What's the Difference? Arc-Flash Warnings and Maximum Available Fault-Current Markings

The National Electrical Code (NEC) has evolved in recent development cycles to address marking requirements for arc-flash hazard warnings and maximum available fault current. Lately, there have been questions regarding the differences between these required markings. This column provides clarity and simplifies the process of differentiating between the two.

Appearing first in the NEC’s numbering sequence, Section 110.16 has specific criteria that drives the marking requirements. First, in subdivision (A), the general arc-flash hazard warning requirement applies to equipment such as switchboards, switchgear, panelboards, industrial control panels, meter socket enclosures, and motor control centers in other than dwelling units, and it is likely to require examination, adjustment, servicing or maintenance while energized. This equipment shall be field- or factory-marked to warn qualified people of potential electric arc-flash hazards. This marking uses the signal word “warning,” which drives the requirements in ANSI Z535.4, thus the reference to 110.21(B).

Subdivision (B) in 110.16 is specific to service equipment rated at 1,200 amperes or greater in other than dwelling units. The same arc-flash hazard warning markings apply to these services, but additional information must be provided: the nominal system voltage, the available fault current, the clearing time of the service overcurrent protective device, and the date the label was applied. The exception to (B) relaxes the above information on the arc-flash hazard warning marking, if a label in accordance with acceptable industry practices was already applied.

An example of this is a label produced to meet the requirements in Section 130.5(D) of NFPA 70E, Standard for Electrical Safety in the Workplace. The important thing to remember is the NEC arc-flash hazard warning label requires installation-related detail that assists users in applying the NFPA 70E Table Method of determining what level of incident energy is present and what level of personal protective equipment (PPE) is required. Three informational notes follow Section 110.16 and provide valuable guidance for users. These notes reference NFPA 70E, where the rules related to determining severity of potential exposure, planning safe work practices, arc-flash labeling, and selecting PPE are provided.

NFPA 70E also provides specific criteria for developing arc-flash labels for equipment that provides nominal system voltage, incident energy levels, arc-flash boundaries, minimum required levels of PPE, and so forth. All of these items mentioned are clearly beyond the scope and purpose of the NEC as described in 90.1 and 90.2. The arc-flash hazard warning marking required by Section 110.16 of the NEC differs significantly from the maximum available fault current general marking requirement in Section 110.24.

Section 110.24 is a mandatory general marking requirement for service equipment in other than industrial installations by exception and dwelling units. This marking requirement differs from Section 110.16 in a few ways. First, the requirement markings of 110.24 are not hazard marking requirements. They do not mandate use of a hazard signal word such as “danger,” “warning,” “caution,” and so forth. The Z535.4 criteria are not required to be met with these markings. This section requires the maximum available fault current to be marked on the equipment and the date of the calculation (if applicable). Often, the maximum available fault current values can be obtained from the local utility, which could be by calculation or by published maximum available fault current values within their documented requirements for electric service.

The maximum value of available fault current must be known to design a project and to be sure that the service equipment is installed within the maximum available fault current rating marked on the equipment. Subdivision (B) in 110.24 addresses system modifications that could cause the available fault current to increase; this is intended to assist owners and authorities having jurisdiction in assuring that equipment is suitably rated for the amount of available fault current at the terminals of the equipment. Section (B) requires new values of maximum available fault current to be marked if a change in maximum value is identified. This is to raise the level of awareness that service equipment must always be applied within its maximum available fault current ratings. If it is not, it is in violation of the NEC.

There are unique differences between the markings required by sections 110.16 and 110.24, both of significant importance. The markings required in 110.16 are directly related to warning qualified people of potential arc-flash hazards, while the requirements in 110.24 are directly related to service equipment being used within its marked available fault current ratings. Each section should be applied independently of the other to ensure full compliance with the NEC.

About the Author

Michael Johnston

Executive Director of Standards and Safety, NECA

Michael Johnston is NECA’s executive director of standards and safety. He is chair of the NEC Technical Correlating Committee. He served as a principal representative on NEC CMP-5 representing IAEI for the 2002, 2005, and 2008 cycles and is currently...

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