Please tell me if I understand this right. if I do a service change and can't get to a ufer then i use 2 ground rods for a typical 200amp service. 250-66 says the gec can be a #6 cu if it is the sole connection. maybe i dont get the meaning of the sole connection bit, but was told that it might have to be a #4 cu. if thats true, could someone give me a case where i would have to use the #4. here's 250-66 thanks, C
250-66.(a) Connections to Made Electrodes. Where the grounding electrode conductor is connected to made electrodes.. that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than No. 6 copper..
Cindy, No one can reference the code for more than a #6 to a ground rod. It is all the earth can possibly take for fault (Circular Mil Area/42.25 = 5 second fault rating for a conductor) and is sufficient for equalizing potential which is its intended purpose in our service side system anyway. Unless you are creating a system for the supply side, you never need larger than a #6 to a ground rod and your code quotation is correct.
Sorry guys, sometimes I assume everyone has a Soares to read. The fault level (ampacity)of a grounding conductor is calculated for a 5 second bolted fault. That is what 250-122 (99 code) is showing. You can do this calc yourself fairly easy by going to Chap 9, Table 8, look up the circular mil area, and divide that by 42.25. That means a #6 with a cma of 26240 divided by 42.25 has a 5 second fault rating of 621 amps. Any fault is CODE designed to be cleared within 5 seconds, after that, either your ground has gone away (burned through) or your Overcurrent Protective Device has opened. That is assuming you've installed your grounds properly, of course. Do you think the earth can 'dissipate' or return to XO a fault level greater than that ? A 120 volt circuit fault, feeding into a ground rod, let's say is 20 ohms would deliver a fault level of 120/20=6 amps. It doesn't get much higher than that EVER for a ground rod. I also realize I used another term up there that someone may want 'splained. Most electricians have had a short circuit happen fairly close to them. You know that an arcing wire tends to 'bounce and jump' therefore not staying in one place too long. The real fault bounces up and down so much as to be incalculable. The code, and most electrical calculations, are geared to the fault as if it were 'bolted' to the ground. It just makes it easier and yields the highest figure, giving you your margine of safety. Too much, not enough? sorry for the length.
One tiny more thing, the 5 second calc is because it is the maximum current a conductor can withstand for 5 seconds without permanent damage.
[This message has been edited by George Corron (edited 01-09-2002).]
Re: #6 G-rod, #4 Ufer#6619 01/09/0210:00 AM01/09/0210:00 AM
Sparky66wv: #4 solid is for unprotected grounding conductors. If protected, then #6 is ok. George, what happens during a lightning surge? Your explanation works for a ground fault, tho the ground electrode and conductor has just a small parallel current compared to the equipment ground current during a ground fault. The real issue of grounding electrode sizing comes in when dealing with lightning, doesn't it? In the IEEE green book it calls for all sorts of multiple ground rods in order to keep the earthing resistance low enough to keep people from getting killed. The more ground rods you install, the higher the current you can dissipate from a lightning surge, so the grounding electrode conductor rightfully should be increased in size to keep from limiting that dissipation capacity.
YO, The 'lightning' dissipation capability of the low voltage electrical system has been greatly exaggerated.
Why is a transformer grounded, to limit voltage to ground of course. Where is a transformer grounded ? At the transformer.
We have no equipment to capture or dissipate, that is the function of the arrestors at the transformer on the pole. While it is true that some houses have arrestors, most do not, nor is it required. The rare overvoltage that comes from the neutral into the house will dissipate into the ground rod (water/sprinkler pipe, ufer..etc.)and a #6 is way more than sufficient.
The grounding electrode at the structure does not come into play at all for an overvoltage that would come across the phase conductors, that's how people get computers burned up that don't have their surge protectors on them. In other words, by that time the utility equipment has failed. If it did it very often, you would not protect it with the $20.00 protectors picked up at Wal-mart.
The real issue of grounding may be lightning, but not at the structure. As for calculation, impossible, since it isn't even designed for it ie: no required surge arresting equipment, no 25 ohm requirement only that if it doesn't meet 25 ohms you drive another rod, not make it 25 ohms or less, the utility side is a different story. So calculation based on the hypothetical strike that is not supposed to get through anyway would be a 'crap' shoot at best.
Ok Jorje, consider this. Why do we have Table 250.66? Is the grounding electrode conductor size increase, per phase conductor size, only relative to ground fault calculations? If so, why do we have minimum requirements for #2 to ground rings, and #4 to Ufer grounds? If there is no concern about the electrodes' having varying capacities for dissipating surges, then there should just be one size for all grounding electrode conductors to whatever type of electrode we choose (not taking into consideration corrosion resistance or need for physical protection). Another way to look at this issue: 250.122 deals with minimum EGC's per O.C. protection rating. Why don't we just use THAT Table to figure our grounding electrode conductor size? Why do we go to larger sizes when dealing with GEC's? For example, 250.66 at 2/0-3/0 calls for #4GEC. Looking in 310.16, 3/0 equates to 200Amps. Jumping to 250.122, 200A O.C. requires a #6 EGC. Why the difference? Why are the GEC's consistently larger than the EGC's, if all that we are really concerned about is the amount of ground fault before clearing? Perhaps it is a complex mix of both concerns, ground faults & surges, I don't know. Mind you, I'm not trying to one-up you, I'm just out to learn..like everyone else here, I suppose. Maybe the GEC's are sized larger because we don't have a definite size for our O.C. protection device at the pole or vault, but we DO at the service (as well as the downstream O.C. devices). But that still doesn't explain away the code minimums which change relative to what type of grounding electrode we use (250.52).