Quick answer
THHN wire is a single insulated conductor, usually copper, with thermoplastic insulation and a thin nylon outer layer, rated for 600 volts and built to be pulled through conduit.
Most THHN sold today is dual-rated THHN/THWN-2, which lets it run at 90°C in dry or wet locations. Unlike NM-B cable, each THHN wire is separate, so it needs a raceway around it.
Quick facts
The key facts about THHN Wire, with sources:
- Letters
- Thermoplastic, High Heat-resistant, Nylon-coated
- Temperature rating
- 90°C dry or wet (as THHN/THWN-2) 1
- Outer layer
- Nylon (polyamide), color-coded 1
- Listing standard
- UL 83 1
- Installed in
- Conduit, raceway or cable tray (larger sizes) 1
- Other markings
- VW-1 flame test on 14 to 1 AWG; gasoline and oil resistant 1
Key takeaways
- THHN is one insulated wire, not a cable. It goes inside conduit or another raceway.
- The nylon skin makes it slick and tough, so it pulls through pipe without tearing.
- Most THHN on sale is also THWN-2, rated 90°C in wet or dry places 1.
- A higher temperature rating does not mean a bigger breaker; terminals and code limits still apply.
- You will see THHN in garages, outdoor runs, service work and commercial buildings.
What THHN wire is
THHN is the most common single-conductor building wire in the US. Each wire has a copper core, a layer of thermoplastic insulation and a thin, hard nylon jacket over that. The letters describe it: Thermoplastic, High Heat-resistant, Nylon-coated.
One maker describes it simply as thermoplastic-insulated copper wire with a nylon covering, rated 600 volts and listed to UL 83 1. It comes in solid and stranded forms and in many colors, so an electrician can tell hot, neutral and ground apart inside a pipe.
THWN is a close cousin. The "W" means it is rated for wet locations. Most wire today carries both marks, printed as THHN/THWN-2. The "-2" means it holds the 90°C rating in wet locations too 1.
How THHN works in a raceway
Because each wire is separate, THHN needs a pathway: conduit, a raceway or, for larger sizes, cable tray. The conduit guards against damage. In metal conduit such as EMT, the pipe itself can serve as the ground path when installed per code.
The nylon layer lowers friction, so long pulls through bends are easier. Some products add a built-in lubricant for the same reason 1. The insulation resists heat, gasoline and oil, which suits garages and shops.
Electricians pick the number of wires, the size and the colors, then size the conduit so the wires fit without overheating. Too many current-carrying wires in one pipe forces derating, which lowers the amps each may carry.
- Nylon jackettough, slick, oil resistant
- PVC-type thermoplastic insulationthe main electrical insulation
- Copper conductorsolid or stranded
Source: [1]
THHN, THWN-2 and related wire types
| Marking | What it means | Wet location? | Top rating |
|---|---|---|---|
| THHN | Thermoplastic, high heat-resistant, nylon-coated | No | 90°C dry |
| THWN | Thermoplastic, heat- and water-resistant, nylon-coated | Yes | 75°C wet |
| THWN-2 | THWN rated 90°C wet | Yes | 90°C dry or wet |
| THHN/THWN-2 (common) | Dual-rated wire sold today | Yes | 90°C dry or wet 1 |
| XHHW-2 | Cross-linked polyethylene insulation, no nylon | Yes | 90°C dry or wet |
Example: why 12 AWG THHN is still a 20-amp wire
A 12 AWG copper THHN wire on a branch circuit shows how ratings stack. The values come from a manufacturer chart. It is for understanding quotes, not designing circuits.
| Check | Value | What it means |
|---|---|---|
| 12 AWG copper, 90°C column | 30 A 2 | The table number for THHN |
| 12 AWG copper, 75°C column | 25 A 2 | Most breaker and device terminals are rated 75°C or 60°C |
| 12 AWG copper, 60°C column | 20 A 2 | Used where terminals are 60°C |
| NEC 240.4(D) small-conductor rule | 20 A maximum breaker | The cap for 12 AWG copper |
| Result | 20-amp breaker | Same as 12 AWG NM-B |
The 90°C number is useful as a starting point for derating, for example when nine wires share one conduit. It does not let you put 12 AWG on a 30-amp breaker.
Where THHN shows up in a home
In houses, THHN appears wherever wire runs in conduit: exposed garage and basement walls, outdoor runs to AC units and EV chargers, pool equipment, and the service entrance. In many commercial buildings it is the main wiring.
On a quote it may read "pull 3 #8 THHN/THWN-2 Cu in 3/4-inch EMT". That means three 8 AWG copper wires in three-quarter-inch steel tubing. Outdoors, the THWN-2 part matters, because any outdoor conduit is a wet location.
