Available Fault Current

hardhat_insights.pngAvailable fault current is an important parameter to consider when reviewing a new or even an existing installation of electrical equipment. When standing in front of a line up of switchgear, panelboards, or switchboards, you may be amazed at how many labels you see. These labels are there for a reason. They can be very helpful if you just take the time to understand them. A label that includes the available fault current just may be one of those labels, as it is a requirement of National Electric Code (NEC) Section 110.24, "Available Fault Current." Let's review this section and a few other associated sections to understand this requirement and the various ways it impacts safety.

Available Fault Current

Available fault current to many simply means maximum available fault current because of the fact that we have always had to ensure equipment was rated properly and could handle the interruption or could withstand the maximum the system could provide. It's been a requirement for years in the NEC. In a copy of NEC 1940 for example, Section 1114, "Interrupting Capacity," states, "Devices intended to break current shall have an interrupting capacity sufficient for the voltage employed and for the current which must be interrupted." I'm sure this requirement goes much further back than 1940. We know this requirement today as Section 110.9, "Interrupting Rating," of the NEC, and even as recent as NEC 2014, this section continues to receive attention. NEC 2014 language for Section 110.9 reads as follows:

"Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that is available at the line terminals of the equipment."

"Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted."

The second paragraph above was added as part of NEC 1978. The substantiation for the proposal that was made and accepted by the panel noted, “The concept of ‘at fault levels’ removes from this consideration simple disconnect switches which may break charging or magnetizing current. ‘System’ voltage may be different from ‘employed.’ ‘Available current’ is a more adequate definition than ‘that must be interrupted.’ The difference between a fault interrupter and a simple disconnect switch needs bringing out in this section.” Section 110.9 has seen changes ever since to ultimately be what we know in NEC 2014 as written above.

Another important equipment rating involving available fault current is an equipment short-circuit current rating. While an interrupting rating applies to the ability of an overcurrent device to safely open an overcurrent or the ability of a motor controller to open locked rotor current, a short-circuit current rating applies to the ability of electrical equipment to safely carry short-circuit current, not open short-circuit current. NEC Section 110.10, “Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics,” requires that equipment have a short-circuit current rating that is equal to or greater than the maximum available short-circuit current.

Available fault current is an important parameter for designers, installers, and inspectors to ensure equipment is being applied within its rating. The requirement of labeling the available fault current as part of Section 110.24 though did more than just elevate the awareness of meeting Sections 110.9 and 110.10 when it was introduced as part of NEC 2011. This section packs a punch when it comes to safety.

Field Marking Requirements
There are various sections in the code that require a field marking to be applied to equipment. A marking would have to be field applied and not applied by the manufacturer prior to shipping for various reasons. One example can be found in Section 450.14,