The second article in the National Electrical Code (NEC), Article 100, contains definitions that are essential to the proper Code application. Article 110 contains many requirements that are essential to understanding and applying requirements located throughout the Code. Some of the subjects that Article 110 covers are mentioned numerous times throughout the NEC. The NEC does not mention other subjects as often, such as current transformers (CTs). 


At first, it seems odd to have a provision for CTs in Article 110—especially when there is a specific article covering transformers. But as stated in Exception No. 1 under 450.1, installations of CTs are not covered in Article 450. Since Article 450 does not cover CTs, the best article for this provision is Article 110. As stated in 110.23, unused CTs associated with potentially energized circuits shall be short-circuited. Figure 1 shows a CT with an ammeter in the secondary circuit. This is a permitted installation because the ammeter is the load and the secondary circuit is not open (see Figure 1).


In the electrical world, the result of having a short circuit is usually bad. But when it comes to CTs, having an open secondary circuit while the transformer is energized can be dangerous. It is possible to have high voltage produced on the secondary side of a CT if the secondary terminals or conductors are not short-circuited. This potential can be dangerous to personnel and can also damage the CT. In Figure 2, the ammeter was removed without secondary-circuit conductors being short-circuited. As long as there is current flowing through the conductor (primary) passing through the CT, there will be a voltage potential on the secondary-circuit conductors. Because the ammeter was removed and the secondary-circuit conductors have not been short-circuited, this is a Code violation (see Figure 2).


When an ammeter needs to be disconnected and removed from a CT, make sure to short-circuit the CT’s secondary- circuit conductors first. For example, an ammeter supplied by secondary-circuit conductors of a CT needs to be replaced. A jumper is installed to short-circuit the secondary-circuit conductors of the CT before the ammeter is removed from the circuit. After the secondary-circuit conductors have been short-circuited, it is safe to disconnect and remove the ammeter (see Figure 3).


In installations where CTs are used, shorting blocks are often installed. Shorting blocks help make the task of short-circuiting the secondary conductors of a CT easy and safe.


The next section in Article 110 pertains to available fault current at the service. As stated in 110.24(A), service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. Except for dwellings (single-, two- and multifamily), service equipment shall be marked to show the maximum available fault current at that point in the electrical system. Knowing the maximum available fault current is important for those responsible for designing, installing, inspecting and maintaining that particular electrical system.


To determine the maximum available fault current at the service equipment, a calculation must be performed. This section says the field markings shall include the date the fault-current calculation was performed and be of sufficient durability to withstand the environment involved. While this section says the sign or label must be able to endure the environment, it does not say the marking shall meet the requirements in 110.21(B). That is because 110.21(B) is for caution, warning or danger signs or labels. The marking required by 110.24(A) is not a caution, warning or danger marking (see Figure 4).


The informational note under 110.24(A) provides guidance as to why the marking of the maximum available fault current is necessary. The first sentence says the available fault-current marking(s) addressed in 110.24 are related to required short-circuit current ratings of equipment. Two of the requirements pertaining to short-circuit current ratings of equipment are in 110.9 and 110.10. 


In accordance with 110.9, 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. In accordance with 110.10, circuit protective devices must be able to clear a fault without extensive damage to the electrical equipment. Therefore, the overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected must be considered in order to select the correct electrical equipment. 


When service equipment is marked with the maximum available fault current, it is easier to ensure the equipment’s interrupting rating and the short-circuit current rating is equal to or higher than the available fault current. The second sentence of this informational note says NFPA 70E, 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 (PPE).


Maximum available fault current is part of the data needed for an arc flash calculation. Knowing the maximum short-circuit current is essential when using Table 130.7(C)(15)(A)(b) Arc-Flash Hazard PPE Categories for Alternating Current (AC) Systems in NFPA 70E to select the correct personal protective equipment.


The second subsection in 110.24 pertains to modifications to the electrical system. When modifications to the installation occur that affect the maximum available fault current at the service, the maximum available fault current shall be verified or recalculated as necessary to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment [110.24(B)]. Although these field markings have to be updated because of modifications to the installation, there is no minimum or maximum specified time. 


Changes to service equipment are not the only reason why the maximum available fault current would increase. Changes on the utility side of the facility’s service equipment can also increase the available fault current. For example, the electrical system in a small commercial occupancy was in compliance with the required maximum available fault current marking and the interrupting rating provisions when the system was installed in 2012. This electrical system was installed in accordance with the 2011 edition of the NEC.


When this commercial building was constructed, no other buildings were in the area. Over the last few years, more buildings have been erected. Because of a new building next door, the utility company changed the transformer supplying power to the original small commercial building. Now a larger transformer supplies power to the new building and the original commercial building. Because of these modifications, the maximum available fault current shall be verified or recalculated as necessary to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment.


If the maximum available fault current is different, then—as stated in the last sentence of 110.24(B)—the required field marking(s) in 110.24(A) shall be adjusted to reflect the new level of maximum available fault current. If the interrupting rating or short-circuit current rating of the existing electrical equipment is not equal to or greater than the available fault current, the electrical system will be in violation of 110.9 or 110.10. Therefore, some action will be required because of the unsafe condition. If the maximum available fault current is higher than the fault current when the electrical equipment was installed but it is not higher than the interrupting rating and short-circuit current rating of the existing electrical equipment, no action is required other than updating the previous field marking.


At the bottom of the section covering required field marking, there is an exception: The field marking requirements in 110.24(A) and 110.24(B) shall not be required in industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment [110.24(A) and (B) Exception].


Next month’s column continues the discussion of electrical installation requirements.