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, which is a deliberate design choice: past that energy level, PPE alone can't reliably protect a worker regardless of how heavy the suit is.
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.
Where the category number actually comes from
The category isn't something a worker chooses on site - it's read directly off the equipment's arc flash label, which states either the calculated incident energy or the category itself, depending on the analysis method used. That's also why the category can differ from one panel to the next in the same building: incident energy depends on available fault current and clearing speed, not on how important the equipment looks. A small panel with a large fault current can land in a higher category than an imposing piece of switchgear with fast upstream protection.
Buying to the standard, not just the number
Selecting the right category number only matters if the PPE itself actually meets the standard behind it - arc-rated clothing is required to comply with ASTM F1506, and the arc rating has to appear on the garment's label for the number to mean anything. A garment with the right cal/cm² printed on a non-compliant label isn't verified PPE; it's a guess with a number attached.
Full category tables and label-reading practice are in the Study Guide, with exam questions in the Exam Companion.