UL Yellow Card and Flammability Ratings Explained

Ask for a flame-retardant plastic and you will be shown a grade with "UL 94 V-0" on the data sheet. Specify it, build the tool, and later discover that the V-0 rating only applies above a minimum wall thickness the part does not have, or in a color the part is not, or at an RTI below the part's operating temperature. The yellow card is the document that prevents those surprises — provided you read it properly.

Figure 1. UL 94 flammability rating ladder from HB to 5VA with yellow card fields.

What a yellow card is

A yellow card is Underwriters Laboratories' record of a specific material's tested properties, published in the UL Prospector / iQ database and identified by the manufacturer and a UL file number (the "E number"). It is not a marketing claim; it is a test record tied to specific thicknesses, colors, and conditions.

A yellow card typically contains:

• UL 94 flammability ratings — the horizontal and vertical burn ratings (HB, V-2, V-1, V-0, 5VB, 5VA) with the minimum thickness at which each rating was achieved.
• Relative Thermal Index (RTI) — the maximum continuous service temperature for electrical, mechanical with impact, and mechanical without impact properties, usually given for several thicknesses.
• Hot wire ignition (HWI) and high current arc ignition (HAI) ratings — ignition resistance under electrical fault conditions.
• Comparative Tracking Index (CTI) and arc resistance — surface tracking and arc resistance for insulators.
• Dielectric strength and volume resistivity.
• Glow wire ignition temperature (GWIT) and glow wire flammability index (GWFI) for appliance and IEC-based applications.
• Color and thickness coverage — which colors and which thicknesses the data applies to.
• Manufacturer and grade identification, revision dates and any notes or conditions.

Reading the flammability rating correctly

UL 94 ratings climb from HB (slow horizontal burn) through V-2, V-1, V-0 (vertical burn, self-extinguishing within progressively shorter times and without flaming drips), up to 5VB and 5VA (higher severity).

Three details decide whether a rating applies to your part:

1. Thickness. A grade rated V-0 at 1.5 mm may only be HB at 0.8 mm. If your wall is 0.8 mm, the 1.5 mm rating is irrelevant. Ratings are given per thickness band, and they generally get better as thickness increases.
2. Color. Pigments change combustion behavior. A grade may be listed as V-0 "all colors" or only in specific colors — often natural, black and a limited palette. A custom color match may not be covered at all.
3. Conditioning and test specimen. Ratings come from molded test bars, not from your part. Flow-induced orientation, weld lines, gate position and regrind content can all reduce flame performance versus the test bar.

Where the yellow card meets the design

• Wall thickness is a compliance parameter. If the rating you need exists only at 1.5 mm and above, then the design must have no flame-critical wall below 1.5 mm. Check the thinnest wall in the flame-critical area, not the nominal wall.
• Thin features are a hazard. A thin rib, a louver, a snap arm or a shut-off edge can become the ignition path even if the main wall is thick enough.
• Keep the grade honest. If the drawing says "UL 94 V-0 at 1.5 mm, black," the material certificate should say the same, and purchasing should not substitute a visually identical grade that is only HB.
• Regrind limits apply. Most listings cap regrind. If the molder adds more, the listing no longer covers the part.
• RTI drives temperature-rated applications. For a part that runs continuously at elevated temperature, the relevant number is the RTI for the applicable property (often mechanical with impact), not the HDT and not the UL 94 rating. Compare RTI against the maximum continuous operating temperature with a margin.
• Electrical clearances and CTI. For high-voltage or high-tracking applications, CTI and the electrical RTI determine whether the grade is usable at the design's creepage and clearance.
• Additive and filler effects. Flame retardants, glass fiber, mineral fillers and impact modifiers all change the card values. Do not assume that a family of grades shares the same ratings.

Compliance workflow

1. Identify the requirement. Which standard and rating does the end product need — UL 94 V-0 for an enclosure, 5VA for a large appliance housing, GWIT/GWFI for an IEC-based product, RTI for a continuous-temperature application?
2. Pull the actual yellow card for the exact grade, from the UL database, and check the revision date. Do not rely on a data sheet summary or a distributor's slide.
3. Match thickness and color against the design: minimum flame-critical wall and production color.
4. Confirm the design temperature against the correct RTI column with margin.
5. Specify on the drawing the grade name, manufacturer, color, and the rating with its minimum thickness and the UL file number.
6. Control the supply chain — incoming inspection or a certificate of compliance per lot, and a formal change process if the grade or color must change.
7. Notify and re-qualify if the material, color or thickness changes. A grade change is a compliance event, not a purchasing decision.
8. Keep the traceability the customer and the certification body will ask for: file number, lot, dates, and material certificates.

Common mistakes

• Quoting V-0 and building a part with a wall thinner than the rated minimum.
• Using a custom color that is not covered by the listing.
• Substituting a "same family" grade that has different ratings.
• Reading only the RTI for mechanical without impact when the application is impact-loaded.
• Assuming the rating transfers to a welded or bonded assembly — the joint is not the tested material.
• Letting regrind exceed the listing limit because the part is not visible.
• Never checking the revision date: listings are revised, and an old print may no longer be valid.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Required rating and standard identified from the end-product requirement
• Yellow card obtained for the exact grade and revision from the UL database
• Minimum thickness at the rating compared against the thinnest flame-critical wall
• Production color confirmed as covered by the listing
• Correct RTI column selected (electrical, mechanical with/without impact) and compared to service temperature
• HWI, HAI, CTI, GWIT/GWFI reviewed where the application requires them
• Grade, color, rating, minimum thickness and UL file number written on the drawing and in the purchase specification
• Regrind percentage limits communicated to the molder and enforced
• Certificate of compliance required per material lot
• Change control in place for material, color, thickness or supplier changes
• Welded, bonded or assembled joints assessed separately from the raw material listing

The yellow card is a precise engineering document, and every one of its numbers is conditional. Read it with the design's actual wall thickness, actual color and actual service temperature in hand, specify exactly what the card supports, and control the supply chain so the material that arrives matches the material that was qualified. That is what makes a flame-retardant claim survive an audit.