When a project must live with dew, fog, or driving rain, the jacket on the wire matters as much as the copper inside it. Three trade names-THW, THHN and THWN-fill the same conduit, but they are not created equal once humidity climbs. The short answer is:
• If the location is continuously wet, choose THW or THWN-2;
• If the location is dry or only occasionally damp, THHN/THWN is acceptable;
• When you want one reel that covers both cases, specify dual-rated THHN/THWN-2.
Below is the reasoning.
1. What the acronyms really promise
• THW = Thermoplastic, Heat- and Water-resistant. A single layer of PVC gives 75 °C in both dry and wet locations at 600 V .
• THHN = Thermoplastic High-heat-resistant Nylon-coated. Rated 90 °C dry / 75 °C wet, but only if the reel also carries a THWN stamp.
• THWN adds the "W": 90 °C dry / 75 °C wet, plus a thin nylon skin for easier pulls .
• THWN-2 pushes the wet rating to 90 °C, matching the dry rating.
2. Humidity and temperature interaction
PVC in THW swells slightly when soaked; the nylon in THWN/THWN-2 shrugs water off. In a coastal greenhouse where air is 30 °C but relative humidity stays >85 % all year, THW's 75 °C ceiling is still safe, yet THWN-2 gives an extra 15 °C buffer if a motor overload causes conductor heating.
3. Mechanical differences you will notice
• Flexibility: THW's thicker PVC wall makes it a little stiffer-handy in direct-bury trench where kinking is a risk.
• OD (overall diameter): For 4 AWG, THWN-2 is ~2 mm larger than THW. If the conduit is already >40 % full, the smaller THW may be the only code-legal choice.
• Cost: THW is typically 5–8 % cheaper per foot, but the gap disappears when bulk THHN/THWN-2 is bought on reels.
4. Installation examples
• Pump house in Florida Keys: Condensation drips off chilled-water lines all night. Inspector asked for "wet-rated 90 °C". We pulled THWN-2.
• Arizona carport lighting: Dry desert air, conduit on roof deck. THHN/THWN dual-marked wire passed inspection at minimum cost.
• Underground feeder to dock: Soil stays saturated; local code allows THW or THWN-2 in conduit. We chose THW because the trench was narrow and we needed the smaller OD to fit three 2 AWG conductors plus ground.
5. Modern reality-one reel does it all
Almost every distributor now sells THHN/THWN-2 under a single SKU. Unless you have an unusual need (extreme cold embrittlement, chemical wash-down, etc.), buying the dual-rated wire eliminates the decision fatigue of "THW vs. THWN".
6. Quick selection checklist
1. Confirm ambient temperature and moisture class from NEC Table 310.104(A).
2. If the wet column says ≤75 °C, THW or THWN is fine.
3. If the wet column says ≤90 °C, or you want margin for future load growth, jump straight to THWN-2.
4. Check conduit fill; if tight, verify the OD of the chosen style.
5. Request mill test reports showing UL-83 listing for the chosen temperature column.
Bottom line: in outdoor humid environments, THW will do the job at 75 °C, THWN-2 does it better at 90 °C, and plain THHN should only be used if the conduit never floods. When in doubt, let the reel label decide: if it says THHN/THWN-2, you are covered rain or shine.
