We’re building a house in rural Arizona and I’ve encountered a problem in installing electrical service. Our electic coop says we can run underground cable (3 conductor, 220v with neutral) from the power pole to a pedastal with the meter. Then run from the pedastal to the house. The house has a poured founddation and the contractor installed ground wire to the rebar to provide a ground to the house electircal panel. The local electrical material supplier sold me all the goods: pedastal, conduit, 40 3 conductor wire, a ground rod and a ground cable.
The supplier says I need to run a seperate ground cable from the pedastal to the house. He says the ground wire tied to the rebar in the foundation is not code. THe ground rod will be stuck in the ground at the pedastal and will tie to the neutral lead going to the house.
To my thinking, extending the ground from the pedastal to the house is a waste of time when I have a ground already to go in the foundation. i.e. a ground is a ground. I have checked with the county bldg inspectors and the electric coop and they both say I don’t need the seperate ground cable to the house.
Any thoughts or comments / insight appreciated.
Robert
Replies
The ground to the rebar is called a Ufer ground. In general it is one of the better grounding systems that can have.
And I don't believe that it has to have a 2nd grounding electrode like a ground rod has to have.
That said I can't answer your question directly as I don't have any experience with pedistal type of installations.
But a few questions.
Is there a main disconnect at the pedistal or is it at the house?
Installations that I am familar with, where the power comes straight to the house the main disconnect is where the neutral is bonnded to the ground electrode system. If this happens to be separate from the main panel then 4 wires are run to the panel and panel is treated as a sub-panel with the neutral bus being isolated from the ground.
I believe that if the main disconnect is away from the house then there might be a couple of options about how to handle this.
But, in general I would follow the BI and the PO and not the counter clerk.
The pedistal is a mobile home pedistal (even though it's not a mobile). It has the main breaker / disconnect as well as the meter. In the install diagram for the pedistal it shows the neutral bonded to the ground rod at that location. THe interesting thing is that the diagram also shows only 3 leads going to the house / subpanel.
I think I will go with just the 3 wires (2 hot, 1 neutral) and use the ground at the house, but have it tested once I get some wiring in. I suspect the test can't be all that complicated. After 30 years in switching systems in the phone company, I should be able to figure it out.
If there is a main breaker/disconnect at the pedestal, I'd go with the ground at the pedestal, and run a ground wire from the pedestal to the SUB-panel in the basement. To me, that limits the confusion. But I'm sure there are other ways to do it.
Just curious. Are you employing a professional electrical contractor?
Shout out to Andy C. Namaste my friend.
http://www.hay98.com/
Mostly DIY. I'll hire the pros when and where it's necessary.
Ohhh I see.
Shout out to Andy C. Namaste my friend.
http://www.hay98.com/
........and the list gets longer, eh Gunner?
What list?
Riverfest 2005. Be there, or be square.
http://www.hay98.com/
the one with the HOs doing their own elec work
The ones that will be back in six months wanting to know why they're going through light bulbs like crazy.
Riverfest 2005. Be there, or be square.
http://www.hay98.com/
I can appreciate your doubting the average HO doing their own electrical work. Most HOs should limit themselves to the plug outlet interface. With 30 years at the phone company working in switching systems, power rooms, cable TV and a myriad of other projects & systems, I don't think I fall into the average category.
Just curious...why is the meter being mounted on a pedestal, instead of right on the side of the house? There's nothing wrong with doing it that way, as long as the installation is done per Code and to the satisfaction of your local inspector and power company, but putting it on the house is more typical (at least in my neck of the woods) and probably would have simplified this discussion.
By the way, as an electrical engineer I've been specifying Ufer grounds on construction projects (commercial and municipal, but mainly water and wastewater treatment plants) for years now with good results.
Well it started with wanting to have power underground all the way from pole to house. Initially, the power co. said the run was too long for a secondary line. (about 350'). So we were going to have the power Co extend the aerial primary line to within 150' of house and take the secondary underground from there. That would have put the meter on the house and life would have been good. In order to extend the primary line we needed to get an easement from a neighbor which didn't happen. After that the power Co came back and offered the current configuration (buried secondary halfway to house into a mobil home pedestal and buried the rest of the way to house) as an acceptable installation.
I suspect this config will allow the meter reader to only walk half as far onto our prop. to read. We've been going around with the power Co. now for over 3 months to get to this point. All things considered, in hindsight I would have rather had the meter on the house.
Ah, I get it. The power companies aren't always easy to deal with.
