Fixture Wire
NEC® Article 402, Table 402.3

F Fixture wire, standard stranding
FF Fixture wire, flexible stranding
G Glass braid
H 75°C insulation
HH 90°C insulation
K Aromatic polyimide tape insulation
N Nylon jacket
P Fluorinated ethylene propylene insulation
R Thermoset insulation
S Silicone (Thermoset) insulation
T Thermoplastic insulation
X Cross-linked synthetic polymer insulation
Z Modified tetrafluoroethylene insulation
-1, 2, 3 Insulation thickness, thinnest to thickest for some types. Actual insulation
thickness varies, with insulation types and AWG size.
Examples: SF-1–Silicone rubber fixture wire
TFF–Thermoplastic, flexible stranded fixture wire