Available Fault Current
Available 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, “Disconnect Means,” for transformers. The language in this section states, “for transformers other than Class 2 or Class 3 that are required to have a disconnect, where that disconnect is in a remote location, the disconnecting means shall be lockable, and the location shall be field marked on the transformer.” This field marking is required due to how the installation was conducted. Other examples in addition to Section 450.14 of field marking requirements include the following sections of the NEC:
- Section 240.86, “Series Ratings”
- Section 408.3, “Support and Arrangement of Busbars and Conductors.” Item (F), “Switchboard or Panelboard Identification.”
- 550.33, “Feeder.” Item (A)“Feeder Conductors”
Section 110.24, “Available Fault Current,” which was first introduced in NEC 2011, is another example of a field marking requirement that cannot be applied by the manufacturer as the system dictates the available fault current. The field marking language of Section 110.24 states, “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."
In addition to the available fault current, the requirement here is for the date that the fault current calculation was performed. As far as this author can tell, there are only two other areas in NEC 2014 that require a date to be field marked. Those can be found in Articles 640 for “Audio Signal Processing, Amplification, and Reproduction Equipment” and Article 645, which addresses “Information Technology Equipment.” Section 640.6, “Mechanical Execution of Work” item (D), “Installed Audio Distribution Cable Identified for Future Use,” requires cable tags on these future use cables to include the date the cable was identified for future use and the date of intended use. In a similar manner in Article 645, Section 645.5, “Supply Circuits and Interconnecting Cables,” has item (H), “Installed Supply Circuits and Interconnecting Cables Identified for Future Use,” which requires labeling of these cables with tags that include a date the cable was identified for future use and the date of intended use. Including a date on these labels makes a statement. Including a date on the label required as part of 110.24 does this as well.
The field marking of available fault current raised the awareness of meeting the requirement of 110.9 and including the date raised the awareness that the available fault current can change. Changes in available fault current could be due to changes on the utility side of the equipment and on the customer side of the equipment. Lighting loads and similar will not add to the fault contribution but those that add motors for example are adding sources of fault current. Major changes in facilities can increase the available fault current. Changes on the utility side of the facility can also increase the available fault current. As an example of when a utility available fault current could change and cause a problem for existing equipment, let’s consider a strip mall of stores that experiences growth by adding twice as many stores to the existing structure. In this case, if the existing service is used, the utility may have to increase the size of the transformer supplying the entire load. A larger transformer with the same impedance would translate into higher fault current. This new fault current could put the existing service and all existing electrical equipment at risk of having their ratings exceeded. Good planning ahead of time could avoid problems like this. When the existing labels are updated and inspections proceed, awareness of the problem may be raised.
110.24 and Arc Flash
There has been a bit of confusion regarding the use of the 110.24-required maximum available fault current marking for arc flash protection. When first introduced, many in the industry rose to the floor in concern that this number would be used for the calculation of incident energy. Others pointed out the original intent of verifying short circuit ratings but also noted it could be used with the “Table Method” in NFPA 70E to determine the necessary personal protective equipment. These discussions led to the addition of an Informational Note to 110.24(A) that stated, “The available fault-current marking(s) addressed in 110.24 is related to required short-circuit current ratings of equipment. NFPA 70E-2012, 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.”
The Informational Note clarifies the purpose of the marking, which is to assure that service equipment has the right interrupting ratings and short-circuit current ratings. The second part of this informational note simply says that, “NFPA 70E-2012...provides assistance in determining the severity of potential exposure, planning safe work practices, and selecting personal protective equipment.” This additional note about 70E is helpful as it directs us to the appropriate place for addressing arc flash safety. So that’s where I went to understand whether or not I can use this label for any activities surrounding safe work practices and arc flash.
