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Standard cross-reference · North American building wire · 2026
UL 83 vs UL 44: THHN, THWN-2, XHHW-2 and Every Other Type, Decoded
Every North American building wire type is assigned to one of two standards by a single letter in its name: T for thermoplastic, X or R for thermoset. That letter decides which temperature column applies, whether the wire may be used wet, and — through the termination rule in the electrical code — how much of the insulation rating you are actually allowed to use. This page decodes all fifteen type letters against UL 83 and UL 44, and gives the published ampacity and conduit-fill figures for the sizes North American jobs actually buy.
The short answer
UL 83 and UL 44 split the North American building-wire family by insulation chemistry. UL 83 covers thermoplastic wire — the type letters beginning with T: TW, THW, THHW, THW-2, THHN, THWN and THWN-2; its scope is 600 V single-conductor. UL 44 covers thermoset wire — the letters beginning with X or R: XHHW, XHHW-2, RHH, RHW, RHW-2 and SIS; its scope runs to 600 V, 1000 V, 2000 V and 5000 V. Four more types sit in their own standards: USE-2 under UL 854, UF under UL 493, MTW under UL 1063 and fixture wire under UL 66. The suffix -2 means 90 °C in wet locations as well as dry; without it, a wet-rated type such as XHHW or THWN drops to 75 °C wet. And the number that sizes your circuit is usually not the 90 °C column at all: the termination rule limits most circuits of 100 A or less to the 60 °C column.
01One letter decides which standard applies
Reading a jacket marking is a two-step exercise. The first step is finding the letter that assigns the wire to a standard; the second is reading the rest of the string for its temperature and location ratings. Most wrong orders happen at step one, because THHN and XHHW-2 are sold side by side as "90 °C building wire" and are not the same product at all.
Underlying that is a material difference. A thermoplastic insulation — practical terms, PVC — softens as it is heated and relies on a thin nylon jacket to survive abrasion, gasoline and oil. A thermoset insulation — cross-linked polyethylene (XLP/XHHW) or ethylene-propylene rubber (EPR/RHW) — is cured into a three-dimensional network, so it does not melt, stays flexible in the cold and tolerates higher surface temperatures. That is why the letters T and X/R are the first thing to read.
The two standards are not mirror images of each other. Their scopes differ in reach: UL 83 is written for 600 V single-conductor thermoplastic wire, while UL 44 covers thermoset wire and cable at 600 V, 1000 V, 2000 V and 5000 V. Both documents are jointly maintained by the US, Canadian and Mexican standards bodies, which is why the same wire is listed under UL 83 in the United States, under a CSA C22.2 number in Canada and under an NMX-J number in Mexico.
02Reading the letters in order
The type string is a compressed specification. Read it left to right and each character answers a question a specifier would otherwise have to look up.
- T — thermoplastic insulation, usually PVC. Softens when heated. Sends the wire to UL 83.
- X — cross-linked polymer insulation (XLP). A thermoset: it does not melt, and it stays flexible in cold weather. UL 44.
- R — rubber; in modern products a thermoset such as EPR or XLP that meets the rubber requirements. UL 44.
- H — heat-resistant: 75 °C.
- HH — high heat-resistant: 90 °C.
- W — moisture-resistant, rated for wet locations. Without the -2 suffix the wet rating is 75 °C; for TW it is 60 °C.
- N — nylon outer jacket. Thin and tough; resists abrasion, gasoline and oil, and lets a smaller outside diameter fit more conductors in a raceway.
- -2 — rated 90 °C in both wet and dry locations. This is the single most valuable two characters in the string.
- U / UF / USE — underground. UF is underground feeder cable; USE is underground service entrance.
- F — fixture wire (TF, TFN). A second F denotes flexible stranding (TFF, TFFN).
- M in MTW — machine tool wire. Not built from the letter rules; read as a whole.
Two names are not letter codes at all
MTW and SIS are read as whole words, not decoded letter by letter — MTW is machine tool wire, SIS is switchboard wiring. And the letter rules are a guide, not the authority: the governing values come from the table the electrical code points to, plus the listing behind the jacket marking. A manufacturer can print less than a type permits, never more.
