Conductor types and insulation: THHN, THWN, XHHW

The conductor carries current, the insulation sets the rules

A conductor is the metal that carries current. The insulation is everything else you are paying for: the temperature it can run at, whether it survives a wet location, and which ampacity column you are allowed to read. Two reels of #4 copper can carry the same current and still be the wrong wire for the job, because the insulation on one is rated for the place and the termination and the insulation on the other is not.

Picking the conductor is two questions answered together. What is the environment, wet or dry, hot or cold, exposed to oil or sun. And what does the wire land on, because the breaker lug or the equipment terminal has its own temperature rating that caps what the wire is allowed to deliver. Get the environment right and the termination right and the rest is sizing.

This guide is about reading the insulation and matching it to the place and the lug. The ampacity math, the derating, and the column rules live in the companion ampacity guide, and which raceway or cable assembly the conductor runs in lives in the wiring-methods guide. Here the question is narrower and it comes first: is this the right wire at all.

What do the markings printed on the wire mean?

Every listed building conductor carries a printed legend repeated down the jacket, and reading it is the fastest way to know what you are holding. The legend gives the conductor size in AWG or kcmil, the insulation type letters, the voltage rating, the metal, and the listing mark. A typical THHN reads something like 12 AWG, the type letters THHN or THHN/THWN-2, 600V, and the manufacturer and UL mark.

Read it in that order on the reel before you pull, not after. The size tells you the gauge. The type letters tell you the temperature and the wet rating, which is the part this guide decodes. The voltage tells you it is 600V building wire and not something rated lower. The metal matters because aluminum is usually marked AL and copper is often unmarked or stamped CU, and the resistance and the termination both change with the metal.

The legend is also how you settle an argument on site. When someone swears the wire in the pipe is rated for a wet location, you do not guess. You find the print on the jacket and read the letters. If it says THWN or THWN-2 it is wet rated. If it says only THHN it is not, and no amount of confidence changes the stamp.

What do the letters in THHN and XHHW mean?

The insulation letters are a code you can decode on sight once you know the parts. T is thermoplastic, the PVC-type insulation. H is heat resistant to 75C. A second H, written HH, raises that to 90C. W means moisture resistant, rated for wet locations. N is a nylon jacket over the insulation. X is cross-linked polyethylene, a thermoset insulation instead of thermoplastic.

Put the letters together and the name reads itself. THHN is thermoplastic, high heat to 90C, with a nylon jacket, rated for dry locations. THWN is thermoplastic, heat to 75C, wet rated, with nylon. XHHW is cross-linked, high heat to 90C, wet rated. The trailing -2 on any of them, as in THWN-2 or XHHW-2, means the 90C rating applies in wet locations as well as dry.

Decode the type and you know three things without a table: the dry temperature rating, whether it survives water, and the base material. That is most of what the conductor selection turns on. The size and the ampacity are a separate question answered in the ampacity guide.

THHN, THWN, and THWN-2: the building wire workhorse

THHN and its relatives are the single-conductor building wire you pull through conduit on most commercial work. THHN by itself is 90C in dry locations only. THWN is 75C in wet or dry. THWN-2 is 90C in wet and dry, which is the best of the three because it gives you the 90C column for derating and still survives water.

Here is the part that confuses people at the counter. Almost nothing sold as plain THHN anymore is single-rated. The common product is dual-marked THHN/THWN-2, printed with both type names on the same jacket, so the one reel is 90C dry as THHN and 90C wet as THWN-2. When you buy THHN from a major supplier today you are very likely getting that dual-rated wire. Read the print to confirm it, because a job that actually has single-rated THHN in a damp pipe is a problem.

On the job this is the wire in the EMT, the rigid, and the PVC. It pulls well because of the nylon, it takes the 90C column for derating, and the dual rating covers the wet runs. The catch is always the same one and it is not the wire: the termination caps the ampacity, covered below.

What does XHHW mean?

XHHW is cross-linked polyethylene insulation, high heat and water resistant. The cross-linked material is a thermoset rather than the thermoplastic in THHN, so it holds up better to heat and handles a harder pull and a rougher environment. Plain XHHW is 90C dry but only 75C wet. XHHW-2 is 90C in both wet and dry, which is why XHHW-2 is the version you actually see.

