The 2017 NEC and Arc Flash

By Jim Phillips

The 2017 National Electrical Code (NEC) contains several changes regarding arc flash:

• 110.16(B) Arc-Flash Hazard Warning of Service Equipment
• 240.87 Arc Energy Reduction (Circuit Breakers)
• 240.67 Arc Energy 
Reduction (Fuses)

The severity of an arc flash is largely dependent on two key variables which include the available short-circuit current and the duration of the arc flash. The short-circuit current is determined by performing a short circuit study involving extensive calculations. The duration is normally defined by determining how long it takes an upstream overcurrent device, such as a circuit breaker or fuse to operate and clear the arc flash. Time-current curves of the protective device are analyzed to calculate the duration. Recognizing the importance of these variables, three significant changes were made to the 2017 NEC.

110.16(B) Arc-Flash Hazard Warning of Service Equipment


The existing 110.16, Arc-Flash Hazard Warning, was divided into two sections—General and Service Equipment. The original text from the 2014 edition, regarding field or factory marking warning of the arc-flash hazard, is now in 110.16(A), General. This includes labeling requirements to warn qualified people of the arc-flash hazard. NFPA 70E is referenced to provide guidance for developing arc-flash labels, which includes listing the nominal system voltage, incident energy levels, arc-flash boundaries and minimum required levels of personal protective equipment (PPE) for arc-flash protection.

If equipment does not have a label that provides this necessary information for determining appropriate arc-flash PPE, NFPA 70E Table 130.7(C)(15)(A)(b) and 130.7(C)(15)(B) arc-flash PPE categories may be used. These tables list types of equipment and an associated minimum PPE. For each listing in the tables, the PPE category also contains “parameters,” which includes the maximum short-circuit current and maximum fault-clearing time permitted for the specific category. Sometimes the information needed to evaluate the parameters may not be readily available, which can make using the PPE category tables difficult.

To address this situation, a new requirement, 110.16(B) Arc-Flash Hazard Warning—Service Equipment, was added. It applies to service equipment rated 1,200 amperes (A) or more that is installed in other than dwelling units and requires, in addition to the requirements in 110.16(A), a permanent label to be field- or factory-applied.

The service-equipment label is required to meet 110.21(B) requirements and contain the following:

• Nominal system voltage
• Available fault current at the service overcurrent protective devices
• The clearing time of service overcurrent protective devices based on the available fault current at the service equipment
• The date the label was applied

If an arc-flash label is applied in accordance with acceptable industry practices, such as NFPA 70E, which would include listing the incident energy, arc-flash boundary and other details, then the service equipment labeling is not required.

240.87 Arc Energy Reduction (Circuit Breakers)


Normally, overcurrent devices with higher continuous current ratings require greater current to trip their instantaneous function. Operating with no intentional time delay, the instantaneous function can significantly reduce the arc-flash duration, resulting in a lower arc (incident) energy. However, if the arcing fault current is not of sufficient magnitude, the instantaneous function would not trip, resulting in the device operating in its time-delay mode and likely creating a greater hazard.

A method for reducing the arc energy must be provided where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted to 1,200A or higher. The 2014 NEC previously listed methods for reducing the clearing time and resulting arc energy, such as zone-selective interlocking, differential relaying, energy-reducing maintenance switching and energy-reducing active mitigation systems.

The 2017 NEC expands this list and permits the use of the device’s instantaneous trip unit or instantaneous override for arc energy reduction as long as there is sufficient arcing short-circuit current for it to trip. IEEE 1584, IEEE Guide for Performing Arc-Flash Hazard Calculations, is referenced as a method for calculating the arcing short-circuit current.

“Arcing Short-Circuit Current” (­Electrical ­Contractor, March 2016), illustrates how to perform arcing short-circuit calculations.

240.67 Arc Energy 
Reduction (Fuses)


This new language for fuses requires reducing the clearing time where a fuse is rated 1,200A or higher with a fault clearing time greater than 0.07 seconds for the available arcing short-circuit current. It is similar to 240.87 for circuit breakers listed above.

This requirement does not take effect until Jan. 1, 2020, but it is included in the 2017 NEC to provide ample advance notice.

Based on and article by Jim Phillips originally Published in Electrical Contractor Magazine.

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