110.24(A) Field Marking.
Code Change Summary: Revised code language and a new informational note regarding the marking of the available fault current.
One of the most common questions about available fault current (AFC) is how can the electrician determine how much AFC is present at the electrical service? Afterall, the NEC® requires that the AFC must be field marked on services at other than dwellings as well as the date that the AFC value was determined.
New informational note 2 added to this section makes it clear that this is the responsibility of the electrical utility provider. The final AFC value is based on factors generally known only by the utility company such as the actual length and size of transformer secondary conductors or whether the conductors are installed in a ferrous metal raceway or a nonmagnetic raceway.
Many utility transformers are not visibly marked on the exterior with the size and % impedance of the transformer which also plays a role in determining the final AFC at the secondary spades of the transformer.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC® edition of NFPA 70.
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.
Informational Note: The available fault-current marking(s) addressed in 110.24 is related to required short-circuit current ratings of equipment. NFPA 70E -2015, Standard for Electrical Safety in the Workplace, provides assistance in determining the severity of potential exposure, planning safe work practices, and selecting personal protective equipment.
2020 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 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.
Informational Note No. 1: The available fault-current marking(s) addressed in 110.24 is related to required short-circuit current and interrupting ratings of equipment. NFPA 70E-2018, Standard for Electrical Safety in the Workplace, provides assistance in determining the severity of potential exposure, planning safe work practices, and selecting personal protective equipment.
Informational Note No. 2: Values of available fault current for use in determining appropriate minimum short-circuit current and interrupting ratings of service equipment are available from electric utilities in published or other forms.