What is the proper thread engagement for NPT (National Tapered Pipe Threads) to ASME B.1.20.1 Standard?

ANSI B1.20.1 Standard provides both “Hand Tight” engagement (L1) and “Wrench Make-Up” (L3) dimensions. The L-1 basic hand tight engagement applies to NPTF threads. It is the theoretical length of assembly (hand-tight) if both parts had perfect thread form and taper and were exactly at the nominal size. The diagram and table below show standard L1 lengths (Hand-Tight Engagement) for pipe threads as well as the L-3 (Wrench Make-Up). For convenience we have added L1 + L3 which is the total thread engagement when wrench tight.

Because of allowable tolerance of plus or minus one turn on both the male and female threads the actual thread engagement length / threads of the assembly will vary from the engagement dimensions shown in the chart which are based on “perfect” threads.