Solid THHN is common in the smaller branch sizes, such as 14, 12 and 10 AWG, because it is easy to push onto device screws.
Stranded THHN is more flexible, so it is the usual choice for larger sizes and for long pulls with many bends. Either form carries the same marking and rating when the label matches.
Color matters inside a pipe. In US practice, white or gray marks the neutral, green or bare marks the ground, and black, red and blue are used for hot wires. Larger wire may be all black with colored tape at each end.
If you ever see a white wire taped black at a box, it is being used as a hot wire, which is a job for an electrician to read, not to change.
THHN versus Romex
Romex (NM-B) is a cable: several wires in one plastic jacket, for dry interior walls. THHN is single wire for conduit. You cannot strip NM-B to use its wires as THHN, and THHN is not allowed loose inside walls.
Why electricians choose it
Where it falls short
- THHN must be in a raceway; it cannot run loose in walls or be stapled to framing.
- Plain THHN (without THWN-2) is not for wet locations.
- The nylon can be damaged by sharp edges during a pull, so fittings need bushings.
- Conduit adds labor compared with cable in a house.
- Ampacity is capped by terminals and the small-conductor rule, not the 90°C rating alone.
What drives the cost
The wire itself is priced by size and metal, and copper prices change often. The bigger cost in a THHN job is usually the conduit, fittings and labor to bend and install the pipe. Distance, number of bends and indoor versus outdoor work all change the total.
We do not publish prices without dated, sourced quotes. Use the project cost estimator or the EV charger installation cost guide, where conduit runs are common. THHN in conduit can last for decades; failures usually come from water in conduit or damage during the pull.
How a THHN run is installed
A summary of the professional process, so you know what you are paying for.
- Size the wire from the load and the breaker, then size the conduit for the number of wires.
- Pull a permit where required.
- Install conduit and boxes first, with supports and fittings.
- Pull the wires with lubricant if needed, keeping colors for each circuit.
- Terminate at rated lugs and devices, torqued per the maker's instructions.
- Pass inspection.
Maintenance
THHN in sealed conduit needs no routine care. Outdoor conduit can collect water from condensation, which is why wet-rated wire is used there. Damaged or loose conduit, missing covers or broken fittings expose the wire and should be repaired by an electrician.
What you can watch for: rust or separated joints on exposed conduit, open boxes, and breakers that trip during rain.
Safety warning
Safety and risk
Never pull wire out of conduit, open boxes or touch exposed conductors. Wire in conduit can still be live even if a nearby breaker is off.
If you see bare or melted wire, a breaker that trips when it rains, or water in an outdoor box, keep clear and call a licensed electrician. If anyone is shocked, call 911.
Signs of a problem
- Plain THHN (no wet rating) in an outdoor or underground conduit.
- Conduit with missing fittings, so wire is exposed.
- Scorched or discolored wire at a box or lug.
- GFCI or breaker trips tied to wet weather.
Standards and code
THHN and THWN-2 are tested to UL 83, the standard for thermoplastic-insulated wires 1. Insulation types and their temperature and location ratings are listed in Article 310 of NFPA 70, the National Electrical Code 3; ampacity is in Table 310.16 of the 2023 edition.
Your state or city decides which NEC edition applies 4. The full code text can be read free on NFPA's site 5.
THHN compared with related terms
| Term | Form | Needs raceway? | Wet locations |
|---|---|---|---|
| THHN/THWN-2 | Single wire | Yes | Yes |
| NM-B (Romex) | Multi-wire cable | No | No |
| UF-B | Multi-wire cable, solid jacket | No | Yes, direct burial |
| Wire gauge | Size, not a type | n/a | n/a |
| Aluminum conductors | Metal choice | Depends on type | Depends on type |
Myths about THHN
- Myth THHN can run in a wall without conduit.
- Reality It is single wire and must be in a raceway.
- Myth 90°C wire means a bigger breaker.
- Reality Terminals and NEC 240.4(D) usually cap it lower 2.
- Myth All THHN is fine outdoors.
- Reality Only wire also marked THWN or THWN-2 is wet rated 1.
- Myth The nylon is the insulation.
- Reality The nylon is a protective skin over the main insulation.
State and city differences
Some cities require conduit and THHN where others allow cable, and the NEC edition differs by state 4. Check local rules, and confirm the electrician is licensed where you live.
When THHN matters to you
THHN matters on outdoor runs, garage circuits, EV chargers mounted on exterior walls, and any time conduit appears on a quote. Ask whether the wire is THWN-2 rated if any part of the run is outdoors.
