The National Electrical Code (NEC) contains the minimum requirements for a safe installation. One must do at least that much when installing electrical equipment and systems. This means understanding how to size electrical conductors of circuits, including the equipment grounding conductors (EGCs). For circuits to operate or perform effectively, conductor sizing must by addressed in longer runs of feeders and branch circuits. Increasing conductor sizes from the minimum required sizes of current-carrying conductors reduces circuit impedance, because there is more wire to do the necessary work. EGCs of the wire type must also be increased to effectively perform during a ground-fault or short-circuit event.
Section 250.4(A)(5) indicates that electrical equipment, wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source.
This performance requirement addresses installing electrical materials that form part or all of an effective ground-fault current path. An important aspect of this requirement is that it is related to the manner in which the materials are installed. Because the effective ground-fault current path is intentionally installed, its effectiveness is measurable based on NEC meeting minimum requirements and being installed in workmanlike fashion. Wire-type EGCs must meet specific criteria directly related to performance and be of the lowest possible impedance. This will vary because of different lengths, sizes and other installation characteristics of branch circuits and feeders.
Higher current levels can cause significant damage to conductive paths or current-sensitive circuit components. Impedance is in opposition to current in an alternating current (AC) circuit, so keeping the impedance as low as possible means that sufficient current will be present under ground-fault conditions, allowing fast operation of fuses or circuit breakers.
Section 250.122(B) includes requirements to adjust the size of wire-type EGCs for conditions such as voltage drop. EGCs are required to be increased in size to address voltage drop. In addition, a proportionate increase of the associated ungrounded circuit conductors is required.
If a circuit’s ungrounded conductors are increased from the minimum size that has sufficient ampacity for the intended installation, the wire-type EGCs must also be increased proportionately according to the circular mil area of the ungrounded conductors. This can be verified by following the steps below, using a 400-ampere (A) feeder as an example.
The 400A feeder is installed using 600 kcmil copper circuit conductors and a minimum size 3 AWG copper EGC. For voltage-drop reasons, or due to circuit capacity needs, the 600 kcmil conductor is increased in size from 600 kcmil to two paralleled 400 kcmil copper conductors for each ungrounded phase conductor and the neutral conductor. The circular mil values are added together (400 plus 400) to result in 800 kcmil copper now required for the circuit conductors in this installation. The adjusted size (800 kcmil) is then divided by the originally required size (600 kcmil) to determine the proportionate value of circular mil area adjustment:
800 ÷ 600 = 1.3 (multiplier)
The minimum size wire-type EGC for a 400A feeder is normally a 3 AWG copper, according to Table 250.122. The circular mil value of a 3 AWG conductor is 52,620 as provided in NEC Table 8, Chapter 9. Multiply the value 52,620 centimeters by 1.3 to come up with 68,406 circular mils:
52,620 x 1.3 = 68,406 circular mils
Take this value back to NEC Table 8, Chapter 9 and round up to the next higher value, in the third column, to determine the minimum size EGC as adjusted proportionately to the increase in size for the ungrounded phase conductors of the circuit. The next higher circular mil value in NEC Table 8 is 83,690. The proportionately adjusted minimum size required for this EGC is a 1 AWG copper based on the adjustment. It is always best to perform this simple calculation to verify that the EGC’s adjusted size meets or exceeds the minimum requirements. Simply increasing the EGC’s size to the next higher size is not adequate in all cases.
As a reminder, Sections 250.4(A)(5) and (B)(4) are performance requirements and indicate that the earth is not permitted as an effective ground-fault current path. The restriction of using the earth for this purpose is necessary because, although the earth is conductive, it offers variable levels of opposition to current. The earth should not be depended on for any steady-state current, so obviously it can never be depended on for fault current levels or to serve as an effective ground-fault current path.