Lesson 29 · Field Skills

Reading Wire & Cable Markings

The letters and numbers printed on a wire tell you everything — size, insulation, temperature and voltage rating. Here is how to decode them.

The printing on the jacket means something

Every conductor and cable has markings printed along it, and learning to read them tells you instantly whether the wire is right for the job — its size, its insulation type, its temperature and voltage rating, and where it’s allowed to be used.

Wire gauge (AWG)

The size is given in AWG (American Wire Gauge) for common sizes, then kcmil for large conductors. Remember the backwards rule: smaller number = bigger wire. 14 and 12 AWG are typical branch-circuit sizes; as the number drops (10, 8, 6...) the wire gets physically larger and carries more.

Insulation type letters

Those letter codes (like THHN, THWN, XHHW) describe the insulation, and each letter means something:

T — Thermoplastic insulation.
H — Heat resistant (one H ≈ 75°C); HH — high heat resistant (90°C).
W — rated for Wet locations.
N — Nylon outer jacket (tough, resists oil/gas).

So THHN = thermoplastic, high-heat (90°C), nylon-jacketed — a very common building wire. THWN = thermoplastic, heat-resistant, wet-rated, nylon. Many wires are dual-rated THHN/THWN for both dry-high-heat and wet use.

Why the temperature rating matters: the insulation’s temp rating (60/75/90°C) sets which ampacity column you’re allowed to use, and it must be compatible with the terminals you land on. A 90°C wire is great, but if the breaker lugs are rated 75°C you generally size to the 75°C column. Wire, insulation, and terminations all have to agree.

The rest of the print

Voltage rating (e.g. 600V) — the maximum the insulation is rated for.
Conductor material — CU for copper, AL for aluminum.
Manufacturer, standards/listing marks (UL, etc.) and sometimes the gas/oil resistance rating.

What to take away

Wire markings tell you size (AWG/kcmil, smaller number = bigger wire), insulation type (THHN = thermoplastic/high-heat/nylon, W = wet-rated), temperature rating (sets your ampacity column and must match your terminals), voltage rating, and material. Reading them fluently lets you confirm a wire is right for the job at a glance.

Concept lesson: this one is field knowledge rather than a wiring exercise, so there’s no Sandbox build. How these ratings feed into code-compliant ampacity selection belongs in the NEC section we add later.