XHHW carries no nylon jacket. The cross-linked insulation is tough enough on its own, and it is thicker than THHN insulation, so the same size XHHW takes up more room in a conduit. That fill difference is real on a packed pipe and it surprises people who size the conduit for THHN and then field-change to XHHW.

XHHW-2 shows up on feeders, on larger conductors, and very commonly on aluminum, where the durability and the full 90C wet rating earn the slightly larger footprint. For a long wet feeder run, XHHW-2 and dual-rated THWN-2 both give you 90C wet, so the choice often comes down to pull conditions, fill, and what the spec calls out. When the spec names one, install that one.

The 60, 75, and 90C temperature columns

The ampacity tables list three columns by insulation temperature rating: 60C, 75C, and 90C. The insulation type tells you which column the wire is built for. A 60C wire reads the 60C column. A 75C wire like THWN reads the 75C column. A 90C wire like THHN, THWN-2, or XHHW-2 is built for the 90C column.

The 90C rating is not a license to load the wire to the 90C ampacity in service. The higher rating buys you headroom for derating. You start derating from the 90C value when correcting for ambient heat or bundling, then compare the result against the termination cap, and the lower of the two is what you use. That whole calculation is the subject of the ampacity guide, and it is where most of the real work happens.

So the insulation does two jobs in the table. It sets the starting column for derating, and it sets the ceiling the wire itself can tolerate. The termination, not the wire, usually sets the ceiling you actually live under. That rule is next, and it is the one that catches people.

What is the termination temperature rule?

The termination temperature rule is the gotcha that costs people inspections. The NEC, commonly at 110.14(C), limits the conductor ampacity to the temperature rating of the lowest-rated termination in the circuit, even when the wire is rated higher. Your THHN may be 90C wire, but if the breaker lug is rated 75C, you size that conductor from the 75C column. The 90C on the jacket does not raise the lug.

The reason is heat at the connection. Equipment terminals, breakers, and lugs are listed and tested at a temperature rating, commonly 60C or 75C for gear rated 600V and under. Run the wire hotter than the termination is built for and the heat cooks the connection, which is exactly where failures concentrate. As one widely stated point puts it, no general distribution or utilization equipment is listed to use 90C wire at its full 90C ampacity, so you do not get to use that column in service.

The practical sequence is this. You may begin derating from the 90C column because the insulation allows it. You then compare the derated result to the conductor ampacity at the termination rating, 60C or 75C. The smaller number wins. This is the link between this guide and the ampacity guide, and it is the rule that turns a correctly chosen insulation into a correctly sized conductor. Confirm the section and the edition with the adopted code, because the numbering shifts between cycles.

Can you use THHN wire in a wet location?

No. Plain THHN, with no W in the type and no -2, is rated for dry locations only, and a wet location calls for a W-rated insulation like THWN, THWN-2, or XHHW. The W is the whole point of the letter code. Insulation without it absorbs moisture, breaks down, and you get a fault where the conductor sits in water.

The trap is what counts as a wet location. The NEC treats underground raceway, conduit that can fill with water, and exposed outdoor runs as wet locations, and that includes the inside of a conduit run underground or outdoors even though it looks dry the day you pull it. A raceway below grade fills. A run on a roof sees rain driven into the fittings. The code reads those as wet whether or not there is standing water the moment you look.

In practice this is why dual-rated THHN/THWN-2 took over. The plain dry-only THHN problem mostly disappears when the wire on the reel is already wet rated. But you still confirm it by the print, because the difference between THHN and THWN-2 in a wet pipe is the difference between a wet-location violation and a clean install. The wiring-methods guide covers which raceways count as wet and how to keep water out of the ones that do not.

The nylon jacket, and why N earns its letter

The N in THHN and THWN is a thin, tough nylon jacket over the PVC insulation, and it does more than its thickness suggests. It resists abrasion on a hard pull, it cuts the friction so the wire slides through bends, and it stands up to oil and gasoline better than bare thermoplastic. That is why THHN pulls cleaner through a long conduit run than insulations without it.

The oil and gas resistance is not a small thing in the right building. In a vehicle service bay, a fuel station, or a mechanical room where hydrocarbons are around, the nylon is the layer keeping the insulation from softening and swelling. XHHW skips the nylon because the cross-linked insulation is tough on its own, which is a different way to get to durability.

