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110.24 Available Fault Current.
There are many programs to determine the available fault current at a given location but most just provide the final AFC value, not the math to back it up. What would be the available fault current at the service if the service entrance conductors are only 80 feet?
110.24 Available Fault Current.
There are many programs to determine the available fault current at a given location but most just provide the final AFC value, not the math to back it up. What would be the available fault current at the service if the service entrance conductors are only 80 feet? |
Code Change Summary: Additional language makes it clear that the calculation is now needed for the available fault current.
Read carefully, this is a BIG change.
Since the 2011 edition of the NEC®, section 110.24 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 intent of this section is to ensure that equipment short circuit current ratings (SCCR’s) are considered and equipment is properly selected based on the available fault current 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® now 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 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.
Below is a preview of Article 110. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC® edition of NFPA 70.
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.
Which of the following is an NEC requirement for an electrical service at a commercial building?
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