Article 250 is arguably the most important article in NEC 70, and much of its content is devoted to the proper sizing and installation of equipment grounding conductors. First, let us consider the purpose of an equipment grounding conductor (EGC). We know that EGC’s provide a low impedance path from the equipment to the overcurrent protective device (or ground detector for high-impedance grounded systems) so that a fault condition can be quickly cleared. Hence, EGC’s are a critically important safeguard to persons and equipment.
If you turn to Section 250.122 in your NEC, you find information that has assisted electricians sizing equipment grounding conductors (EGC) for longer than most electricians have even been twisting wire.If you were on the job in 1968, you would have turned to Table 250.95 in your NEC for this information instead of Table 250.122, but the information was essentially the same.In 1968 you also would have found that an ungrounded (hot) branch circuit conductor protected by a 6000-amp overcurrent device required no less than a 1200 kcmil aluminum equipment grounding conductor… assuming you were using an aluminum equipment grounding conductor. That information remained unchanged for 50 years. But as the saying goes, “All good things must come to an end.”
The Isolated Ground (IG) is a type of equipment ground that, in theory, reduces interference experienced by electronics and instrumentation from radio frequency (RF) noise, by connecting that equipment directly to the grounding terminal of the service equipment, without ever making contact with another metal component or grounded surface, that could potentially be serving as an antenna for airborne RF noise.
For our typical AC electrical system, the purpose of the “Main Bonding Jumper” is to connect the equipment grounding conductors (E.G.C.) contained in each branch-circuit and the metal (conductive) service-disconnect enclosure to the Neutral Conductor of the electrical service. The reason for this is: a driven or buried earth ground […]
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