The field payoff of the nylon is the pull. On a long, bent run, the slick nylon is the difference between a conductor that pulls in clean and one that hangs up and gets damaged. Wire pulling lube still goes in the pipe, but the jacket is doing half the work.

Building wire vs cable assemblies

There are two ways the conductors get to the load. As single insulated conductors pulled into a raceway, which is the THHN, THWN-2, and XHHW-2 building wire above. Or as a cable assembly, where the conductors come bundled inside a common sheath or armor that you run as a unit. Which one you use is a wiring-method decision, and the wiring-methods guide covers the raceways and the rules in depth.

Single conductors in conduit give you flexibility. You pick the exact insulation for the location, you can pull more later if the raceway has room, and the conduit itself is the physical protection. That is the commercial and industrial default. The cost is labor: you bend pipe, you pull wire, and that takes time.

Cable assemblies trade that flexibility for speed. The conductors, the grounding conductor, and sometimes the armor come as one product you staple or strap into place, which is faster on the kind of work where it is permitted. The named types are NM-B, MC, AC, UF, and the service-entrance cables SE and USE, each built for a different place. The next sections take them one at a time.

NM-B cable (Romex)

NM-B is the nonmetallic-sheathed cable that wires most houses. The conductors inside it carry 90C-rated insulation, but the cable is limited to the 60C ampacity column, commonly under NEC 334.80, so 14 AWG is 15 A, 12 AWG is 20 A, and 10 AWG is 30 A regardless of the higher-rated wire inside. The 90C rating inside is there for derating headroom, not for loading the cable harder.

NM-B is for dry, indoor, protected locations. It is not wet rated, not for direct burial, and not for exposed outdoor use. People run it where it does not belong because it is cheap and fast, and that is one of the common ways a job fails inspection. The rules on where NM is allowed, and the height and protection requirements, sit in the wiring-methods guide and in the code article that governs it.

On commercial and industrial work NM-B is usually off the table entirely, replaced by conduit and building wire or by MC cable. Where it is permitted, the 60C ampacity limit is the number that matters, and it is lower than people expect from wire that has 90C insulation printed on the conductors.

MC and AC cable

MC, metal-clad cable, and AC, armored cable, both run insulated conductors inside a flexible metal armor, which gives the assembly physical protection without a separate raceway. They are common in commercial work, in the kind of building where NM is not allowed and full conduit is more labor than the job needs. The armor lets you run cable through structure and above ceilings fast.

The two are not interchangeable, and the difference is in the grounding path. AC cable uses its armor, with an internal bonding strip, as the equipment grounding path. MC cable includes a separate equipment grounding conductor inside, and its armor is generally not the ground. That distinction changes how you terminate them and how the inspector checks the ground, so do not treat the two names as the same cable.

The conductors inside are typically 90C wire, and the same termination rule applies: you still size to the lowest-rated termination. The wiring-methods guide covers where each is permitted, the connector and anti-short bushing details, and the support rules, because installing the armor wrong is a more common failure than the conductor choice.

UF, USE, SE, and SER: underground and service-entrance cable

These are the cables for going underground and for landing the service, and the names get mixed up constantly. UF is underground feeder cable, rated for direct burial, used for branch circuits and feeders run in the ground without a raceway. It is the cable to the detached garage, the well, or the yard light. UF is not a service-entrance cable.

USE is the underground service-entrance cable, the one that carries power from the utility underground to the service equipment. USE-2 is the 90C wet-rated version. USE is an outdoor and underground product. A point worth knowing is that USE is generally not permitted as interior wiring inside the building, so it terminates where it enters and the wiring changes type from there.

SE is the aboveground service-entrance cable, commonly seen as SER, the round version with a separate grounding conductor, running from the meter to the panel or feeding a subpanel. The two letters that change the install are E for service entrance and U for underground. Mix them up and you put the wrong cable in the wrong place, which the wiring-methods guide treats in more detail along with burial depths and the cover requirements.

Copper or aluminum conductors?

Both carry current. Copper has lower resistance at the same size, so it drops less voltage and runs a bit cooler, and it is the default for branch circuits and smaller conductors. Aluminum costs less and weighs far less, which is why it dominates larger feeders, service conductors, and long runs where the material savings are real. The trade carries both, and the choice is cost and weight against size, run by run.