03All fifteen types, side by side
The table below is the decoder. The dry and wet columns are what the type is rated for; a dash means the type carries no rating for that location at all, and may not be installed there.
| Type | Standard | Insulation | Dry | Wet | Typical use |
|---|---|---|---|---|---|
| TW | UL 83 | PVC | 60 °C | 60 °C | Basic 60 °C building wire; found in older installations |
| THW | UL 83 | PVC, thicker wall | 75 °C | 75 °C | Older general-purpose building wire; larger outside diameter than THHN of the same size |
| THHW | UL 83 | PVC | 90 °C | 75 °C | Often multi-rated with THW-2 |
| THHN | UL 83 | PVC + nylon | 90 °C | — | Building wire in conduit, dry and damp locations; usually sold dual-rated THHN/THWN-2 |
| THWN | UL 83 | PVC + nylon | 75 °C | 75 °C | The 75 °C predecessor of THWN-2, now largely replaced |
| THWN-2 | UL 83 | PVC + nylon | 90 °C | 90 °C | The wet-location rating carried on most THHN building wire; feeders and branch circuits indoors and out |
| XHHW | UL 44 | XLPE | 90 °C | 75 °C | Feeders and services; 90 °C dry but only 75 °C wet |
| XHHW-2 | UL 44 | XLPE | 90 °C | 90 °C | Feeders, services and large conductors; common in aluminium. Tough insulation, good cold and wet performance |
| RHH | UL 44 | XLPE or EPR | 90 °C | — | Dry-location high-temperature industrial wiring; often co-rated with RHW-2 |
| RHW | UL 44 | XLPE or EPR | 75 °C | 75 °C | The 75 °C wet predecessor of RHW-2 |
| RHW-2 | UL 44 | XLPE or EPR, with or without an outer covering | 90 °C | 90 °C | Feeders, services and industrial wiring in wet locations; often multi-rated RHH/RHW-2/USE-2 |
| SIS | UL 44 | XLPE | 90 °C | — | Wiring inside switchboards and switchgear only |
| USE-2 | UL 854 | XLPE or EPR | 90 °C | 90 °C | Underground service entrance and direct burial. A single USE-2 conductor is not permitted inside a building unless it is also rated RHH or RHW-2 |
| UF | UL 493 | PVC, integral jacket | 60 °C | 60 °C | Direct-buried feeders and branch circuits. Ampacity is taken at the 60 °C value whatever the insulation says |
| MTW | UL 1063 | PVC, optional nylon | 90 °C | 60 °C | Industrial machinery and control panels, wired to NFPA 79; finely stranded, often co-rated THHN/THWN-2 and AWM |
| TFN / TFFN | UL 66 | PVC + nylon | 90 °C | — | 18 and 16 AWG fixture wire; luminaire wiring and, where the code permits, control circuits |
Dry and wet ratings compiled from the type-to-temperature assignments in the conductor tables of the electrical code and from two distributor references, which agree type by type. Damp ratings follow the dry rating for most types — check the listing. Oil and sunlight resistance are product markings, not part of the type name: look for "OIL RES I", "OIL RES II", "GASOLINE AND OIL RESISTANT" or "SUNLIGHT RESISTANT" on the jacket. Neither UL 83 nor UL 44 publishes its type tables openly, so no clause or annex value from either document is quoted here.
04What UL 44 and UL 83 actually test
The two standards look similar from the outside — both run the same family of tests — but the thermoset document asks for more, because thermoset insulation is expected to survive harsher service. The published test sets differ in four places that matter to a buyer reading a certificate.
| Test | UL 44 (thermoset) | UL 83 (thermoplastic) |
|---|---|---|
| Vertical flame | VW-1, five cycles of flame application; no fire propagation, no flaming drips | VW-1: must not ignite cotton, sustain flame or drip flaming material |
| Dielectric withstand | 2000 V AC rms for 1 min at 600 V; higher for 1 kV and above | 2500 V+ for 1 min with no breakdown |
| Cold bend | Bent at low temperature, as low as −25 °C; insulation must not crack | Bent at 0 °C or −10 °C without damage |
| Heat ageing / heat shock | Post-oven ageing: insulation must retain tensile strength and elongation | Heat shock at 150–200 °C: jacket must not shrink, blister or crack |
| Sunlight resistance | 720 hours of UV exposure where applicable | Not part of the base type requirement |
| Insulation-specific | — | Nylon jacket adhesion: nylon must stay bonded to the PVC insulation under stress |
Single distributor reference summarising both standards; presented as the published overlap between the two test sets, not as a clause-by-clause comparison. UL 1581 is the separate reference standard that supplies the test methods that UL 44, UL 83 and the other product standards call up — it certifies no cable of its own. Where a project needs a definitive statement, the certificate and the listing for the exact construction govern.
