NEC Article 110.24 Available Fault Current
Fault current is the amount of electric current that flows through a circuit when a fault, such as a short circuit or ground fault, occurs. It is important to be aware of the fault current as it can cause damage to electrical components and cause injury to personnel. The fault current is typically much higher than normal operating current, and it must be taken into consideration when designing and installing electrical systems. The fault current is determined by the impedance of the circuit, as well as the voltage of the power source. The maximum fault current a system can handle can be determined by measuring the impedance of the system using an ohmmeter. It is important to take into consideration the effects of fault current when installing electrical systems in order to protect electrical components and personnel from potential damage or injury.
(A) Field Marking NEC Article 110.24
Field marking of service equipment in non-dwelling units must include the maximum available fault current, the date the fault current calculation was done, and must be durable enough to survive the environment.
(B) Modifications NEC Article 110.24.
When changes are made to the electrical system that affect the highest level of fault current available at the service, the highest level of fault current must be checked or recalculated as needed to make sure the service equipment ratings are adequate for the highest fault current level at the equipment line terminals. The required labels in 110.24(A) must be adjusted to reflect the new level of highest available fault current.
Exception NEC Article 110.24
In industrial installations where only qualified persons are allowed to service the equipment, the field marking requirements in 110.24(A) and 110.24(B) are not necessary.
NEC Article 110.24 Main Points Regarding Fault Current
Equipment must have an interrupting rating or short-circuit current rating that is at least as high as the available fault current in order to be used safely. Equipment with ratings that are lower than the available fault current can be unsafe. As electrical distribution systems age, the supply network connected to them can be impacted by growth and other changes, leading to an increase in the available fault current. This increase can exceed the ratings of the original equipment, which violates 110.9 and 110.10 and poses a danger to personnel. 110.24 was added to the 2011 Code to require the initial marking of the maximum available fault current, as well as updating the information when the system is altered.
In order to complete a hazard analysis of arc-flash per Section 130.3 of NFPA 70E, the available fault current must be known. This analysis is used to determine the arc-flash protection boundary and the required Personal Protective Equipment (PPE) according to NFPA 70E, Sections 130.3(A) and 130.3(B). The equipment must then be labeled with the incident energy or the necessary level of PPE per 130.3(C). Additionally, an arc-flash hazard analysis must be updated when major modifications or renovations are made, as outlined in 130.3. The analysis should also be reviewed regularly, not exceeding a 5-year period, to take into account changes in the electrical distribution system that could affect the original arc-flash analysis.