Aluminum is not a drop-in for copper at the termination, and that is where the failures come from. Use a lug listed for aluminum, marked AL/CU or AL9CU for larger lugs or CO/ALR for devices, never a copper-only lug. Torque the connection to the manufacturer's value with a calibrated wrench, because under-torque and over-torque both fail, and many manufacturers call for an antioxidant compound on the conductor, brushed in, though the code does not universally mandate it. Follow the listing for the specific connector.

The classic aluminum failure is the loose termination. Aluminum moves under thermal cycling, the lug backs off if it was not torqued and treated right, and you find it by the discoloration on the insulation or the smell before the breaker ever trips. Copper-on-aluminum on a device not rated for it is the same story. The conductor sizing math for the two metals lives in the ampacity guide; the part that matters here is the insulation and the termination, and aluminum demands the right one of each.

Stranded vs solid conductors

Solid is one piece of metal. Stranded is many smaller wires bundled into the same size. Small building wire, the 14 and 12 AWG that lands on receptacles and switches, is commonly solid, which holds a screw terminal and a backstab well and is stiff enough to push into a box. Larger conductors are stranded because solid wire above a certain size is too stiff to bend and pull.

Stranded flexes, pulls easier around bends, and is what you want on anything that moves or anything large. The catch is the termination. Some devices and lugs are listed for solid, some for stranded, some for both, and a stranded conductor under a screw can splay and lose strands if it is not handled right. Match the conductor type to what the termination is listed to accept.

On the resistance and the ampacity, solid and stranded of the same size are close enough that the table value covers both for field work. The choice is about handling and termination, not about carrying capacity.

Conductor color code

Color identifies what a conductor does, and only some of it is mandated. The grounded conductor, the neutral, is white or gray. The equipment grounding conductor is green, green with a yellow stripe, or bare. Those two are fixed by code. The ungrounded phase conductors are not assigned colors by the NEC for most systems, so the phase colors are convention and project standard rather than a rule.

Common practice on a 120/208V system is black, red, and blue for the three phases. On a 277/480V system the common convention is brown, orange, and yellow. These are widely used but they are standard practice, not a code mandate for the phases, so a project spec or the local standard can set something else and that governs. Where two voltage systems share a building, the code does require a means to identify each system, often by a consistent color scheme posted at the panel.

One color is a hard rule worth stating plainly. Do not use white, gray, or green for an ungrounded conductor. White and gray are the neutral, green is the ground, and putting a phase on them is the kind of mistake that gets someone hurt by a wire they trusted to be dead.

Voltage rating and special insulations

Standard building wire, THHN, THWN-2, and XHHW-2, is rated 600V, which covers the great majority of building circuits at 480V and below. The voltage rating is an insulation property, printed on the jacket, and it sets the maximum system voltage the insulation is built to withstand. For medium-voltage work above 600V, the conductor and its insulation are a different class entirely, with their own types and testing, and that is its own subject.

Beyond the common building wire there are insulations for hotter places. Silicone and other high-temperature fixture and appliance wires run well above 90C, used inside luminaires, in high-heat equipment, and where the ambient or the device demands more than thermoplastic can give. These are not general building wire and you use them where the listing or the equipment calls for them, not as a substitute for THHN in a pipe.

The point for selection is to read the voltage rating along with the temperature and the wet letters. A 600V wire is fine for a 480V feeder. A high-temperature appliance wire belongs where the heat is, not pulled through conduit as a feeder. Match the insulation class to the application the same way you match the temperature and the location.

The grounding and bonding conductor

The equipment grounding conductor, the EGC, is the path fault current takes back to the source so the breaker trips. It is identified green, green with a yellow stripe, or bare, and it can be a separate insulated conductor, a bare conductor, or the raceway or cable armor itself where the wiring method is listed to serve as the ground. It carries no current in normal operation, only during a fault.

When the EGC is a wire, its insulation follows the same letter code as the phase conductors, so a green THHN or THWN-2 is common in a conduit run. The size of the EGC is a separate question driven by the overcurrent device protecting the circuit, and it grows when the phase conductors are upsized. That sizing math sits in the ampacity and voltage-drop guides.

The thing to carry from here is that an undersized or open ground means fault current has no clean path back, so the breaker may not trip when it has to. The insulation choice for a green wire is straightforward; the sizing and the integrity of the path are where the attention goes.

