ok here id the situatuation a home owner poured a pad for a house with a wrap around porch poured as well. he put a 2 1/2" pvc into the foundation and stubbed up into a wall along with a 1" stub for communication. he has a 320AMP service at his other building one 200 amp panel and a 200 amp disconnect which will feed new house(2 1/2" pipe runs to here. he didnt allow for the ground rods (i.e. a pipe for the bare #4 outside of poured area)so can i run the ground rod wire with the 4 wires from the disconnect and clamp onto the original service ground rods or is there a code that says it must be its own ground rods at the location of service(plus that dreaded 25 ohms could come into play on a 140' run back to the meter and would have to oversize wire perhaps) i think i might have to tell him that we have to bust up some of his cement to satisfy the grounding requirements for my two supplimentry ground rods. anybody ever have similar case? any thoughts?
I agree in principle with that gfretwell, but I don't recall ever seeing a requirement about distance for electrodes in the NEC (Often its hundreds of feet to the cold water from the switch gear on projects I work on).
Other than the 25 ohm requirement for ground rods, which will not ever be obtainable if you have too far a run with your GEC. But as we all know, you can bypass that requirement by driving a 2nd ground rod.
Maybe I'm missing something here, but what does a concrete slab have to do with ground rods? Why can't you just come out the back of the disconnect with an LB (assuming its inside the structure) go down with a PVC pipe on the exterior of the structure into the dirt and connect to one or two rods?
You could try to run a GEC in the feeder conduit all the way back to the sourcing structure, but seems kind of expensive to run that long of a GEC and it won't be very effective. You might even be able to use the feeder EGC or neutral as the GEC as long as its #4 copper or larger. The inspectors don't seem to like a ground electrode system to be too far from the structure its supposed to protect, but I see no GEC length limitations in the code.
Easy solution: drill 2 holes in the porch slab, 3+ feet either side of the service meter. Drive one 10 foot ground rod through each hole, until at least 8 feet is in contact with the earth (top 8-12 inches are exposed, so inspector can read the length of the rods) Run PVC sleeves from service panel in house to each ground rod, and connect the GECs via acorn clamps. After inspection, a "housekeeping" slab, 7-8 feet wide by 2 feet deep and 8-10 inches thick, may be placed atop and surrounding the ground rods to neaten up the entire service area. Have the telephone on one side of the meter and the cable TV on the other.
it was the aesthetics trying to conceal the #4bare from sub-pnl(no meter fed from 200 A disconect other half of 320 service from existing garage)to ground rods drilled through cement. which would be at the front door of the house so i was wondering if using the conduits back, would be legal and meet the ground rod requirement for seperate building. since i already have the bond from 200 A disconnect (4-wire) anyway a second would be redundit and close impedance truly would not serve its purpose, because of vault in house(20') i drilled around the corner(side) and hit first ground rod and drilled thugh studs to get to second ground rod minimizing bare copper showing. mission accomplished.