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THHN/THWN-2 is a dual-rated building wire combining the characteristics of:
THHN (Thermoplastic High Heat-resistant Nylon-coated)
THWN-2 (Thermoplastic Heat and Water-resistant Nylon-coated - version 2)
Key Characteristics:
Voltage Rating: 600V
Temperature Rating:
90°C (194°F) in dry locations
75°C (167°F) in wet locations
Conductor Material: Copper (stranded or solid) or Aluminum
Insulation: PVC (Polyvinyl Chloride)
Outer Jacket: Nylon (for abrasion and moisture resistance)
Flame Resistance: Flame-retardant (meets UL 83 & NFPA 70)
Water Resistance: Suitable for wet locations (THWN-2 rating)
THHN/THWN-2 wire is approved for:
Primary Applications:
General purpose wiring in conduit or raceways
Commercial and industrial power distribution
Service entrance, feeders, and branch circuits
Machine tools and control panels
Wet locations (THWN-2 rating only)
Direct burial (when specifically marked for such use)
Key Advantages:
Dual rating provides installation flexibility
Nylon jacket offers superior mechanical protection
Resistant to gasoline, oils, and chemicals
Available in multiple colors for easy circuit identification
3.1 Conductor
Material Options:
Bare Copper (most common)
Tinned Copper (for corrosion resistance)
Aluminum (less common, typically for larger sizes)
Stranding:
Solid (Class B): 14 AWG - 10 AWG
Stranded (Class C): 8 AWG and larger
3.2 Insulation System
Base Insulation: PVC compound
Insulation Thickness: Complies with UL 83 requirements
Color Coding:
Black, Red, Blue (ungrounded conductors)
White, Gray (grounded conductors)
Green (equipment grounding)
3.3 Outer Jacket
Material: Nylon
Function:
Provides abrasion resistance
Enhances chemical resistance
Improves moisture protection (THWN-2)
3.4 Standard Sizes & Ampacity
| AWG Size | Stranding | Diameter (mm) | Ampacity (90°C Copper) |
|---|---|---|---|
| 14 AWG | 1 solid | 1.63 | 20A |
| 12 AWG | 1 solid | 2.05 | 25A |
| 10 AWG | 1 solid | 2.59 | 35A |
| 8 AWG | 7 stranded | 3.26 | 50A |
| 6 AWG | 7 stranded | 4.11 | 65A |
4.1 Primary Standards
UL 83 (Thermoplastic-Insulated Wires)
UL 1063 (Machine-Tool Wires)
NFPA 70 (NEC Article 310)
CSA C22.2 No. 75 (Canadian Standards)
4.2 Testing Requirements
Dielectric Strength: 2,000V AC for 5 minutes
Water Immersion: 14 days at 75°C (THWN-2)
Flame Test: UL VW-1/FT1 vertical flame
Cold Bend: -25°C for THWN-2
4.3 International Equivalents
IEC 60227 (PVC insulated cables)
BS 6004 (UK standards)
VDE 0281 (German standards)
5.1 Approved Installations
In conduit (EMT, IMC, PVC)
In cable trays (when permitted by NEC)
Underground (when specifically marked)
Wet locations (THWN-2 rating only)
5.2 Installation Requirements
Minimum Bending Radius: 5 × cable diameter
Ambient Temperature Range: -40°C to +90°C
Pulling Tension: ≤ 0.008 × cmil × number of conductors
5.3 Prohibited Uses
Direct exposure to sunlight (unless marked sunlight-resistant)
As a substitute for NM-B cable in residential wiring
In hazardous locations without proper protection
| Characteristic | THHN/THWN-2 | XHHW | MTW |
|---|---|---|---|
| Insulation | PVC | XLPE | PVC |
| Max Temp (Dry) | 90°C | 90°C | 90°C |
| Wet Location | Yes (THWN-2) | Yes | No |
| Oil Resistance | Good | Excellent | Best |
| Flexibility | Moderate | Stiff | High |
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