Grounding
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The NEC Article 100 definition of Grounded is: “Connected to earth or to some conducting body that serves in place of the earth.”
Earth grounding is required in order to minimize the difference of potential between non-current carrying metal parts of equipment and the earth. When a current carrying conductor is intentionally grounded (to earth), it becomes "the grounded conductor" of the system. This accomplishes two main tasks. First, grounding a system limits the voltage to ground during the normal operation of the system. Second, grounding a system prevents excessive voltages due to lightning, line surges or unintentional contact with higher voltage lines and stabilizes the system voltage to ground during normal operation. ( 2005 NEC 250.4(A)(1))
[edit] Earth grounding itself does not facilitate the operation of overcurrent devices!
Equipment grounding or bonding is intended to connect all non-current carrying conductive materials enclosing electrical conductors or equipment, to the earth, so as to facilitate the operation of the overcurrent device. This is accomplished with equipment grounding conductors or bonding conductors. These conductors are sized based on Table 250.122 for load side bonding conductors or Table 250.66 for line side bonding conductors. Equipment grounding conductors may also be raceways approved for use as equipment grounding conductors.
Equipment grounding or bonding conductors are intended to carry fault current in the event of a ground fault. The equipment grounding conductor or bonding conductor must provide a low impedance path from the point of the fault to the grounded conductor of the system. (NEC 250.4(A)(5))
The ratio inferred in NEC 250.122(B) by the words "increased in size proportionately according to the circular mil area", is simply the circular mil area of the increased ungrounded conductors divided by the circular mil area of the ungrounded conductors normally used for a given ampacity. In other words:
Where:
<amsmath>C_l</amsmath> = The increased or larger phase (ungrounded) conductor, in circular mils, for a given overcurrent device.
<amsmath>C_n</amsmath> = The normal phase (ungrounded) conductor, in circular mils, for a given overcurrent device.
<amsmath>EGC_n</amsmath> = The normal equipment grounding conductor size in circular mils, for a given overcurrent device.
<amsmath>\frac{C_l (circular mils)}{C_n (circular mils)}= ratio</amsmath>
This ratio is then the multipier used on the circular mil area of the normal equipment grounding conductor for the given overcurrent device.
<amsmath>ratio \times EGC_n</amsmath> (in circular mils) = Minimum circular mil area of required equipment grounding conductor. See NEC Chapter 9, Table 8
