Wire and cable markings pack the whole rating into a few letters, and the letters are not arbitrary. Once you know that T is thermoplastic, X is cross-linked thermoset, H is heat-resistant, a second H takes it to 90 C, W means wet-rated, and N means a nylon jacket, most building-wire types read straight off the print. The two things that trip people are the "-2" suffix, which is a wet-location rating and not a version number, and the gap between the insulation temperature rating and the ampacity you are actually allowed to use at the terminals.
This guide explains the markings the Wire & Cable Type Decoder resolves: the building-wire letter system, the temperature and location columns from NEC Table 310.4(1), the NEC 110.14(C) termination rule that caps usable ampacity, the flexible-cord letters from Table 400.4, and the assembly types (MC, AC, TC-ER, NM-B, UF-B) where the grounding and the ampacity basis carry traps of their own.
Reading the Building-Wire Letters
Building-wire type letters follow a consistent system. Read them left to right:
- T = thermoplastic insulation (PVC).
- X = cross-linked synthetic polymer, a thermoset (XLPE). Tougher and more heat-tolerant than thermoplastic.
- R = thermoset rubber insulation.
- H = heat-resistant, 75 C. A second H (HH) takes it to 90 C in dry locations.
- W = moisture-resistant, rated for wet locations.
- N = nylon (or equivalent) outer jacket, which adds abrasion and oil resistance and lets the conductor pull easier.
- -2 = 90 C in both wet and dry locations.
So THHN reads thermoplastic, high-heat (90 C), nylon-jacketed: a 90 C dry conductor. THWN reads thermoplastic, heat-resistant, wet-rated, nylon: 75 C wet and dry. Add the "-2" and THWN-2 becomes 90 C wet and dry. XHHW is cross-linked, high-heat, wet-rated: 90 C dry but 75 C wet, unless it is XHHW-2, which is 90 C in both.
The Temperature and Location Table
| Type | Insulation | Dry C | Wet C | Locations | UL |
|---|---|---|---|---|---|
| TW | PVC | 60 | 60 | dry, damp, wet | UL 83 |
| THW | PVC | 75 | 75 | dry, damp, wet | UL 83 |
| THHN | PVC + nylon | 90 | not wet-rated | dry, damp | UL 83 |
| THWN-2 | PVC + nylon | 90 | 90 | dry, damp, wet | UL 83 |
| XHHW | XLPE | 90 | 75 | dry, damp, wet | UL 44 |
| XHHW-2 | XLPE | 90 | 90 | dry, damp, wet | UL 44 |
| RHW-2 | XLPE / EPR | 90 | 90 | dry, damp, wet | UL 44 |
| USE-2 | XLPE | 90 | 90 | wet, direct burial (not flame-rated) | UL 854 |
| MTW | PVC, oil-resistant | 90 | 60 | dry; wet/oil at 60 C | UL 1063 |
The pattern to notice: a "-2" type holds 90 C wet; XHHW without the "-2" derates to 75 C wet even though it is 90 C dry; and THHN on its own has no wet rating at all. That last point is why nearly all building wire is dual-printed THHN/THWN-2, so it carries the wet rating on the THWN-2 half.
Wire & Cable Type Decoder
Decode a wire or cable marking (THWN-2, XHHW-2, SOOW, MC, NM-B, TC-ER, USE-2) into temperature ratings, ampacity basis, permitted locations, construction, and the governing UL standard. Built around the "-2" wet-rating trap and the NEC 110.14(C) termination ampacity gap.
Why 90 C Insulation Is Not 90 C Ampacity (NEC 110.14(C))
This is the rule that fails inspections. The temperature on the insulation (60, 75, or 90 C) is the rating of the wire itself. The ampacity you are allowed to use is limited by the lowest-rated termination in the circuit, under NEC 110.14(C).
- For most circuits 100 A and under (#1 AWG and smaller), terminations are listed for 60 C, so you size the conductor from the 60 C column.
- Above 100 A (or where equipment is marked 75 C), you size from the 75 C column.
- You may use the 90 C column only for derating (high ambient, bundling), and the derated result can never exceed the termination ampacity.
So a 12 AWG THWN-2 is a 90 C conductor, but on a typical 60 C-terminated branch circuit you size it as 20 A (the 60 C value), not 30 A (the 90 C value). The 90 C rating buys you derating headroom, not a bigger breaker. The decoder puts this caveat on every building-wire result, because it is the most common reason a conductor that "calculated fine" gets rejected.
Those ampacity columns also assume a 30 C (86 F) ambient. Route the conductor anywhere hotter, the top of a boiler room, an attic, a south-facing wall, an unventilated pump house, and you must apply the ambient temperature correction factor from NEC Table 310.15(B)(1) on top of the termination limit (plus a conduit-fill adjustment when more than three current-carrying conductors share the raceway). A conductor sized perfectly for its termination can still overheat if the ambient correction is skipped.
Flexible Cord Letters (Table 400.4)
Flexible cords use a different letter set, from NEC Table 400.4 and UL 62:
- S = service, 600 V, extra-hard usage. SJ = junior, 300 V, lighter duty.
- O = oil-resistant jacket. A second O (OO) means the conductor insulation is oil-resistant too.
- W = weather- and sunlight-resistant, for outdoor use.
- T = thermoplastic (PVC); E = thermoplastic elastomer (TPE); no T or E means thermoset rubber.
So SOOW reads service (600 V), oil-resistant insulation and jacket, weather-resistant: the common outdoor shop cord. SJOOW is the same construction at 300 V. The single-O versus double-O distinction is the one people miss: SO has an oil-resistant jacket but not necessarily oil-resistant insulation, while SOO and SOOW have both.
Armored, Tray, and Service-Entrance Cable
The cable assemblies carry traps in the grounding and the ampacity basis, not just the temperature:
- Type AC (BX), UL 4: interlocked armor with an internal bonding strip as part of the ground path. The strip is not a full-size equipment grounding conductor, and AC is for dry, indoor locations only.
- Type MC, UL 1569: carries a full-size green insulated grounding conductor and, with the right jacket, is rated for wet locations. If you need wet or a full EGC, that is MC, not AC.
- TC and TC-ER, UL 1277: tray cable, 90 C dry / 75 C wet. The "-ER" (exposed run) listing lets a cable with 3 or more conductors make a limited run between a tray and equipment per NEC 336.10, including a short unsupported transition.
- PLTC (UL 13) and ITC (UL 2250): power-limited and instrumentation tray cable, 300 V class. ITC is for instrumentation circuits at or below 150 V and 5 A.
- SE / SER / SEU, UL 854: service-entrance cable. SER is the round style with a separate grounding conductor (feeder use); SEU is the flat style with a concentric bare neutral (service-entrance use, not interior feeders).
The "S" trap is worth stating plainly: in a cord, a leading S means Service (600 V). In SE/SER/SEU it means Service-Entrance, a completely different cable. The decoder branches on the family first so the two never get confused.