What Does an Arc Rating (cal/cm²) Mean?
Jeremy Gordon
An arc rating tells you the maximum amount of heat energy, measured in calories per square centimeter (cal/cm²), that a fabric can absorb before there is a 50% chance it causes a second-degree burn to the skin underneath it. The higher the number, the more thermal energy the garment is designed to handle before that burn threshold is reached. This single figure is what lets a safety manager match a garment to a specific electrical task instead of guessing.
Key takeaways:
- An arc rating, expressed in cal/cm², measures how much heat a fabric can absorb before it's expected to cause a second-degree burn.
- The rating is determined through standardized testing under ASTM F1959, not estimated or self-reported by a manufacturer.
- A garment's arc rating must meet or exceed the calculated incident energy of the task, not just look "high enough."
Where does the arc rating number actually come from?
The number comes from a standardized lab test defined in ASTM F1959, where fabric samples are exposed to a series of controlled electric arcs while sensors measure how much heat passes through to a simulated skin surface. The test is built around the Stoll Curve, a well-established model of the heat exposure that produces the onset of a second-degree burn. The incident energy at which that threshold is reached becomes the reported arc rating. This isn't a marketing figure - it's a specific, repeatable lab result.
Why does a "higher" arc rating matter?
A higher arc rating means the fabric can absorb more heat energy before reaching the burn threshold, which translates to protection against a more severe arc flash event. A 4 cal/cm² garment is suited to lower-energy tasks, while a 40 cal/cm² system is built for the highest-energy work most sites will ever specify. The number isn't a general "quality" score - it's a specific ceiling tied to a specific level of exposure, which is why matching it to the actual hazard matters more than simply buying the highest number available.
How does the arc rating connect to what a worker actually needs?
The arc rating has to meet or exceed the incident energy a worker could realistically be exposed to during a specific task, a figure that comes from a formal arc flash risk assessment or from the NFPA 70E category tables. A qualified person calculates or looks up that exposure level, and the garment's arc rating is then selected to match or exceed it. Buying an arc-rated garment without knowing this number first is like buying a ladder without knowing how high you need to reach - the label alone doesn't tell you if it's the right one for the job.
Does a higher arc rating always mean a heavier, hotter garment?
Generally yes, though not in a strict straight line. Fabric weight and construction both influence arc rating, and heavier, denser fabrics tend to carry higher ratings because there's more material to absorb and dissipate heat. This is part of why CAT 4-rated systems (40 cal/cm²) are noticeably bulkier than CAT 1 garments (4 cal/cm²) - the protection level and the comfort trade-off move together, which is a real factor to weigh against a task's actual, calculated exposure rather than defaulting to the heaviest option available.
Where does the arc rating appear on a garment?
The arc rating appears on the garment's permanent label alongside the statement of compliance with ASTM F1506, which governs how arc-rated clothing must be tested and labeled. If a garment doesn't list a specific cal/cm² number on its label, it hasn't been arc-tested, regardless of what other language appears on the tag or in the product description.