How the insulation picks the ampacity column

The insulation does not size the conductor by itself, but it sets the starting point. The type letters tell you the wire's temperature column, 60, 75, or 90C, and that is the column you begin from. A 90C insulation like THWN-2 or XHHW-2 lets you start derating from the 90C value, which is the headroom that makes those types worth buying.

From there the sizing is the ampacity guide's job. You correct for ambient temperature, you adjust for the number of current-carrying conductors bundled together, and you cap the result at the lowest-rated termination under 110.14(C). The insulation chose the column; the conditions and the termination decide the final number. Picking the right insulation is necessary, and it is not the whole calculation.

Keep the two guides side by side on a real run. This one gets you the right wire for the place and the lug. The ampacity guide turns that wire into a sized conductor that holds up under the heat and the crowding it actually runs in.

What to document

Write down the conductor as installed, because the question that comes back later is always whether the right wire went in the right place. The record is what answers it when a wet-location call or a termination question lands months after the crew is gone.

Capture the insulation type as printed, the size and metal, the temperature rating, the wet or dry rating, the location it runs in, the termination rating it lands on, and the column you sized from. If aluminum, record the lug listing and the torque used. The table below is the short version of what a reviewer needs to confirm the conductor suited the place without standing in front of the pipe.

Common mistakes

• Pulling plain THHN, not wet rated, into an underground or outdoor raceway that the code treats as a wet location.
• Sizing to the 90C column in service when the termination is 60C or 75C, instead of capping at the termination under 110.14(C).
• Landing aluminum on a copper-only lug, or skipping the listing check for AL/CU or CO/ALR.
• No antioxidant where the manufacturer calls for it, and no calibrated torque on the aluminum termination.
• Running NM-B cable outdoors, buried, or in a wet location where it is not permitted.
• Treating NM-B as a 90C cable for ampacity when it is limited to the 60C column.
• Confusing USE with SE, or UF with USE, and putting the wrong cable underground or inside the building.
• Putting a phase conductor on white, gray, or green, which are reserved for neutral and ground.

Field checklist

Standards and references

The NEC, NFPA 70, is the framework. Conductor types, their insulation ratings, and their permitted uses live in Article 310, which carries the conductor application and insulation tables that pair each type with its temperature and wet rating. The termination temperature limitation is commonly at 110.14(C), the rule that caps the ampacity at the lowest-rated termination. The ampacity tables themselves, commonly Table 310.16 for the usual conditions, with the correction and adjustment factors, are the subject of the ampacity guide.

The cable assemblies have their own articles. Nonmetallic-sheathed cable, NM, is governed by its article with the 60C ampacity limit. The metal-clad, armored, underground-feeder, and service-entrance cables each have a governing article that sets where they are permitted. The exact article and section numbers shift between code cycles, so confirm them against the edition the jurisdiction has adopted and any local amendments before citing them on a submittal.

The wire itself is built and listed to UL standards: thermoplastic types like THHN and THWN under one standard for thermoplastic-insulated wire, and thermoset types like XHHW under the standard for thermoset-insulated wire, with separate standards for NM and the service cables. The printed markings and the listing mark are what tie a reel on site back to those standards, which is why you read the jacket rather than trust the label on the box.

Units, terms, and conversions

The same conductor reads differently across a drawing set, a manufacturer sheet, and a spec, so the terms are worth pinning down.

Conductor size is American Wire Gauge, AWG, for smaller conductors, where a larger number is a smaller wire, and thousands of circular mils, kcmil, for larger conductors above 4/0. Metric drawings use the cross-sectional area in square millimeters. Temperature ratings are in degrees Celsius in the code tables, the 60, 75, and 90C columns. Voltage rating is the insulation's maximum system voltage, 600V for standard building wire.

THHN

Thermoplastic, high heat to 90C, nylon jacket, dry locations; usually sold dual-rated as THHN/THWN-2

THWN / THWN-2

Thermoplastic, wet rated, nylon; THWN is 75C wet, THWN-2 is 90C wet and dry

XHHW / XHHW-2

Cross-linked polyethylene, high heat, wet rated; XHHW-2 is 90C wet and dry

Termination rating

The temperature rating of the lug or terminal, which caps the conductor ampacity under 110.14(C)

Wet location

Underground, outdoor, or fillable raceway; requires a W-rated insulation

AWG / kcmil

American Wire Gauge for smaller sizes, thousand circular mils for conductors above 4/0

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