Does the National Electrical Code (NEC) address conductor-withstand ratings for wire-type equipment grounding conductors (EGCs)? Yes, but it is worthy of a more detailed explanation. The Code is the minimum set of wiring requirements for safety, meaning one must do at least that much to be compliant. The minimum sizing requirements for wire-type EGCs are found in Section 250.122 and Table 250.122.


Protecting equipment from damage


Section 110.10 indicates that the overcurrent protection devices (OCPDs), the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical equipment. The word “equipment” includes the conductors in this general requirement. Table 250.122 provides the minimum sizes for wire-type EGCs, and they are related to current-withstand capabilities of the conductor. 


An important note follows Table 250.122 and references the performance requirements in Section 250.4. Notes following tables are applicable as requirements. This table note is an indication that the minimum size required for wire-type EGCs could be larger than sizes included in the table.


Conductor-withstand capabilities


The I2T values (short-time rating or withstand rating) of the EGC sizes are between 13 and 28 times their nominal continuous rating based on a value of 1 ampere (A) for every 42.25 circular mils of conductor. The Insulated Cable Engineers Association (ICEA) established and used this value to develop the 5-second withstand rating for insulated conductors. Using this formula, a 5-second rating can be established for insulated conductors covered in the NEC. This is the proven conductor-withstand formula developed through extensive testing and data collection by the ICEA. The value of 42.25 circular mils is applicable to insulated conductors. The value is 29.1 circular mils for uninsulated conductors based on the ICEA findings.


For example, to determine the 5-second withstand rating of a 4 AWG insulated conductor, use the values in the third column of NEC Table 8, Chapter 9, to determine the circular mil area of a 4 AWG conductor. The value is 41,740 centimeters (cm). Take 41,740 cm and divide it by 42.25 cm, and the result is 987.9A, which is the current that can be carried safely for 5 seconds by a 4 AWG conductor. Five seconds is a long time for any overcurrent device to open. Overcurrent protection typically operates in cycles or fractions of a cycle. Using the actual time characteristics of the overcurrent device in the withstand formula can result in a value of amperes the wire-type EGC can safely carry without damage. The faster the overcurrent device operates, the more fault current the conductor can carry safely without conductor degradation or annealing. This point emphasizes that there are additional factors such as voltage drop and higher amounts of fault current in systems that could result in the EGC sizes in Table 250.122 being insufficient. Good engineering designs sometimes require a careful study and selection of overcurrent protection in conjunction with sufficiently sized EGCs that will ensure fast and effective operation of OCPDs in short-circuit or ground-fault conditions.


EGC sizing to meet Code


In addition to the engineering basics of providing an effective ground-fault current path, meeting the Table 250.122 minimums is essential. The driving text of Section 250.122 is that the minimum size required for wire-type EGCs is not to be less than the values in Table 250.122. In no case are the EGCs required to be larger than the circuit conductors supplying the equipment. 


It is important to mention parallel arrangements and their relationship to the circuit. The EGC is never required to be larger than the circuit conductors. Where circuit conductors are installed in parallel to create the equivalent of a larger circuit conductor, the size of the circuit conductor is the total area of all individual conductors in parallel that create the larger circuit conductor using multiple parallel paths. The NEC does not permit conductors to be installed in parallel to create an EGC. The EGC in each set of a parallel installation must be full size based on the rating of the OCPD protecting the feeder or branch circuit.


Using Table 250.122 requires knowing the rating of the overcurrent device protecting the branch circuit or feeder. The appropriate column for copper as compared with aluminum EGCs should be used. Always meet the minimum sizes required for wire-type EGCs, and when circumstances warrant it, it is a good practice and may be necessary to verify the EGC’s withstand capabilities. For additional information about short-circuit characteristics and capabilities of cables, refer to the ANSI/ICEA publication P-32-382-2007 in addition to the ICEA website at www.icea.net.