THHN vs THWN vs XHHW Wire: Complete Insulation Comparison

Selecting the correct wire insulation type is fundamental to electrical system safety, code compliance, and long-term performance. THHN, THWN, and XHHW represent three of the most widely specified wire insulation types in commercial and residential construction, each with distinct characteristics suited to different environmental conditions and applications. This comprehensive guide compares these three wire types across critical factors including temperature ratings, moisture resistance, installation flexibility, cost considerations, NEC compliance requirements, and optimal use cases. Understanding these differences enables electricians, engineers, and contractors to specify appropriate conductors that meet both code requirements and project-specific environmental demands while optimizing material costs.

Understanding Wire Insulation Designations

What Do the Letters Mean?

Wire insulation designations use standardized letter codes that indicate specific performance characteristics. Understanding these codes is essential for proper wire selection:

• T: Thermoplastic insulation (PVC-based material)
• H: Heat resistant (90°C rating)
• HH: High heat resistant (often indicates higher temperature capability)
• W: Rated for wet locations
• N: Nylon jacket over primary insulation
• X: Cross-linked synthetic polymer (XLPE) insulation

These letter combinations describe the wire's construction and performance capabilities. For example, THWN indicates thermoplastic heat-resistant insulation with a nylon jacket rated for wet locations, while XHHW indicates cross-linked high heat-resistant insulation rated for wet locations.

THHN Wire Explained

THHN (Thermoplastic High Heat-Resistant Nylon-coated) features a PVC insulation base covered with a tough nylon jacket. The nylon outer layer provides excellent abrasion resistance, chemical resistance, and protection during installation. THHN is rated for 90°C (194°F) in dry locations but only 75°C (167°F) in wet locations unless specifically dual-rated as THHN/THWN. The nylon coating makes THHN slippery and easy to pull through conduit, reducing installation labor.

THWN Wire Explained

THWN (Thermoplastic Heat and Water-Resistant Nylon-coated) shares similar construction to THHN but with insulation formulated for moisture resistance. THWN is rated for both wet and dry locations at 75°C (167°F). Most modern THHN wire is actually dual-rated as THHN/THWN or THHN/THWN-2, combining the best characteristics of both designations. The THWN-2 designation indicates the wire meets more stringent heat and moisture resistance standards, allowing 90°C operation in both wet and dry locations.

XHHW Wire Explained

XHHW (Cross-linked High Heat-Resistant Water-Resistant) uses cross-linked polyethylene (XLPE) insulation rather than PVC. This cross-linking process creates molecular bonds that significantly improve temperature resistance, moisture resistance, and resistance to environmental stress cracking. XHHW is rated for 90°C in dry locations and 75°C in wet locations. The newer XHHW-2 designation indicates 90°C rating in both wet and dry locations. XHHW typically doesn't have the nylon outer jacket found on THHN/THWN, making it slightly larger in diameter.

Comprehensive Comparison Table

The following table compares critical specifications and characteristics:

Specification	THHN/THWN	THWN-2	XHHW/XHHW-2
Insulation Material	PVC with nylon jacket	PVC with nylon jacket	Cross-linked polyethylene (XLPE)
Dry Location Rating	90°C (194°F)	90°C (194°F)	90°C (194°F)
Wet Location Rating	75°C (167°F)	90°C (194°F)	75°C (167°F) / 90°C (XHHW-2)
Outer Jacket	Nylon coating	Nylon coating	No jacket (XLPE only)
Abrasion Resistance	Excellent (nylon)	Excellent (nylon)	Good (no jacket)
Moisture Resistance	Good	Excellent	Excellent (XLPE superior)
Chemical Resistance	Very Good (nylon)	Very Good (nylon)	Excellent (XLPE)
Flexibility	Good (especially small sizes)	Good	Excellent (more flexible)
Pulling Ease	Excellent (slippery nylon)	Excellent (slippery nylon)	Good (no slippery coating)
Sunlight Resistance	Fair (limited outdoor)	Fair (limited outdoor)	Good (better UV resistance)
Relative Cost (per ft)	Low to Moderate	Low to Moderate	Moderate to High
Conductor Size Range	14 AWG - 1000 kcmil	14 AWG - 1000 kcmil	14 AWG - 1000 kcmil
Direct Burial	No (unless in conduit)	No (unless in conduit)	Yes (if marked for burial)
Voltage Rating	600V	600V	600V

Detailed Analysis: THHN/THWN Wire

Construction and Materials

THHN/THWN wire features a copper or aluminum conductor surrounded by PVC insulation, then covered with a tough nylon jacket. The PVC insulation thickness varies by wire size, meeting NEC Table 310.104(A) requirements. The nylon jacket adds approximately 4-6 mils (0.004-0.006 inches) to the overall diameter. This dual-layer construction provides excellent mechanical protection while keeping overall wire diameter relatively small, maximizing conduit fill capacity.

