In electrical wiring projects across the United States and Canada, two names often appear together: THHN Wire and NM-B Cable. Electricians, contractors, and homeowners frequently face the same question: What is the difference between THHN and NM-B wire, and which one should I choose?
To answer this, we will explain what THHN is, what NM-B is, compare their advantages, highlight their common features, analyze different specifications and applications, and finally guide you on how to choose the right wire for your project.
What Is THHN Wire?
THHN stands for Thermoplastic High Heat-Resistant Nylon-coated wire. It is one of the most widely used building wires in the U.S., recognized under UL Standard 83.
- Conductor: Bare copper or aluminum
- Insulation: Polyvinyl chloride (PVC)
- Nylon Coating: Provides mechanical strength and extra protection against heat and oil
- Temperature Rating: 90°C dry, 75°C wet (when marked THWN)
- Environment: Must be installed in conduit (PVC, EMT, or flexible metallic)
Applications:
- Industrial and commercial wiring
- Outdoor or wet location installations (when marked THWN)
- Conduit-protected feeders and branch circuits
What Is NM-B Wire?
NM-B Cable stands for Non-Metallic Sheathed Cable, also known as Romex® in the U.S. It is primarily used for indoor residential wiring.
- Conductor: Copper (solid or stranded)
- Insulation: PVC insulation for each conductor, color-coded
- Outer Jacket: Flame-resistant PVC sheath
- Temperature Rating: 90°C dry (indoor use only)
- Environment: For indoor, dry locations only, not suitable for wet or outdoor use
Applications:
- Lighting circuits
- Receptacles in bedrooms, living rooms, and kitchens
- Branch circuits for small to medium appliances
Comparative advantages of THHN and NM-B
| Feature | THHN Wire | NM-B Wire |
|---|---|---|
| Flexibility | Sold as single conductors, easy to pull through conduit, flexible installation | Pre-bundled with sheath, convenient for residential indoor use |
| Environment | Suitable for indoor/outdoor (with conduit), wet or dry | Indoor, dry locations only |
| Durability | Nylon coating resists heat, oil, and abrasion | PVC sheath resists flame and abrasion indoors |
| Cost | Slightly higher due to conduit requirement | Lower cost, easier to install |
| Applications | Industrial, commercial, outdoor wiring | Residential, indoor branch circuits |
Common Features Between THHN and NM-B
Despite differences, both wires share some similarities:
- ✅ Both use copper conductors for high conductivity.
- ✅ Both have PVC insulation for electrical safety.
- ✅ Both comply with UL and NEC standards, ensuring safety and reliability.
- ✅ Both are widely available in multiple AWG sizes to meet different current ratings.
Specifications of THHN and NM-B Wires
THHN Wire Specifications
| Gauge (AWG) | Ampacity (Amps) | Applications |
|---|---|---|
| 14 AWG | 15A | Lighting, control circuits |
| 12 AWG | 20A | General branch circuits in conduit |
| 10 AWG | 30A | Air conditioners, water heaters |
| 8 AWG | 40A | Industrial feeders |
| 6 AWG | 55A | Large motors, feeders |
NM-B Cable Specifications
| Cable Type | Outer Jacket Color | Ampacity (Amps) | Applications |
|---|---|---|---|
| 14/2 NM-B | White | 15A | Bedroom lighting, receptacles |
| 12/2 NM-B | Yellow | 20A | Kitchen/bathroom outlets, garages |
| 10/2 NM-B | Orange | 30A | Water heaters, dryers |
| 8/2 NM-B | Black | 40A | Electric ranges, HVAC |
| 6/3 NM-B | Black | 55A | Sub-panels, heavy appliances |
Real-World Applications of Different Specifications
1.THHN Wire Examples
- 12 AWG THHN: Industrial lighting in conduit systems
- 10 AWG THHN: Outdoor pumps in conduit
- 6 AWG THHN: Large commercial feeders
2.NM-B Cable Examples
- 14/2 NM-B: Bedroom lights and small outlets
- 12/2 NM-B: Kitchens and bathrooms where loads are higher
- 10/2 NM-B: Dedicated circuits for dryers or water heaters
- 6/3 NM-B: Sub-panels for residential or farm applications
What Is the Difference and How to Choose?
The key difference lies in environment and installation requirements:
- THHN Wire → Single conductor, designed for conduit, suitable for both indoor and outdoor (wet/dry) environments.
- NM-B Cable → Pre-bundled, with a non-metallic sheath, designed strictly for indoor, dry locations.
How to Choose?
1.Choose NM-B Cable if:
- Your project is residential indoor wiring (lighting, outlets, appliances).
- You want a cost-effective, quick installation option.
2.Choose THHN Wire if:
- Your project involves industrial, commercial, or outdoor conduit installations.
- You need flexibility in environments (wet, oily, high-temperature).
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- 🌍 Global Export Expertise: Familiar with international trade processes, customs clearance, and shipping logistics - ensuring fast and worry-free delivery.
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Ready to Start Your Project?
Whether you need THHN Wire, NM-B Cable, or other UL certified products, Greater Wire is here to deliver quality, reliability, and full support for your business.
👉 Contact us today and let's build success together.
Dongguan Greater Wire & Cable Co., Ltd.
📞 Tel/WhatsApp/Wechat: +86 135 1078 4550 / +86 136 6257 9592
📧 Email: manager01@greaterwire.com