How to read the markings on a wire jacket
Published Sep 30, 2026
Everything an inspector needs to know about a wire is printed on it. The legend repeats every foot or two along the insulation or jacket, and NEC 310.8 (310.120 in the 2017 edition) requires the key items. Once you know the order, you can read any spool in a few seconds.
A typical THHN legend
Here is the kind of line you will see on a spool of 12 AWG THHN:
MFR 12 AWG 19 STR CU THHN OR THWN-2 600V (UL) GASOLINE AND OIL RESISTANT II VW-1 E123456 2026/08/14 00350 FT
Taken piece by piece:
| Marking | What it tells you |
|---|---|
| MFR | The manufacturer's name or trademark (required) |
| 12 AWG | Conductor size (required) |
| 19 STR | Stranding: 19 strands. SOL means solid |
| CU | Copper. AL means aluminum |
| THHN OR THWN-2 | The insulation types it is listed for (required) |
| 600V | Maximum voltage rating (required) |
| (UL) | The listing mark of the testing lab |
| GASOLINE AND OIL RESISTANT II | Rated for exposure to gasoline and oil at the stated level |
| VW-1 | Passed the vertical wire flame test |
| E123456 | The manufacturer's file number with the testing lab |
| 2026/08/14 | Manufacturing date code |
| 00350 FT | Sequential footage marker |
The file number above is an example. A real one lets you look up the manufacturer in the lab's online directory.
Insulation letters
The type letters come from NEC Table 310.4(1) (Table 310.104(A) in the 2017 edition):
- T: thermoplastic
- H: 75 °C. HH: 90 °C
- W: wet locations
- N: nylon jacket
- X: cross-linked polymer
- R: rubber (as in RHH and RHW-2)
- -2: 90 °C in wet locations too
- USE: underground service entrance
So THWN-2 reads as thermoplastic, 90 °C in wet and dry, nylon jacket. For a side-by-side of the two most common types, see THHN vs XHHW-2.
Other markings worth knowing
- SUN RES or SUNLIGHT RESISTANT: the insulation is rated for UV exposure. Without it, keep the wire out of direct sun.
- DIRECT BURIAL or DIR BUR: can go in the earth without a raceway. You will see it on UF-B, USE-2 and URD, never on plain THHN.
- CT USE or FOR CT USE: permitted in cable tray (1/0 AWG and larger single conductors, NEC 392.10(B)(1)).
- -40C: rated for installation or flexing at low temperature. Check the manufacturer's data for what the number covers.
- RoHS: the manufacturer states the product meets the EU hazardous substances limits. It is not a code requirement in the US.
- c(UL) or CSA: also certified for Canada.
Cable jackets read the same way
NM-B, UF-B, SER and MC cable carry a legend on the outer jacket with the conductor count and size, the cable type and the voltage, for example 12-2 WITH GROUND TYPE NM-B 600V (UL). The first number is the gauge and the second is the number of insulated conductors, not counting the bare ground.
Footage markers
Most building wire has a running footage number. Note the marker at the free end before a pull and again after you cut: the difference is what went into the run. Checking a delivered cut works the same way, by reading the markers at both ends.
Shop wire with the legend you need
- 12 AWG THHN/THWN-2 copper
- 6 AWG USE-2/RHH/RHW-2 aluminum for direct burial
- 2 AWG XHHW-2 copper
- Browse copper building wire
The markings on the wire you receive are the ones that count. If a legend does not show the rating your job needs, ask us before you install it.