Thanks for your responses. The NEC provide 3 requirements for effective grounding:
1. provide a permanent path.
2. Conductor must have the capacity to withstand the fault current and
3. must have a sufficiently low impedance to limit the voltage to ground and to facilitate the operation of the OC device.
Numbers 1 and 2 are straight forward. However #3 leaves some questions to be answered. What does "limit the voltage to ground" mean and what does " facilitate the operation of the OC device" mean.
At the time this requirement was entered into the code I suspect there was not a great deal of information regarding the effects of electrical shock on the human body. We now have information that has provided the use of GFI devices to limit the effects on the human body. In the IAEI Soares Book on Grounding there is a chart that list voltage level and and the human body response.
For body resistance of 500, 1500 and 3000 ohms, the let go voltage is 4.5, 13.5 and 27 volts. For fibrillation the voltages were 11.5, 34.5 and 69 volts. Soares suggests using 40 volts to ground as the limit. This voltage is the result of the fault current x the impedance of the EGC.
Facilitating the operation of the OC device means opening the OC as fast a possible. Therefore we must have a fault current that will open the device instantly or in 1 cycle. Soares shows 5 times the OC rating as the fault current required to open the OC instantly however tests have shown that 4 times is adequate.
First the NEC is not an engineering manual. It provides safety guidelines to be used for the installation of safe electrical systems. Table 250.122 provides the MINIMUM SIZE EGC. I think everyone realizes that distance plays a part in selecting the EGC. A general rule is that over 100 ft the EGC selected from table 122 is increased one size for each 100 ft. The given EGC size for a 300 amp breaker is #4 copper which is good for a circuit of 100 ft. We need to increase the EGC 4 times to meet the general rule. That gives us 3/0 copper. This has nothing to do with the increasing of the feeder for voltage drop. We will need 4 time the OD device rating to insure an instant opening. Given a 300 amp breaker we need a 1200 amp fault. In order to check the voltage to ground we simply multiply the 3/0 impedance by the 1200 amps. 3/0 at 25C x 1200 = Voltage to ground = 1200 amps x .037 = 45 volts. That's very close to the suggested value. If you are interesed in more information visit this site http://www.steelconduit.org/gemi.htm.
[This message has been edited by Bob (edited 12-05-2005).]