CONDUCTORS FOR GENERAL WIRING
NEC® Article 310, Table 310.104(A)

B Braid
FEP Fluorinated ethylene propylene insulation
H 75°C (Note: Lack of "H" indicates 60°C)
HH 90°C
N Nylon jacket
PFA Perfluoroalkoxy insulation
R Thermoset insulation
S Silicone (Thermoset) insulation
T As first letter - Thermoplastic insulation
TFE Polytetrafluoroethylene
U Underground use
W Moisture resistant
X Cross-linked polymer insulation
Z Modified ethylene tetrafluoroethylene insulation

Examples: RHW –Thermoset Insulation, 75°C, Moisture resistant
THHN –Thermoplastic Insulation, 90°C dry, nylon jacket