Conductor insulation types: what it means for the Texas exam

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://bondingjumper.com/charts/bondingjumper/conductor-insulation-types.svg" alt="A length of insulated wire with its printed legend THWN-2 broken into labeled pieces: T for thermoplastic, H for heat-resistant, W for wet locations and the dash 2 for a higher rating, wet or dry, with a second label for N, nylon jacket" width="100%"><p><a href="https://bondingjumper.com/guides/conductor-insulation-types?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=conductor-insulation-types.svg">Read the letters one at a time: material, heat, wet use, jacket. by Bonding Jumper</a></p>

Decoding the letters

• T: thermoplastic insulation.
• H: heat-resistant, a higher temperature rating. HH: more heat-resistant again.
• W: suitable for wet locations.
• N: an outer nylon jacket, which resists abrasion and some chemicals and makes pulling easier.
• X: cross-linked polymer insulation, tougher and more heat-resistant than plain thermoplastic.
• -2: the conductor keeps its higher temperature rating in wet locations as well as dry ones.

So THHN is thermoplastic, highly heat-resistant, with a nylon jacket. THWN is thermoplastic, heat-resistant, for wet locations, with a nylon jacket. Many wires on the market carry both markings, and the conductor then has the rating that matches the location it is installed in. Article 310 holds the table of conductor types with their temperature ratings and permitted uses; read the exact ratings in your own 2026 NEC.

Why the type matters

Ampacity. The ampacity tables are arranged in temperature columns. A conductor may start in the column of its insulation rating, but the lowest-rated part of the circuit, often the terminals on the breaker or equipment, can limit the final value. See ampacity.

Location. Every raceway underground or outdoors is treated as a wet location inside, so its conductors need a W rating. A dry-only type pulled into an underground conduit is a violation however good it is otherwise.

Sunlight, oil and chemicals. Some locations need insulation listed as resistant to sunlight or to specific substances.

Insulation and fill

Thicker insulation takes more room, so the same size conductor fills a raceway faster in one type than another. Conduit fill uses the area of the actual insulation type from Chapter 9, and the type you choose can change how many conductors fit.

Common mix-ups

• Using the 90-degree column as the answer when the terminals are rated lower.
• Thinking THHN is wet-rated. Without a W marking, it is not.
• Treating -2 as a size. It is a rating suffix.

On the Texas journeyman exam

Insulation types appear in branch circuits and conductors, 14 scored items, and wiring methods and materials, 12 items, across both portions under the PSI bulletin. Study to decode a type, pick one for a location, or start an ampacity problem in the right column.

Practice it

The conductor ampacity guide shows the temperature columns in use. Every term in this set is in the electrical terms glossary.

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Sources

Show the list

• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.