How to Read a Building Wire Print Legend

What it is

The print legend is the line of text printed or embossed on the insulation or jacket. NEC 310.8 requires conductors to be marked with the maximum rated voltage, the type letters, the maker's name or trademark, and the AWG size or circular mil area. Most wire also shows the listing mark, the conductor metal, extra ratings and a footage or date code. The legend is the best way to identify wire in the field.

How to read it

A typical legend reads something like: maker, 12 AWG, THHN or THWN-2, CU, 600V, a listing mark, then extra ratings and a footage number.

1. Find the size. It is given in AWG or kcmil.
2. Find the type letters. Wire can carry more than one type, such as THHN and THWN-2. Each type is a separate rating.
3. Find the conductor metal. CU is copper. AL or AA-8000 is aluminum alloy.
4. Find the voltage rating, usually 600V for building wire.
5. Look for the listing mark and file number. The standard (for example UL 83 for thermoplastic wire) may be printed too.
6. Read any extra ratings, such as sunlight resistant or oil and gasoline resistant.
7. Note the footage marks if you need to measure what is left on a reel.

Legend elements

Key points

• Each type letter on a multi-rated wire is its own rating. Use the one that fits the location.
• The "-2" suffix means a 90 °C rating in wet locations. See Building Wire Letter-Code Decoder.
• The legend does not give ampacity. Ampacity comes from NEC Article 310 tables and the terminal ratings.
• Print can wear off in use. Check the clearest section of the run, or the reel label.
• Aluminum and copper look alike under insulation only at the cut end. Trust the legend and the cut end, not the color.

Common mistakes

• Reading the footage number as a part number or size.
• Assuming wire is wet-rated because it is THHN. Check for THWN or THWN-2.
• Ignoring the conductor metal on the legend.
• Treating an imported wire without a listing mark as equal to listed wire.

Governing standards: NEC 310.8 (marking); UL 83 (thermoplastic-insulated wire); UL 44 (thermoset-insulated wire); NEC Article 310 for conductor use.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.