Short answer: The print on a wire tells you what it is and where it may be used: the insulation type letters (like THHN or THWN-2), the size, the voltage rating, the temperature rating and whether it is rated for wet, damp or dry locations. The letters are a code: T is thermoplastic, HH is high heat resistant, W is water resistant and N is a nylon outer coating, and a “-2” on THWN-2 means it is rated for 90 °C in wet locations too. A wire is only right for a job when its ratings match the conditions where it goes, so read the print before you pull it, and ask your supervising electrician when you are not sure.
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Read the print on a wire
What the print on this wire tells you
Hold the wire, the reel label or the maker’s data sheet and check each item you can find and understand. Some ratings come from the type letters or the label, not a separate print. This is a reading drill, not approval to use the wire.
- Insulation type letters (THHN, THWN-2, XHHW-2)
- Wire size (AWG or kcmil)
- Copper or aluminum
- Voltage rating (for example 600 V)
- Temperature rating (for example 90 °C), printed or set by the type letters
- Wet, damp or dry use, printed or set by the type letters
- Maker name and listing mark
- Where this wire is going on the job
What the letters stand for
| Letters | Meaning | What it tells you |
|---|---|---|
| T | Thermoplastic insulation | The insulation material type |
| HH | High heat resistant | A 90 °C conductor rating, usually in dry or damp places |
| H (one H) | Heat resistant | A lower temperature rating than HH |
| W | Water resistant | Rated for wet locations |
| N | Nylon outer coating | A tough jacket that resists scuffing when pulled |
| X | Cross-linked (thermoset) insulation, as in XHHW | Read the full type letters for its temperature and wet or dry rating |
| -2 | As in THWN-2 or XHHW-2 | Rated 90 °C in wet locations too |
Many wires carry more than one type, such as THHN/THWN-2 printed on the same insulation. One maker’s product page for THHN/THWN-2 copper building wire describes it as 90 °C, 600 volt wire for use in both wet and dry locations. A dual-rated wire takes the rating that fits the location: an older plain THWN rating is 75 °C in a wet place, while the THHN rating is 90 °C in a dry place.
Why “conditions of use” matters
OSHA defines ampacity as the current a conductor can carry continuously “under the conditions of use” without going over its temperature rating. So the same wire can be fine in one place and wrong in another. Heat around the wire and how many wires share a raceway change what it can safely carry, and a wet or damp location limits which insulation types and temperature ratings you may use.
OSHA’s definitions also explain the three location words you will see on prints and in the code:
- Dry location: not normally subject to dampness or wetness. A building under construction can still count as dry even if it gets wet for a while.
- Damp location: partly protected places, like under a canopy or a roofed open porch, and inside places with moderate moisture, like some basements.
- Wet location: underground, in concrete slabs or masonry in contact with the earth, places soaked with water, and places exposed to weather.
OSHA’s wet location includes installations underground, so wire in an underground run needs a wet rating. Underground conduits can fill with water, which is one reason wet-rated wire is so common. The exact ampacity and temperature limits come from the NEC tables; look them up in the 2026 NEC with your supervising electrician rather than from memory or a forum post.
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What is the difference between THHN and THWN-2?
THHN is thermoplastic, high heat resistant, nylon coated wire with a 90 °C rating for dry and damp places. THWN-2 adds a water resistant rating that holds at 90 °C in wet places too. Much building wire is printed with both, so check the print on the actual wire.
Can I use any wire that fits in the conduit?
No. The wire has to match the conditions where it goes: wet or dry, the temperature, the voltage and the current it must carry. Underground installations count as wet locations. Check the print and ask your supervising electrician.
Where do the exact temperature and ampacity numbers come from?
From the NEC tables for conductors and from the wire’s own listing and print. Look them up in the 2026 NEC instead of trusting a number from memory, an old book or a video.
Sources checked October 2026
- eCFR, 29 CFR 1910.399, OSHA definitions (ampacity, dry, damp and wet location)
- Cerrowire, THHN/THWN-2 product page
- Wikipedia, Electrical wiring in North America (insulation types THHN, THWN, THWN-2, XHHW)
- PSI Candidate Information Bulletin 1682, Texas electrician exams (PDF)