05THHN vs THWN-2 vs XHHW-2: the three types most jobs actually buy
These three carry most of the volume, and most of the confusion, because all three are advertised as "90 °C". They are not the same wire, and the differences are exactly the three that decide an order.
THHN is not wet-rated on its own
Plain THHN is rated 90 °C in dry and damp locations and has no wet rating. The wet rating that buyers assume they are getting belongs to THWN at 75 °C and THWN-2 at 90 °C. Because the market wants one wire for both, most THHN is shipped dual-marked THHN/THWN-2 and is then usable wet at 90 °C. This is why the conduit rule catches people out: the inside of a raceway outdoors or underground is a wet location even when it looks sealed, so a spool marked THHN only cannot be pulled there.
XHHW-2 is a different standard, not a variant
XHHW-2 sits under UL 44 because its insulation is cross-linked polyethylene, not PVC. It holds 90 °C wet and dry, unlike plain XHHW at 75 °C wet, and it is the usual choice on large and aluminium conductors. It has no nylon jacket, and its outside diameter behaves differently in a raceway from THHN's — more on that in section 08.
| Attribute | THHN | THWN-2 | XHHW-2 |
|---|---|---|---|
| Standard | UL 83 | UL 83 | UL 44 |
| Insulation | PVC + nylon jacket | PVC + nylon jacket | Cross-linked polyethylene |
| Family | Thermoplastic | Thermoplastic | Thermoset |
| Dry / wet | 90 °C / — | 90 °C / 90 °C | 90 °C / 90 °C |
| Voltage | 600 V | 600 V | 600 V; 1000 V listings exist |
| Copper size range as catalogued | 14 AWG – 1000 kcmil | 14 AWG – 1000 kcmil | 14 AWG – 1000 kcmil |
| Practical difference | Smallest diameter; needs the dual marking to go wet | The wet rating on most THHN spools | Different standard, tougher insulation, common in aluminium |
Voltage and size ranges as catalogued by our own building-wire range and as published by the two standards' scopes; the 90 °C wet-and-dry behaviour of XHHW-2 and THWN-2 follows from the type letter definitions, not from a manufacturer's claim.
06The 90 °C you cannot spend
A wire rated 90 °C does not give you a 90 °C circuit. The electrical code caps the ampacity you may use at the lowest temperature rating of anything connected to it — and the terminations on most breakers and equipment are rated 60 °C or 75 °C. The 90 °C rating still earns its keep, but as the starting point for adjustment and correction arithmetic (many conductors in one raceway, high ambient), not as the final number.
The boundary is written as a conductor size, not an amperage, which is what makes it easy to misread. The rule sets 60 °C as the usable column for circuits of 100 A or less or marked for 14 AWG through 1 AWG conductors, and 75 °C for circuits over 100 A or marked for conductors larger than 1 AWG — unless the equipment is listed and marked for a higher rating, in which case the marking wins.
The two code references do not phrase the boundary the same way
Our two code references disagree on where the small-conductor boundary falls: one writes it as 14 AWG through 1 AWG, the other as 3 AWG and smaller. That is a live difference and this page does not reconcile it. What it does say is that the deciding document is neither rule of thumb: it is the marking on the device, followed by the listing information for the equipment. Where the terminal is marked "60/75 °C" the 75 °C column applies; where it carries no such marking, the default above is what you get.
