When adding a subpanel in a detached building, I know we have to run 4 wires. Do we have to drive 2 grd. rods at the separate building like we do at the main service? And when we do drive our ground rods, and run our ground wire to the grounding bar,it will be tying the two ground systems togethers. Is there something about the grounding impedance difference here? I probably didn't word that right, but I'm sure someone will pick it up:) Steve...
A separate building is a separate building; build it the same way you built the fisrt one. The only difference is that you now have a service-sized ground wire connecting the two panels (and, of course, kept the grounds apart from the neutrals).
Remember the purposes of the different types of 'grounding'
The ground rod is there for lightning. A separate building is a separate target, so it needs a ground rod (or Ufer, or other electrode).
The big wire connecting the two panels is there to clear faults, to allow the breakers to trip. Think of it as a 'bonding wire' rather than a 'ground wire.'
The local ground also provides a local ground reference so the case of your tool will be at the same potential as the concrete floor you are sitting on. It is the best reason you can think of to use Ufer grounds in unfinished buildings, making sure to pick up the steel in the slab along with the footer. At that point the footer is the ground electrode and you are just bonding the floor because the floor is usually on visqueen
According to 250.56, a single Electrode consisting of a rod, Pipe or Plate which has a Resistance of 25 Ohms or greater, shall be augmented by an additional Electrode... However, this does not directly address an Electrode at separate structures, which will be connected (bonded in some fashion) to another GES.
This could be tossed back & forth ad nausea, especially with the wording of 250.58
Might be best to drive two rods, minimum of 6'-0" apart, and call it a day!
Looks like a great discussion topic! What do the Inspectors & Plan Checkers in the group, have to say about this scenario?
As an EE, I MIGHT require two driven rods, if a separate structure was really far from the structure with the Service + GES. If the structure is new, the CEE UFER would be good enough for me!
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
I have had an inspector make me remove connection to ground rods at a seperate building. This argument/discussion will be beat to deatth forever. IMO ground rod is not for lightning. Lightning rod is for lightning. Ground rod is for "fault"
I don't see an NEC requirement for two rods anywhere; IMO, one would have to be able to demonstrate that one was NOT enough before there would be a requirement to add another.
I have been told of municipalities that do assume that you need two; if so, that's a local requirement. As such, the AHJ is the one you need to ask.
Using the 'ground rods are for lightning' model, I would say that separate structures need to be treated as, well, separate structures. That is, they would require two rods just like the 'main' building. Or, in NEC terms, that one rod is supposed to have less than 25 ohms of resistance.
It is true that there are other justifications used for ground rods, "reference plane" being one. There's a fair amount of discussion on the topic, and a case can even be made for not having them at all (Just ask Norway)- but that's not the point here.
What is important is to realize that, whatever ground rods do, they are no help at all in clearing faults. I say this for two reasons. First, because we are forbidden by code from considering the dirt under our feet as a conductor. The second is a bit more subtle. Let's assume that you have a very code-compliant ground rod with only 10 ohms resistance to ground. Using Ohm's law, that means that a 120v fault will draw only 12 amps - nowhere near enough to trip even the smallest breaker.
I find it a better model to think of it this way: electricity wants to 'go home' to where it is made. For our homes, this 'home' is the secondary winding of the transformer. If we want the breaker to trip, we want to provide any 'lost' electricity to be able to find a good, solid, low resistance back to that transformer. Hence the desire for having separated grounds and neutrals at that detached building.
I guess we will agree to disagree on the ground rod Reno.
I have seen cllateral damage on the grounding electrode where it is connected to the ground rod. Large fault in a building which eventually tripped the main, before it tripped it cooked the ground wire. Not completely, but there was obviously large amount of current traveling to earth. Where did the current go??? Through the grounding electrode an assumeably back to the "source" through earth.
Oh, I agree with you there ... electricity will take all paths, and I have also seen some really nice arc paths. Not that those scorch marks mean much; a visit to a weld shop will show you just how much energy can be found in a 20-amp circuit.
What I want to stress in my over-simplified model is the critical need for the ground wires to ultimately be connected - at the service only - to the utility neutral.
Otherwise, you'll get all manner of DIY types thinking they can 'make' 120v at their 240v only well house by simply banging in a ground rod. Visit the DIY forums, and you'll see that this brilliant idea comes up on a regular basis.
Other folks will bond the neutral to the pipe, when there's no neutral wire convenient. A very dangerous practice - even if the two are connected at the main service!
What if there is no connection back to the utility neutral? Just a ground rod? Well, I would never actually test this; I can't think of a safe way to do it. I'll leave that for others to research. Terms that come to mind include 'floating neutral' and 'stray voltage.' If nothing else, we want to make sure that fault current has a much, much better path back 'home' than Mother Earth.
That comes from people who grew up around copper plumbing and nothing breaking that solid copper path. The day they sold the first water filter or water softener that became obsolete thinking. When utilities went to non-metallic pipe the code had to change.
The earth is still a significant return path to the main distribution centers though, anywhere that they use wye distribution. Even if your local MV > 120/240 transformer uses a delta primary, you still have a percentage of your neutral current returning via the ground electrode. Dr Ohm says he has to be so.