NFPA 70E sorts arc flash PPE into categories (formerly Hazard/Risk Categories, HRC) that map directly to the incident energy calculated for a piece of equipment. Once you know the cal/cm² number from the arc flash label, picking the category is close to automatic - the harder part is remembering what changes between them.
The five categories
Category 0 covers incident energy below 1.2 cal/cm². At this level, non-melting or treated natural fiber clothing - untreated 100% cotton, for instance - is acceptable. No heavy arc-rated PPE is required, though avoiding synthetics that melt to skin is still good practice regardless of category.
Category 1 covers 1.2 to 4 cal/cm². Required PPE: an arc-rated shirt and pants or coverall rated at least 4 cal/cm², an arc-rated face shield or hood, arc-rated balaclava, a Class E or Class G hard hat, safety glasses, hearing protection, leather gloves, and leather boots.
Category 2 covers 4 to 8 cal/cm². Same clothing list as Category 1, but the minimum arc rating jumps to 8 cal/cm², and an arc flash suit hood becomes required head protection rather than a face shield plus balaclava.
Category 3 covers 8 to 25 cal/cm². Minimum PPE arc rating: 25 cal/cm². This is where a full arc flash suit - jacket and bib overall or coverall - enters the picture, along with matching head, face, hand, and foot protection.
Category 4 covers 25 to 40 cal/cm². Minimum PPE arc rating: 40 cal/cm², worn as a full suit. This is the top of the table.
Above 40 cal/cm² - the tables stop
Work at incident energy levels above 40 cal/cm² isn't addressed by the PPE category tables at all. It's generally prohibited unless special engineering controls are put in place, and it has to be evaluated through a dedicated incident energy analysis. "Add more PPE and proceed" isn't an option the standard offers once you're past 40 - there's no category to select.
Two details worth memorizing separately
An arc rating (ATPV or EBT) is the maximum incident energy the PPE can absorb with a 50% probability the wearer avoids a second-degree burn. A garment rated 12 cal/cm² is fine at 12 cal/cm² - it is not fine at 15. And hard hat class matters more than it looks: Class E hats (formerly Class B) are tested to 20,000V and are required for electrical work; Class G hats (formerly Class A) only go to 2,200V; Class C offers no electrical protection at all. Arc flash work always calls for Class E, full stop.
Full category tables and label-reading practice are in the Study Guide, with exam questions in the Exam Companion.