Flame ratings are the compliance backbone of harness work in North America: they determine which wire may go into which product and building, and they are verified by test, not by datasheet promise. This guide explains the rating families, what each actually measures, and how harness manufacturers stay compliant through material control and process discipline.
The Rating Families
Appliance Wiring Material (AWM) Ratings
Most harness wire is AWM — wire certified to UL's appliance wiring standard (UL 758) with a specific style number, each style carrying its own temperature, voltage and flame requirements. The style number is the spec: a harness built with a stated UL style is certified to that style's full requirement set.
VW-1: The Vertical Wire Flame Test
VW-1 (UL 1581) is the reference flame test for individual wire: a vertical flame application with strict criteria — burning must stop within limits, flags must not ignite, char length is bounded. VW-1 is more demanding than the older 'horizontal' tests and is the default requirement for wire in exposed appliance and equipment applications. When a drawing says 'UL VW-1 wire', it means wire whose style passed this specific test.
FT-Series: Building Cable Ratings
FT1/FT2/FT4/FT6 (CSA-derived, harmonized into UL practice) rate cables for building installation: FT1 vertical (roughly VW-1 class), FT2 horizontal (lighter duty), FT4 vertical array (heavier, riser-class), FT6 plenum (the most demanding — low flame spread and low smoke). The application decides: wire inside an appliance earns AWM/VW-1 territory; cable routed through building plenums must be FT6-class plenum product.
What Makes Wire Flame Retardant
Flame performance comes from the insulation compound: PVC formulated with halogenated flame retardants and antimony synergists, cross-linked compounds with inherent char behavior, fluoropolymers (naturally flame resistant — the plenum and aerospace choice), and halogen-free alternatives (LSZH) where toxicity/smoke rules apply (transit, marine, some building codes). The material selection guide covers the families; the compliance point is that flame rating is a compound property verified per style — no field substitution.
Compliance Practice for Harness Manufacturers
- Material control: buy wire to the drawing's UL style with per-lot certificates; substitute styles only with written approval and re-verification.
- Marking integrity: UL-style wire carries legible style markings; harness work must not obscure them where marking is a compliance requirement.
- Process discipline: heat from stripping, crimping and sleeving must not degrade the insulation that carries the rating — process windows respect the material.
- Component ratings: connectors, sleeving and tubing in the harness carry their own flammability classes (UL 94 V-0/V-1/V-2 for plastics); the harness's overall compliance uses the weakest rated element in the flame path.
Common Compliance Questions
Does harness assembly change the wire's rating? No — the rating follows the wire style; assembly components add their own ratings to the system. Can two wires of the same gauge but different styles be interchanged? No — different styles carry different flame/temperature/voltage packages. Is VW-1 always required? No — it's the common default for exposed equipment wiring; internal harness runs may use lighter-rated styles where the end-product standard allows.
Kaweei UL Harness Manufacturing
Kaweei builds harnesses with UL-recognized materials to customer specifications — style-controlled wire, UL 94-rated components, per-lot certificates and documented processes — from our Dongguan facility for appliance, industrial and equipment customers. Contact our engineering team with your market's compliance requirements; our UL wire harness guide covers the material landscape.