UL 1331 FEP Wire 18 AWG — 150°C at 600 Vac with Litz Cable Conductor Option
UL 1331 in 18 AWG is a single-conductor appliance wiring material with extruded FEP insulation, rated 150°C maximum at 600 Vac with a horizontal flame rating. At 18 AWG the insulation falls in the style’s 30–10 AWG bracket at 20 mils minimum average thickness, 18 mils minimum at any point. Beyond conventional solid and stranded construction, UL 1331 permits a Litz cable conductor built from Recognized Component magnet wire — the capability that most clearly separates this style from UL 1330, which does not list Litz construction at all. CableApex supplies UL 1331 18 AWG as a UL Recognized Component — file on record.
What a Litz Conductor Does at 18 AWG
In a solid or conventionally stranded 18 AWG conductor carrying alternating current, current density is not uniform across the cross-section. As frequency rises, skin effect pushes current toward the outer surface and proximity effect redistributes it further where adjacent conductors interact, leaving the interior of the conductor progressively underused and raising effective AC resistance well above the DC value. A Litz conductor addresses this by subdividing the conductor into many fine magnet wire strands, each individually insulated by its own enamel film so that current cannot simply crowd to the outside of a single mass of copper. Under UL 1331 those strands are cabled together to form one conductor, and because the strands carry enamel rather than being bare, the resulting AWG size is established by the DC Resistance Test rather than by physical measurement.
The magnet wire used must be Recognized Component magnet wire (OBMW2) rated at a minimum of 150°C, matching the temperature rating of the finished wire. Product supplied with Litz conductors carries a mandatory additional marking — the tag or reel must be plainly marked “Litz conductors are manufactured using solderable magnet wire.” This marking is not optional and applies to all Litz-conductor product under this style.
Litz Construction: UL 1331 Compared with UL 1333
| Aspect | UL 1331 (this page) | UL 1333 |
|---|---|---|
| Litz strand definition | Strands cabled together to form one conductor | Each single magnet wire strand is a conductor |
| Magnet wire requirement | R/C (OBMW2), minimum 150°C | R/C (OBMV2), minimum 150°C |
| AWG determination | DC Resistance Test | DC Resistance Test per UL 1581 |
| Voltage rating | 600 Vac | 300 Vac |
| Conductor range | 30 AWG–4/0 AWG | 30–10 AWG |
| Litz marking required | Yes — mandatory on tag or reel | Yes — mandatory on tag or reel |
UL 1331 18 AWG Specifications
| Item | Specification |
|---|---|
| UL Style | 1331 — single conductor with extruded insulation |
| Temperature Rating | 150°C maximum |
| Voltage Rating | 600 Vac |
| Conductor | 18 AWG, solid or stranded, or Litz cable conductor of R/C (OBMW2) magnet wire rated minimum 150°C (style range 30 AWG–4/0 AWG) |
| Litz AWG determination | By DC Resistance Test |
| Insulation | Extruded FEP, 20 mils minimum average thickness, 18 mils minimum at any point |
| Optional Covering | Extruded FEP, 2 mils minimum thickness; or glass braid, 7 mils minimum thickness |
| Flame Rating | Horizontal flame; VW-1 vertical flame test available |
| Oil Rating | Optional — 60°C or 80°C oil resistant |
| Gasoline Resistance | Optional |
| UL Recognition | UL Recognized Component — file on record |
| Marking | Per UL Subject 758 marking requirements; Litz product additionally marked “Litz conductors are manufactured using solderable magnet wire” |
| Permitted Use | Internal wiring of appliances |
| RoHS / REACH | Documentation available on request |
Specifying Litz at Order Stage
Litz construction is not a stock variant that can be substituted late in a build. Because the strand count, strand gauge and cabling pattern determine the effective AWG and the frequency range the conductor is suited to, these need to be settled with the order rather than assumed. When submitting an inquiry for Litz-conductor UL 1331, state the intended operating frequency range and the target effective AWG, along with whether the finished lead will be soldered directly — the magnet wire used is solderable, which is what the mandatory tag marking records, and this affects how the termination is prepared. Solid and stranded construction remain available at 18 AWG for conventional line-frequency and DC applications where Litz offers no benefit.
Applications
Within its permitted use — internal wiring of appliances — UL 1331 18 AWG in Litz construction suits lead-out wiring for transformer and inductor windings, induction heating coil connections and switching power supply magnetics operating well above line frequency, where AC resistance rather than DC resistance governs conductor losses. In solid or stranded construction the same size serves as general high-temperature internal wiring at 600 Vac in zones reaching 150°C. Optional 60°C or 80°C oil resistance and optional gasoline resistance are available where the wiring contacts lubricants or fuel. For applications outside internal appliance wiring, confirm application suitability before specifying.
Key Features
- Litz cable conductor option from R/C magnet wire rated minimum 150°C
- Effective AWG established by DC Resistance Test, not physical measurement
- 150°C maximum at 600 Vac; FEP wall 20 mils average at 18 AWG
- Solid and stranded construction also available across 30 AWG–4/0 AWG
- Optional 60°C or 80°C oil and gasoline resistance
- UL Recognized Component — file on record
Request a Quote
CableApex supplies UL 1331 18 AWG from 100 m per color and responds to B2B inquiries within 12 hours. Specify conductor construction (solid, stranded or Litz), and for Litz the target effective AWG and operating frequency range, along with colors, covering option and any oil or gasoline requirements through the form below.
Related FEP styles: UL 1330 FEP Wire (200°C / 600 Vac) · UL 1333 FEP Wire (150°C / 300 Vac thin wall) · UL 1331 FEP Wire category