07The ampacity columns and what you can bill for
The published ampacity table for conductors in a raceway, cable or direct burial is set out in three columns — 60 °C, 75 °C and 90 °C — against copper and aluminium or copper-clad aluminium. The type list attached to each column is the authoritative shortcut: which wire types are allowed to use which column.
| Size (Cu) | 60 °C column | 75 °C column | 90 °C column | What the insulation says | What you can usually bill the circuit for |
|---|---|---|---|---|---|
| 12 AWG | 20 A | 25 A | 30 A | 90 °C (THHN / THWN-2 / XHHW-2) | 20 A circuit: 25 A is the ceiling at 75 °C, 20 A at 60 °C |
| 10 AWG | 30 A | 35 A | 40 A | 90 °C | 30 A water heater: fine at 75 °C; the 40 A figure is derating arithmetic only |
| 8 AWG | 40 A | 50 A | 55 A | 90 °C | A 50 A range circuit sits on the 75 °C column |
| 6 AWG | 55 A | 65 A | 75 A | 90 °C | 60 A feeder: 65 A at 75 °C, 55 A at 60 °C |
| 4 AWG | 70 A | 85 A | 95 A | 90 °C | 70 A feeder on the 60 °C column, 85 A at 75 °C |
| 3 AWG | 85 A | 100 A | 115 A | 90 °C | The classic 100 A feeder answer — and it is a 75 °C-column answer |
| 1 AWG | 110 A | 130 A | 145 A | 90 °C | Past 1 AWG the 75 °C column is the working default |
| 1/0 AWG | 125 A | 150 A | 170 A | 90 °C | Above 100 A you are working on the 75 °C column by default |
| 4/0 AWG | 195 A | 230 A | 260 A | 90 °C | 200 A service: 230 A at 75 °C; the 260 A figure exists to be derated from |
The three ampacity columns are the published table for not more than three current-carrying conductors in a raceway, cable or direct burial, for copper at 30 °C ambient; the aluminium or copper-clad aluminium half of the same table carries lower figures. The last column is the practical consequence of the termination rule, not a table value. Neither this table nor the type list should be applied without the code edition adopted where the work is done.
Send the type and the loading, and we will check the build
If your schedule names a type and a size and you need to know what that commits you to, send us the type letters, the conductor size, the installation method and the ambient condition. We will answer against a building wire we manufacture, and tell you where the type string and the ampacity column disagree. Ask a quote for the corresponding THHN/THWN-2 construction.
08Does XHHW-2 fit more conductors in the same conduit?
Conduit fill is decided by the cross-sectional area of the finished wire, not by its type name, and the two families behave differently. Below 4 AWG the thermoset wire is the fatter of the two; from 4 AWG upward it turns slightly thinner than THHN of the same size. That single crossover decides which conduit size a retrofit needs.
| Size | THHN area | THHN OD | XHHW-2 area | XHHW-2 OD | XHHW-2 against THHN |
|---|---|---|---|---|---|
| 12 AWG | 0.0133 in² | 0.130 in | 0.0181 in² | 0.152 in | About 36 % more area |
| 10 AWG | 0.0211 in² | 0.164 in | 0.0243 in² | 0.176 in | About 15 % more area |
| 8 AWG | 0.0366 in² | 0.216 in | 0.0437 in² | 0.236 in | About 19 % more area |
| 6 AWG | 0.0507 in² | 0.254 in | 0.0590 in² | 0.274 in | About 16 % more area |
| 4 AWG | 0.0824 in² | 0.324 in | 0.0814 in² | 0.322 in | Roughly equal — the crossover |
| 1/0 AWG | 0.1855 in² | 0.486 in | 0.1825 in² | 0.482 in | Slightly less area than THHN |
Conductor areas and outside diameters as published for the two types in the conduit-fill dimension tables of the electrical code; the last column is the percentage difference between the two area figures, computed here. No conductor count per conduit size is quoted: those are derived values in a different part of the code, and they depend on the raceway type.
09Mapping to GB/T and IEC types
A Chinese or IEC project names the same physical wire differently, and the mapping is at the level of construction rather than type letter. The nearest Chinese relative of a single-core PVC building wire is a BV (rigid copper conductor) or BVR (flexible) under the GB/T 5023 series, with the current two-digit national numbering now carrying the equivalent PVC-insulated 450/750 V fixed-wiring types. The European equivalent of the same construction is written H07V-U (solid), H07V-R (stranded) or H07V-K (flexible), or as the older 6491X under the British designation.
Two mapping traps are worth naming. The first is that the temperature basis differs: the North American types in this page are built around a 600 V, 90 °C conductor in a 30 °C ambient with the three-column ampacity scheme, while the GB/T 5023 and H07V families are 450/750 V construction rated 70 °C, so a type letter swap is not an ampere-for-ampere swap. The second is that the PVC/thermoplastic-versus-thermoset split does not exist in the same place: the Chinese YJ prefix marks XLPE insulation, which lands on the thermoset side of the UL 44 line, while the Chinese building-wire family is largely PVC and therefore lands on the UL 83 side.
