This information comes from UL's Marking Guide for Wire, Cable and Cord. It can be downloaded from the UL web site.

APPENDIX A
WIRE, CABLE AND CORD
DESIGNATIONS
In general, the letter designations assigned to wire, flexible cord and cable in the NEC ® , for identification purposes, are established according to a coding system that provides information on intended use, insulation type and insulation temperature rating. This coding system, to which there are exceptions, does not cover all NEC designations. The coding system is as follows:

CONDUCTORS FOR GENERAL WIRING
NEC ® Article 310, Table 310-13

A — Asbestos (obsolete; now must be glass fiber or similar material)
FEP — Fluorinated ethylene propylene insulation
H —75 o C (Note: Lack of “H” indicated 60 o C)
HH — 90 o C
L — Lead sheath
N — Nylon jacket
PF — Perfluoroalkoxy insulation
R — Thermoset insulation
S — Silicone (Thermoset) insulation
T — Thermoplastic insulation
U — Underground use
W — Moisture resistant
X — Cross-linked synthetic polymer insulation
Z — Modified tetrafluoroethylene insulation

Examples: RHW THHN


[This message has been edited by Joe Tedesco (edited 12-13-2001).]
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Joe Tedesco, NEC Consultant