What does the ISA 5.1 first letter N stand for?

In ISA 5.1, the first letter N stands for User's choice. It can also stand for No standard assignment.

What second letters commonly follow N in ISA 5.1 tags?

Common combinations include NT (Transmitter, whatever the operating company has assigned N to mean).

N - User's choice

Q: What are example ISA 5.1 tags starting with N?

NT-105 (Whatever N has been defined to mean on the project legend).

Reference for the ISA 5.1 first letter N. Of all the user's-choice letters, N is the one with the least convergence across operating companies. There's no convention to learn. If you see N on a drawing, the legend defines it for that project and that project only. Don't carry a meaning across job boundaries. That's how you ship a misclassified loop on a new project.

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Also stands for.

No standard assignment

Common second letters.

Transmitter, whatever the operating company has assigned N to mean

Example tags.

NT-105

Whatever N has been defined to mean on the project legend

Typical placement.

Wherever the project legend allows.

Related

GlossaryISA 5.1 ISAISA letter O - User's choice ISAISA letter X - Unclassified