Advantages of THHN/THWN

Disadvantages of THHN/THWN

Best Applications for THHN/THWN

THHN/THWN is ideal for general-purpose commercial and residential wiring in conduit systems, panel feeders, branch circuits, equipment connections, and virtually any application requiring conductors in raceways. The dual-rated THHN/THWN-2 version is particularly versatile, approved for both wet and dry locations at full 90°C rating. This wire type dominates North American electrical construction due to its excellent balance of performance, cost, and installation ease. Use THHN/THWN-2 as your default choice unless specific environmental conditions require XHHW's superior moisture or chemical resistance.

Detailed Analysis: XHHW/XHHW-2 Wire

Construction and Materials

XHHW features cross-linked polyethylene (XLPE) insulation without an outer nylon jacket. The cross-linking process chemically bonds polymer molecules, creating a three-dimensional network that significantly improves mechanical strength, heat resistance, and resistance to environmental stress cracking. XLPE insulation maintains flexibility and physical properties across a wider temperature range than PVC. The insulation thickness meets the same NEC requirements as THHN but the absence of a separate nylon jacket means XHHW has a slightly larger overall diameter.

Advantages of XHHW/XHHW-2

Disadvantages of XHHW/XHHW-2

Best Applications for XHHW/XHHW-2

XHHW excels in wet locations, underground installations (when marked for direct burial), chemical plants, wastewater treatment facilities, outdoor exposed applications, and anywhere superior moisture or chemical resistance is required. It's particularly valuable for service entrance conductors, underground feeders, marine environments, and installations subject to frequent moisture exposure. Specify XHHW-2 when you need 90°C rating in wet locations without derating. While more expensive than THHN/THWN-2, XHHW provides critical performance advantages in challenging environmental conditions that justify the cost premium.

Which Should You Choose? Decision Guide

Ampacity Comparison and Temperature Derating

Understanding Temperature Ratings

Wire ampacity depends on the insulation's temperature rating and whether the wire is installed in wet or dry locations. The 90°C rating allows higher current capacity, but NEC often requires using lower temperature ratings depending on application:

• THHN/THWN (non-2): 90°C in dry, 75°C in wet locations
• THWN-2: 90°C in both wet and dry locations
• XHHW: 90°C in dry, 75°C in wet locations
• XHHW-2: 90°C in both wet and dry locations

Ampacity Derating Factors

Even with 90°C rated wire, NEC often requires using 75°C or 60°C ampacity values due to terminal temperature ratings. Most standard circuit breakers, switches, and receptacles have terminals rated only for 75°C. Per NEC 110.14(C), you must use ampacity values consistent with terminal ratings. This means:

• For circuits 100A or less: Usually limited to 60°C ampacity
• For circuits over 100A: Usually limited to 75°C ampacity unless terminals are specifically marked for 90°C
• However, 90°C wire rating allows better performance under derating conditions (more than 3 current-carrying conductors in conduit, ambient temperature over 86°F, etc.)

Practical Ampacity Example

For 12 AWG copper conductor in conduit with three current-carrying conductors:

• THHN in dry location: 90°C rating = 30A (Table 310.16), but limited to 20A by 60°C terminal rating
• THWN in wet location: 75°C rating = 25A, limited to 20A by terminal rating
• THWN-2 in wet location: 90°C rating = 30A, but still limited to 20A by terminal rating

The advantage of 90°C wire becomes apparent when derating is required. If four current-carrying conductors are in the conduit, apply 80% derating factor to the base ampacity, then apply terminal limitations.

Cost Analysis and Budget Considerations

Material Cost Comparison

Wire costs vary by size, market conditions, and copper prices, but relative relationships typically follow these patterns (per 100 feet of copper conductor):

• 12 AWG THHN/THWN-2: $25-45 (baseline)
• 12 AWG XHHW-2: $30-55 (10-25% premium)
• 10 AWG THHN/THWN-2: $45-75
• 10 AWG XHHW-2: $55-90 (10-20% premium)
• 6 AWG THHN/THWN-2: $110-170
• 6 AWG XHHW-2: $130-200 (10-18% premium)

The cost premium for XHHW decreases as wire size increases, making the difference less significant on large conductor applications.

Installation Labor Considerations

THHN/THWN's slippery nylon jacket reduces pulling friction, potentially saving labor on long or complex conduit runs. The difference becomes significant on large projects with hundreds or thousands of feet of wire. However, XHHW's superior cold-weather flexibility can offset this advantage in cold climates where PVC-based wire becomes stiff and difficult to work with.

Long-Term Value

XHHW's superior environmental resistance may provide better long-term value in challenging environments despite higher initial cost. Wire failures require expensive troubleshooting, conduit access, replacement labor, and downtime costs that far exceed the material cost difference. In wet or chemical environments, XHHW's reliability justifies the price premium.