Where the mapping is exact, and where it is not
The mapping is exact at the layer level: annealed copper conductor, a PVC or cross-linked insulation, a 600 V or 450/750 V class, and a fixed installation in a raceway. It is not exact at the standards level. GB/T 12706.1-2020 is modified from IEC 60502-1:2004 — the adoption code is MOD, not IDT — and neither document is related to UL 83 or UL 44. A certificate against one system does not establish approval in another, and a project specified to a UL type needs evidence against that type.
10What this page publishes, and what it will not
The type table, the scope statements, the ampacity columns and the conduit-fill figures above come from the published scopes of UL 83 and UL 44, from the code tables that govern the ratings, and from named references, all listed in Sources. Where a document was not available, the page stops at the name: neither UL 83 nor UL 44 publishes its type tables openly, so no clause number and no annex value from either document appears here. The type-to-temperature assignments are given because they are cross-checked between the code tables and two independent references. The test-requirement comparison in Table 2 rests on a single distributor summary and is presented as such.
What the page does not do is approve an installation. It will not tell you that a given wire satisfies your project — that depends on the listing, the marking on the cable, the marking on the equipment it lands on, and the code edition adopted by the authority having jurisdiction. Bring us the schedule and the installation conditions, and we will work the comparison against a construction we actually build.
Name the type and the size, and we will read it back to you
Send the type string, the conductor size, the voltage class and the installation method. We will come back with the construction it corresponds to, the temperature column it is entitled to, and the points where the letters and the table disagree.
How this was written: every figure in this article is checked against the standard or datasheet listed in Sources before publication. Where a value is our own measurement, it is labelled as such.
Sources checked for this article
- UL 44, Thermoset-Insulated Wires and Cables — Edition 19, active; scope clause 1.1 (single- and multiple-conductor thermoset-insulated wires and cables rated 600 V, 1000 V, 2000 V and 5000 V, for use with CSA C22.1, NOM-001-SEDE and NFPA 70), 1.3 (submersible pump cable, no type-letter designations) — UL Standards & Engagement record.
- UL 83, Thermoplastic-Insulated Wires and Cables — Edition 16, active; scope clause 1.1 (600 V single-conductor thermoplastic-insulated wires and cables) and 1.3 (multiple-conductor thermoplastic cable covered in other standards in Canada and the United States) — UL Standards & Engagement record.
- UL 493, Thermoplastic-Insulated Underground Feeder and Branch-Circuit Cables — Edition 10; scope clauses 1.1–1.3, including the UF-B and UF temperature provisions and the statement that the ampacity of all cables covered is that of 60 °C conductors — UL Standards & Engagement record.
- Which standard covers which type — UL 44 for XHHW, XHHW-2, RHW, RHW-2, USE, USE-2 and SIS; UL 83 for THHN, THWN and THWN-2, and the published test requirements for both — Nassau National Cable, Key UL Standards for Wire and Cable.
- Type letters, dry and wet ratings, and insulation construction for TW, THW, THHW, THHN, THWN, THWN-2, XHHW, XHHW-2, RHW-2, USE-2, UF, MTW, TFN/TFFN and SIS, together with the letter-by-letter decoding rules — Galco, Wire Insulation Types. The ratings this reference summarises follow the conductor tables in the electrical code.
- Thermoset and thermoplastic material recognition — thermoset-insulated wires tested to UL 44, service-entrance cables using thermoset-insulated conductors to UL 854, and thermoplastic building wire for outdoor, wet and oil-exposed locations to UL 83 — UL Solutions, Wire and Cable Material Recognition Program.
- Conductor ampacities and conduit-fill dimensions — the three-column ampacity table for not more than three current-carrying conductors in a raceway, cable or direct burial, and the conduit-fill area and diameter table for THHN and XHHW/XHHW-2, as published in the electrical code adopted in California.
- Termination temperature limits — the equipment provision setting 60 °C for circuits of 100 A or less or marked for 14 AWG through 1 AWG conductors, and 75 °C above that, as published in the same code.
