Guide to Selecting Electrical Wires THHN THWN Romex XHHW

When facing electrical wiring projects for home renovations or industrial equipment upgrades, the array of wire types can be overwhelming. THHN, THWN, Romex, and XHHW - these similar-looking letter combinations represent distinct performance characteristics and applications. Choosing the wrong wire can affect electrical performance at best, or create serious safety hazards at worst. This guide will clarify the properties of these four common wire types to help you make informed decisions.

THHN (Thermoplastic High Heat-resistant Nylon-coated wire) features a single conductor with PVC insulation and nylon outer coating. This construction provides excellent heat and oil resistance, making it suitable for various applications.

Common Applications:

• Building electrical wiring: Frequently used for conduit wiring in construction projects
• Appliances and machinery: Performs well in high-temperature and oily environments

Structurally similar to THHN, THWN wire also features a single conductor with PVC insulation and nylon coating. While often used interchangeably with THHN, key differences exist.

Key Differences:

The "W" in THWN stands for "Water-resistant," indicating superior performance in wet conditions. THWN maintains a 90°C rating in dry environments and 75°C in wet conditions, compared to THHN's 75°C wet rating.

Romex refers to the brand name for Thermoplastic Sheathed Cable (TPS), specifically Non-Metallic Sheathed Cable (NM-B), primarily used for residential electrical circuits and lighting.

Common Applications:

• Home electrical systems: Widely used for residential wiring and lighting circuits

Differences from THHN:

Romex essentially bundles multiple THHN wires within a single PVC sheath. However, significant regulatory differences exist. Romex isn't permitted for commercial use or residential buildings above three stories. Installation in conduit may require larger sizes, and some jurisdictions prohibit its use entirely.

XHHW (Cross-Linked Polyethylene High Heat-resistant Water-resistant Wire) features a single conductor primarily used in commercial and industrial applications.

Comparison with THHN:

The "X" (often abbreviated as XLPE) denotes cross-linked polyethylene insulation, offering superior moisture resistance (90°C vs. 75°C) and thicker insulation to prevent breakdown in extreme conditions.

Comparison with THWN:

XHHW's XLPE insulation provides better thermal stability and chemical resistance, maintaining 90°C ratings in both wet and dry conditions. THWN uses nylon-coated thermoplastic insulation with 90°C dry and 75°C wet ratings. XHHW suits high-temperature and chemical-exposure environments, while THWN excels where physical durability and flexibility are priorities.

XHHW vs. XHHW-2:

Both use XLPE insulation, but XHHW-2 features enhanced 150°C dry rating (maintaining 90°C wet rating) for demanding industrial applications with higher ambient temperatures.

What does THHN stand for?
Thermoplastic High Heat-resistant Nylon-coated wire, a single-conductor wire mainly for conduit/raceway applications requiring heat resistance and durability.

What are the key differences between THHN, THWN, Romex and XHHW?
THHN and THWN are single conductors rated for dry and wet locations respectively. Romex (NM-B) is non-metallic sheathed cable for dry residential circuits. XHHW offers superior heat/chemical resistance for industrial/wet applications.

When should I choose THHN over THWN or XHHW?
Select THHN for dry conduit applications where cost matters. Choose THWN or XHHW for wet locations. XHHW is ideal for industrial/high-stress environments.

Can Romex (NM-B) be used in wet locations?
No. Romex is only rated for dry indoor residential use. Use THWN or XHHW for wet/outdoor applications.

Are THHN and THWN interchangeable in conduit applications?
Often yes, as many conductors carry dual THHN/THWN ratings. Always verify markings and NEC compliance before substitution.

What are the temperature and voltage ratings?
• THHN: 90°C dry (600V max)
• THWN: 75°C wet (600V)
• XHHW-2: 90°C wet/dry (600V)
• NM-B (Romex): 90°C (limited to 60°C terminations in dry residential at 600V)