The letters name the insulation and its ratings: T is thermoplastic, X cross-linked, R rubber; H is 75 C and HH 90 C; W means wet locations; N a nylon jacket; and -2 means 90 C in wet locations too. THWN-2 is therefore thermoplastic, nylon jacketed, 90 C wet or dry.
The letters
Letter
Meaning
T
Thermoplastic insulation (PVC)
X
Cross-linked polymer insulation, which does not soften with heat
R
Rubber (thermoset) insulation
H
Rated 75 C
HH
Rated 90 C
W
Rated for wet locations
N
Nylon outer jacket
-2
Rated 90 C in wet locations as well as dry
U, SE
Underground; service entrance (as in USE)
Common types, NEC Table 310.4(1)
Type
Maximum temperature
Where it may be used
TW
60 C
Dry and wet locations
THW
75 C
Dry and wet locations
THHW
90 C dry, 75 C wet
Dry and wet locations
THHN
90 C
Dry and damp locations
THWN
75 C
Dry and wet locations
THWN-2
90 C
Dry and wet locations
XHHW
90 C dry, 75 C wet
Dry, damp and wet locations
XHHW-2
90 C
Dry and wet locations
RHH
90 C
Dry and damp locations
RHW-2
90 C
Dry and wet locations
USE-2
90 C
Dry and wet locations, underground service entrance
SIS
90 C
Switchboard wiring only
Most wire carries more than one type in its legend, for example THHN/THWN-2 or USE-2/RHH/RHW-2; it may be used under any type it is marked with.