NEC Code Requirements and Compliance

Wire Type Listings (NEC Table 310.4)

NEC Table 310.4(1) lists approved conductor applications and insulations. THHN, THWN, THWN-2, XHHW, and XHHW-2 are all listed for general use. Key NEC requirements:

• THHN: Approved for dry and damp locations at 90°C
• THWN: Approved for dry and wet locations at 75°C
• THWN-2: Approved for dry and wet locations at 90°C
• XHHW: Approved for dry and damp locations at 90°C, wet locations at 75°C
• XHHW-2: Approved for dry and wet locations at 90°C

Wet Location Requirements

NEC defines wet locations as installations subject to saturation with water or other liquids. Examples include outdoor exposed installations, underground conduit systems subject to moisture infiltration, and concrete-encased conduit exposed to moisture. For wet locations, use THWN-2 or XHHW-2 to maintain full 90°C ampacity rating.

Direct Burial Applications

Only conductors specifically listed for direct burial may be installed without raceway. Some XHHW-2 products are rated for direct burial, but verify specific manufacturer listings. THHN/THWN is not approved for direct burial—it must be installed in conduit rated for burial (PVC Schedule 40/80, RMC, IMC, etc.).

Installation Best Practices

THHN/THWN Installation Tips

• Take advantage of the slippery nylon jacket—use pulling lubricant sparingly or not at all on short runs
• In cold weather, warm wire or stage it indoors before installation to maintain flexibility
• Verify dual-rating markings (THHN/THWN-2) on wire jacket before using in wet locations
• Don't over-tighten termination screws—nylon jacket can compress, eventually loosening connection
• Protect wire during installation—while nylon is abrasion-resistant, sharp conduit burrs can still damage it
• Check conduit fill calculations using THHN dimensions from NEC Chapter 9 Table 5

XHHW Installation Tips

• Use quality pulling lubricant on long runs—XHHW lacks THHN's slippery nylon coating
• XLPE's flexibility advantage is most noticeable in cold weather—good choice for winter installation
• Verify terminal compatibility with XLPE insulation before installation
• For direct burial applications, confirm specific product is rated for burial before installing without conduit
• Take advantage of XHHW-2's 90°C wet rating when running feeders in wet locations
• Use slightly larger conduit if possible—XHHW's larger diameter reduces fill capacity vs THHN
• Handle carefully during pulling—without nylon jacket protection, abrasion damage is more likely

General Installation Guidelines

• Always ream conduit ends to remove sharp burrs before pulling any wire type
• Use proper pulling tension limits based on conductor size (don't exceed manufacturer specifications)
• Install conductors with correct color coding per NEC (white/gray for grounded, green/bare for grounding, other colors for ungrounded)
• Verify sufficient conduit fill capacity using NEC Chapter 9 tables before installation
• Make sure all conductors of a circuit are grouped together to minimize inductive heating

Common Mistakes and How to Avoid Them

Temperature Rating Errors

• Mistake: Using 90°C ampacity for circuits with 75°C terminals
• Solution: Always verify terminal temperature rating and use appropriate ampacity table

Wet Location Misapplication

• Mistake: Using THHN (non-THWN) in wet locations
• Solution: Verify dual-rating markings or specify THWN-2 for any potentially wet location

Direct Burial Confusion

• Mistake: Direct burying THHN/THWN without conduit
• Solution: THHN/THWN requires conduit; use only direct-burial-rated XHHW or UF cable for burial without raceway

Conduit Fill Calculation Errors

• Mistake: Using THHN dimensions when actually installing larger XHHW wire
• Solution: Use correct wire dimensions from NEC Chapter 9 Table 5 for the actual wire type being installed

Related Tools and Resources

After selecting your wire type, use these calculators to ensure proper conduit sizing and electrical design:

• Wire Size Calculator - Determine proper conductor size based on load and distance
• Voltage Drop Calculator - Calculate voltage drop and verify NEC compliance
• Ampacity Calculator - Determine conductor ampacity with derating factors
• Conduit Fill Calculator - Calculate maximum wire capacity for conduit systems
• EMT Conduit Calculator - Specialized calculator for EMT conduit fill

Conclusion and Recommendations

THHN/THWN-2 and XHHW-2 both provide excellent performance in properly selected applications. For the majority of commercial and residential electrical installations in conduit, THHN/THWN-2 offers the optimal combination of performance, cost-effectiveness, installation ease, and availability. Its slippery nylon jacket speeds wire pulling, its compact size maximizes conduit capacity, and its wide availability ensures you can source material quickly for any project size.

XHHW-2 earns its cost premium in wet locations, direct burial applications, chemical environments, and anywhere its superior moisture and chemical resistance provide tangible reliability advantages. The cross-linked polyethylene insulation maintains properties across wider environmental ranges and resists degradation from moisture, chemicals, and UV exposure better than PVC-based alternatives. For service entrances, underground feeders, and harsh-environment installations, XHHW-2's enhanced durability justifies the additional investment.

The most successful approach uses THHN/THWN-2 as your standard specification for typical conduit installations and XHHW-2 for specific applications requiring enhanced environmental protection. This strategy optimizes material costs while ensuring each conductor performs reliably in its intended environment. Always verify dual-rating markings (THHN/THWN-2 or XHHW-2) to ensure full 90°C performance in wet locations, and remember that terminal temperature ratings often limit usable ampacity regardless of conductor insulation rating.