What I learned was that the marked maximum available fault current cannot be used in the calculation of incident energy as an incident energy calculation actually needs the actual available fault current but this number can be used with the “Table Method” that is a part of NPFA 70E. Using the maximum available fault current in the calculation method could result in a calculation that significantly underestimates or overestimates the incident energy, either of which could result in serious injury or death to the worker if an arc flash incident occurs. If the calculation results in a significant underestimate, it is pretty obvious that the worker might not have enough personal protective equipment (PPE) for the arc flash that could occur. On the other hand, overestimating the incident energy could also be hazardous. My initial thought of this was that overdressing for an arc flash event is a good thing. But as many in the industry have pointed out to me that an overestimate of incident energy could result in a worker wearing too much PPE, which could result in heat exhaustion or an accident from poor visibility and/ or poor dexterity. After donning a 40 Cal suit, you’ll understand as well.
The “Table Method” outlined in NFPA 70E does offer one way that these 110.24 labels could be used for arc flash safety. In the 2012 edition of NFPA 70E, Table 130.7(C)(15)(a) can be used to determine the Hazard Risk Category for specific tasks with specific equipment under specific operating conditions. For example, assume that an enclosed, 200-ampere molded case circuit breaker, with a 42,000-ampere interrupting rating, is the service disconnecting means and service overcurrent protective device feeding a main-lug-only240-volt panelboard immediately next to the enclosed circuit breaker. Assume the task is to remove a bolted cover on the 240-volt panelboard. An energized electrical work permit is obtained after determining that performing the work de-energized would introduce additional or increased risk.
When an incident energy value is not available, the very first part of Table 130.7(C)(15)(a) might be utilized as long as the equipment and operating conditions are met. Those conditions include a maximum available short-circuit (fault) current of 25,000 amperes and a maximum of a two cycle clearing time for the class of overcurrent protective device protecting the panelboard (at the 25,000 ampere fault level). In this example, the maximum available fault current is 19,829 amperes. This fault current level addresses the first of the conditions (25,000 amperes or less). Standard thermal magnetic 200-ampere (and less) molded case circuit breakers, as a class, will have clearing times about ½ cycle at their interrupting rating, so the two cycle clearing time requirement is also met. Finally, the working distance is determined to be 18 inches or greater. With all of the specific conditions met, the table can be used to determine that the task would be a Hazard/Risk Category 1. Table 130.7(C) (16) then shows that arc-rated clothing with a minimum arc-rating of 4 cal/cm2 would be required (see Note 3). This would include (1) arc-rated shirt and arc-rated pants (or arc-rated coveralls), (2) arc-rated face shield (See Note 2) or arc-flash suit hood, (3) arc-rated jacket, parka, rainwear, or hard-hat liner, (4) hard hat, (5) safety glasses or safety goggles, (6) hearing protection, (7) heavy duty leather gloves (See Note 1), and (8) leather work shoes.
Available fault current is a very important parameter to consider in your design, installation, and inspection. There are tools available on the market to help you calculate the available fault current. Leverage your resources and ensure proper labels are installed to ensure products are applied within their listing. With respect to how this maximum available fault current value, marked per 110.24, may or may not be used with respect to arc flash, note that it may not be used to calculate incident energy, but it may be used with the “Table” method in NFPA 70E.
As always, keep safety at the top of your list and ensure you and those around you live to see another day.
Thomas Domitrovich, P.E., is a National Application Engineer with IEC Platinum Industry Partner Eaton Corporation in Pittsburgh, Pennsylvania. He has more than 20 years of experience as an Electrical Engineer and is a LEED Accredited Professional. Domitrovich is active in various trade organizations on various levels with IEC, International Association of Electrical Inspectors, Institute of Electrical and Electronic Engineers (IEEE), National Electrical Manufacturer’s Association (NEMA), and the National Fire Protection Association (NFPA). Thomas is involved with and chairs various committees for NEMA and IEEE and is an alternate member on NFPA 73. He is very active in the state-by-state adoption process of NFPA 70, working closely with review committees and other key organizations in this effort.