Next, check the size the load needs with the wire size tool and compare quotes on the same wiring method.
- If Exposed run on a garage or basement wall
- Then THHN in conduit is common
- If Outdoors or underground
- Then THWN-2 in conduit, or UF-B
- If Inside finished interior walls
- Then NM-B cable is typical
- If Future upgrades likely
- Then Conduit lets wire be re-pulled
Source: [1]
Questions about THHN Wire
What does THHN stand for?
THHN stands for Thermoplastic, High Heat-resistant, Nylon-coated. It describes a single wire with thermoplastic insulation and a nylon outer jacket. The "high heat" part refers to its 90°C dry rating.
It is made for pulling through conduit and raceways. On the insulation you will usually see the full marking, such as THHN/THWN-2, the size, the voltage rating and the listing mark.
What is the difference between THHN and THWN-2?
THHN is rated for dry locations at 90°C. THWN-2 is rated 90°C in wet locations as well. Most wire sold now is dual-marked THHN/THWN-2, so one product covers both.
If part of a run is outdoors or underground, the wire must carry the wet rating. Plain THWN without the -2 is rated 75°C in wet locations, so the -2 matters when wire is derated for a crowded conduit.
Can THHN wire be used outdoors?
Yes, if it is also marked THWN or THWN-2 and runs inside conduit. Outdoor conduit counts as a wet location because it collects moisture. Plain THHN without the wet mark is not allowed there.
It still cannot be run loose outdoors without a raceway. If you see single colored wires strung along a fence or siding with no pipe around them, ask an electrician to look before the next storm.
Can I use THHN wire without conduit?
No, not for building wiring. THHN is a single conductor and needs conduit, a raceway or, in larger sizes, cable tray.
Inside walls without conduit, electricians use cable such as NM-B. A raceway protects the wire from damage.
If you find loose single wires stapled in an attic or basement, that is a sign of unpermitted work worth an inspection.
Is THHN wire copper or aluminum?
THHN is usually copper. Cerrowire, one of the large US makers, sells its THHN/THWN-2 only with copper conductors 1.
The metal is printed on the insulation, as CU for copper or AL for aluminum, so read the marking rather than guessing from the color.
Any aluminum conductor needs a larger size for the same load and connectors rated for aluminum, which an electrician checks at every lug.
How many amps can 12 AWG THHN carry?
On a normal branch circuit, 12 AWG copper THHN is protected at 20 amps. The NEC table lists 30 amps at 90°C, but terminal ratings and the small-conductor rule cap the breaker at 20 amps.
The 90°C value is used as a base for derating. For example, when many current-carrying wires share one conduit, the electrician starts from 30 amps and reduces it, which often still leaves enough to keep the 20-amp breaker.
Is THHN better than Romex?
Neither is better in general; they suit different places. THHN in conduit is tougher, can be re-pulled and works in wet locations. NM-B is faster and cheaper inside dry walls.
Many homes use both: NM-B inside finished walls and THHN in conduit for the garage, the outdoor AC disconnect and the service. A quote that mixes them is normal; ask which method goes where.
Why is THHN wire coated in nylon?
The nylon coat makes the wire slick and resistant to scuffs, oil and gasoline. That lets it slide through conduit and around bends without tearing the insulation.
The maker's data lists it as gasoline and oil resistant 1. That rating is one reason THHN in conduit is common in garages and shops, where spills and fumes are part of daily life.
What voltage is THHN wire rated for?
Standard THHN/THWN-2 building wire is rated 600 volts. That covers all normal home circuits at 120 and 240 volts and most commercial circuits. The rating is printed on the insulation.
Because 600 volts is far above a 240-volt home circuit, the voltage rating is rarely what limits THHN; heat and current are. The real choice is the size, which the breaker and the load decide.
Why does THHN come in different colors?
Colors help identify each wire inside a conduit. In homes, white or gray marks the neutral and green or bare marks the ground. Black, red and blue are common for hot wires. Colors keep circuits sorted.
Larger sizes are often sold only in black and marked with colored tape at each end. A white wire taped black or red is being used as a hot wire, so treat every wire as live and leave it to an electrician.
Sources
- Cerrowire, THHN/THWN-2 product page, retrieved .
- Cerrowire, Ampacity Charts (2017 NEC basis), retrieved .
- NFPA, NFPA 70 (National Electrical Code) code development page, retrieved .
- NFPA, Where the NEC is enforced (adoption maps), retrieved .
- NFPA, Free access to NFPA codes and standards, retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Ratings and standards read from the manufacturer page; ampacity from the manufacturer chart; NEC articles named by edition. No prices (G-COST).
Suggest a correction. We fix errors and say what changed.