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Building Wire and Cable Designations

Technical Information & Technical Bulletins

THW-2 Thermoplastic Insulation (usually PVC), Heat Resistant (90°C rating), suitable for Wet locations.
THWN-2 Same as THW except Nylon jacket over reduced insulation thickness. Also rated THHN.
THHN Thermoplastic Insulation (usually PVC), High Heat Resistant (90°C rating), dry locations only, Nylon jacket. Also rated THWN.
TFFN Thermoplastic Insulation (usually PVC), Fixture wire, Nylon jacket. Also rated MTW and AWM.
TFN Thermoplastic Insulation (usually PVC), Fixture wire, Nylon jacket. Also rated AWM.
XHHW-2 Cross-linked Polyethylene Insulation (X) High Heat Resistant (90°C rating) for Wet and dry locations.
RHH Rubber Insulation (we actually use cross-linked polyethylene because it qualifies for rubber), High Heat Resistant (90°C rating) for dry locations only.
RHW-2 Rubber Insulation (again, cross-linked polyethylene is used by most manufacturers), Heat Resistant (90°C), Suitable for Wet locations.
USE-2 Underground Service Entrance. Most utilize cross-linked polyethylene insulation rated for 90°C in direct burial applications. Product is usually triple rated RHH—RHW—2-USE-2.
MTW Machine Tool Wire. Usually thermoplastic insulation (PVC) or thermoplastic insulation with nylon jacket. Most MTW is rated 60°C. Southwire's copper type THHN is also rated MTW.
AWM Appliance Wiring Material. Usually thermoplastic insulation (PVC) or thermoplastic insulation with nylon jacket. Southwire's stranded copper type THHN in AWG sizes 14 through 6 is also rated AWM. As AWM, the product carries a 105°C rating.
NM-B Nonmetallic-Sheathed Cable. The "B" denotes that individual conductor insulation is rated 90°C; however, ampacity is limited to that for a 60°C conductor (see NEC Section 336-26, 1999 edition). Thermoplastic (PVC) conductor insulation-nylon jacketed, with overall PVC cable jacket.
UF-B Undergound Feeder. The "B" denotes that individual conductor insulation is rated 90°C; however, ampacity is limited to that for a 60°C conductor (see NEC Section 339-3(a)(4), 1999 edition.) Thermoplastic (PVC) conductor insulation-nylon jacketed, with overall PVC cable jacket.
SEU Service Entrance Cable, Unarmored. Usually type XHHW insulated conductors with overall PVC jacket. As such, the cable is rated for 90°C dry, 75°C wet locations. Another school of thought is the "U" in the designation denotes the shape of the cable (if you look at the end, it resembles a closed "U" shape).
SER Service Entrance Cable, Round. Same construction material-wise as SEU, but round construction affords use of more than 3 conductors to which SEU is limited