thins: nominal 1/2 runs about 0.528 inch bore in K, 0.545 in L and 0.569 in M - and since flow capacity scales with roughly the square of the bore, M carries noticeably more water than K at the same pump pressure. That is the economic logic of M in low-pressure heating: thinner walls give back bore, and where pressure is low the strength loss is affordable. The flip side is durability accounting: K's extra wall is corrosion budget, which is why codes spend it underground where the budget gets consumed from the outside. So the honest sizing workflow is: pick the type the code and exposure demand, then read the actual ID from the table for any flow or friction-loss calculation - the nominal size alone tells you almost nothing about either.