Arc rating is defined in Article 100 and is expressed in what unit?

• A. cal/cm2
• B. J/cm2
• C. W/cm2
• D. N/cm2

Answer: (A)

Arc rating represents how much thermal energy per unit area an arc flash PPE can withstand before a dangerous burn could occur. Article 100 defines this rating in calories per square centimeter (cal/cm^2), an energy-per-area unit used for arc flash protection levels. This aligns with how incident energy is measured in arc flash testing and labeling. The other units don’t fit: joules per square centimeter (J/cm^2) is also energy per area but is not the unit NFPA 70E uses for arc ratings; watts per square centimeter (W/cm^2) is power per area; newtons per square centimeter (N/cm^2) is pressure per area. Since the standard specifies cal/cm^2, that is the correct unit.