110.24(A) Available Fault Current.
Since the 2011 edition of the NEC®, section 110.24(A) has required the maximum available fault current (AFC), and fault current calculation date, to be legibly field marked on service equipment in other than dwelling units. This marking requirement has created both awareness and confusion in the field.
The AFC marking is to ensure that overcurrent device AIC ratings and equipment short circuit current ratings (SCCR’s) are considered and equipment is properly selected based on the amount of AFC in order to comply with 110.9 and 110.10. These two code sections have been in the NEC® for many years.
Since section 110.24 arrived in 2011, many people have been using the numeric AFC values for other purposes such as determining potential arc-flash exposure, or selecting levels of personal protective equipment (PPE) based on the task categories found in NFPA 70E Standard for Electrical Safety in the Workplace.
Determining incident energy or selecting PPE was never the intent of section 110.24. The informational note lets the user know that this section is only for equipment selection and that NFPA 70E should be used to determine proper levels of PPE.
The 2017 NEC® requires that the actual calculation used to determine the AFC be made available to those authorized to design, install, inspect, maintain, or operate the system. This is a big change. Just knowing the AFC value is not enough. The math behind the calculation is key in making sure that the one installing the electrical system did not cut any corners.
Simply installing a foot or two shorter of a service conductor run can have a drastic effect on the final value of AFC at the service equipment. When the AFC goes up from the values shown on the approved plans because of less resistance in a shorter run of wire, equipment AIC ratings may need to be increased in order to comply with NEC® 110.9 and 110.10.
See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).
2014 Code Language:
110.24 Available Fault Current.
(A) Field Marking. Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault-current calculation was performed and be of sufficient durability to withstand the environment involved.
2017 Code Language:
110.24 Available Fault Current.
(A) Field Marking. Service equipment at other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault-current calculation was performed and be of sufficient durability to withstand the environment involved. The calculation shall be documented and made available to those authorized to design, install, inspect, maintain, or operate the system.