- Sorivo building wire — our own THHN/THWN-2 12 AWG and THHN/THWN-2 8 AWG constructions, whose catalogued type markings are the reason the dual-rated form is treated here as the practical default.
Related product · the building wire this page decodes
THHN / THWN-2 12 AWG Stranded Copper Building Wire, 600 V
ApplicationBranch circuits in conduit, indoors and out, where the wet rating of the dual marking is needed
StandardsUL 83; conductor stranded to the applicable ASTM B-series designation
ConductorStranded copper, 12 AWG, 600 V class
InsulationPVC with a nylon jacket; dual-marked THHN/THWN-2
Voltage Rating600 V
Ampacity25 A at 75 °C, 30 A at 90 °C, 20 A at the 60 °C column — the column actually usable is set by the termination
Temperature90 °C dry and, under the THWN-2 marking, 90 °C wet
CertificationsConfirmed against the listing for the exact construction
THHN / THWN-2 8 AWG Stranded Copper Building Wire, 600 V
ApplicationFeeders and larger branch circuits in raceway, including wet and outdoor runs under the dual marking
StandardsUL 83; conductor stranded to the applicable ASTM B-series designation
ConductorStranded copper, 8 AWG, 600 V class
InsulationPVC with a nylon jacket; dual-marked THHN/THWN-2
Voltage Rating600 V
Ampacity50 A at 75 °C, 55 A at 90 °C, 40 A at the 60 °C column — the column actually usable is set by the termination
Temperature90 °C dry and, under the THWN-2 marking, 90 °C wet
CertificationsConfirmed against the listing for the exact construction
Which type does your schedule name? Send the type string, the size and the installation method and we will answer against them — request a quote.
Q1What is the difference between UL 83 and UL 44?
UL 83 covers thermoplastic-insulated building wire — the types beginning with T: TW, THW, THHW, THW-2, THHN, THWN and THWN-2 — and its scope is 600 V single-conductor. UL 44 covers thermoset-insulated wire — the types beginning with X or R: XHHW, XHHW-2, RHH, RHW, RHW-2 and SIS — and runs to 600 V, 1000 V, 2000 V and 5000 V. The letter that assigns a wire to a standard is the first one in its type string.
Q2What does the -2 suffix mean?
It means the wire is rated 90 °C in wet locations as well as dry. Without it a wet-rated type is limited in a wet location: XHHW is 90 °C dry but 75 °C wet, and THWN is 75 °C in both. The two characters are the difference between a wire that may be pulled into an outdoor or underground raceway at full rating and one that may not.
Q3Can THHN be used outdoors or underground?
Not as plain THHN. The inside of a raceway outdoors or underground is a wet location, and THHN on its own is rated for dry and damp locations only. Most building wire sold in North America carries the dual marking THHN/THWN-2, in which case the wet rating comes from the THWN-2 half and the full 90 °C wet rating applies. Check the jacket marking before ordering for a wet run.
Q4If the wire is rated 90 °C, can I use the 90 °C ampacity?
Usually not for the final number. The usable ampacity is capped by the lowest temperature rating of anything connected to the conductor, and most breakers and equipment terminals are rated 60 °C or 75 °C. The rule sets 60 °C as the column for circuits of 100 A or less or marked for 14 AWG through 1 AWG conductors, and 75 °C above that, unless the equipment is listed and marked otherwise. The 90 °C column is the starting point for adjustment and correction arithmetic.
Q5Does XHHW-2 fit more conductors in the same conduit than THHN?
It depends on the size. Below 4 AWG the XHHW-2 conductor has the larger cross-sectional area for the same size — about 36 % more at 12 AWG — so fewer will fit. From 4 AWG upward the two are effectively equal, and at 1/0 AWG XHHW-2 is marginally smaller. Conduit fill is worked from the finished area of the wire, so the size range decides, not the type name.
Q6What is the Chinese or European equivalent of a THHN building wire?
The nearest relatives are a single-core PVC building wire: BV or BVR under the GB/T 5023 series, and H07V-U, H07V-R or H07V-K, or the older 6491X designation, in Europe. The construction maps but the ratings do not: those families are 450/750 V PVC constructions rated 70 °C, whereas THHN is a 600 V type worked on the three-column ampacity scheme. GB/T 12706.1 is modified from IEC 60502-1 — MOD, not IDT — and neither is related to UL 83 or UL 44.