What the supplier is saying is that the meter pedestal is the main service entrance for the house, which therefore makes the panel in the house the subpanel. That requires the grounding to be done at the pedestal and a grounding conductor (i.e. the "fourth wire") run from the pedestal to your breaker panel. That would also require that in your breaker panel the ground bus and the neutral bus be separated.
Grounding to rebar is a fairly hokey way of grounding. You would be depending on the concrete to be wet, and to be a good conductor. In a dry climate such as Arizona it can be difficult to get a good ground. You may have to go quite deep and have a bit of surface area to the conductor, or use a ground enhancement chemical. Testing to see if the ground is good is easy and can be done by a good electrician.
Having a good ground is critical for surge suppressors to work well. If the grounding of the building wiring is weak, more of the surge goes through your electronics.
Where we're building is a lot of rock. I won't even be able to drive the ground rod straight down at the pedistal as there's rock about 16" below the surface. I'll have to drive it in at an angle.
I presume the testing to see if the rebar groud is good can be done with a VOM to check for potential differences in voltage between hot to neutral and hot to ground. If the readings are the same or nearly so then it should be an acceptable instalation.
First of all, in this situation it does not matter what Master Electricain says, what the committe that wrote Article 250 of the NEC have to say, it does not matter what Mr Ufer has to say, nor UL, nor people that wrote the IEEE reference book on grounding, what the clerk at the supply house has to say.There are two "persons" and only two persons that have to be satified. The power company and the BI. No, more, no less.And you have heard from them.(That is assumign that you have a BI that knows what they are talking about. Unlike the one that Brownbag had that seemed to make up code as he went along).Now lets look at some details.If you did not have the main disconnect elsewhere having a ufer grounding electrode system, with the main disconnect at the house and the neutral bonded to the grounding electrode system at the main disconnect and no "ground rod" is a perfectly legal and acceptable system. Not only that is the now the prefered system.Also under the code there are applications that allow for a separat main disconnect to be used, but then to treat other locations to also have main disconnects where the neutral is bonded to a local ground electrode system. Those are in outbuildings, common distribution farm system, and mobile homes. But those are not blanket applications, in each case a number of conditions need to be met before you can do that.Now the second on service equipment and bonding is outside of my study and I am not going even try and figure out all of the options and permutation to see how it covers a situation like yours.HOWEVER, you have another, but related, problem.You don't have any ground electrodes at the pedistal. A ground rod can't be more than 45 degrees from vertical 8t long. With that rock you aren't going to make it.Ground rings and plate electodes need to be 2.5 ft or deeper into the ground.20 ft of concrete encased electrode is the only thing that I know that would make it.For the pedestail the main thing is that the pedestail be grounded so that it can'b become hot.Possible the BI might allow you to use the ground from the house.This is what I would ask them and the power company. What is acceptable way to ground the pedistal.
Bill Harmann,
Wow, I didn't think I would have opened a can of worms. Obviously this is another subject where people can have many opinions.
Regarding the ground rod at the pedestal, what I will attempt to do is have a deep hole dug with a backhoe so that I can drop the ground rod into it nearly vertical and adjacent to the pedestal. That should care for providing the ground at the pedestal.
It appears from all you've said that I'll be fine using the ufer ground at the house. I think I will still measure for a potential difference in the grounds.
Thanks for all your information and insight and links.
Actually you will need 2 ground rod at least 6 ft apart at the pedistal.Unless you can test it and show less than 25 ohms resistance.That takes a special tester.Now is might be that either the BI will give you an except if the ground is that hard or it might be std practice in your area to only use one.And unless you qualify for an excemption you will need to run the ground wire and have an isolated neutral bus at the house.But with or with that ground wire from the pedistal you need a local ground at the house and the ufer ground is fine."I think I will still measure for a potential difference in the grounds."If you measure anything between the ground at the pedistal and the ufer ground at the house (before connecting the two then it is caused by stray current, usually from the power company having a bad neutralm but it can also be from neighbors problems.
And unless you qualify for an excemption you will need to run the ground wire and have an isolated neutral bus at the house
No he doesn't.
First off, the inspector told him he doesn't have to.
Secondly, I think this would fall under 250-32(b)(2) (1999 code), where the pedestal and the house are considered separate "structures". In that base no grounding conductor needs to be run between the two, and the ground and neutral bars in the house panel are both connected to the grounding electrode conductor.
He does need a grounding electrode system at the pedestal, though.
ThanksI never though of the pedistal as a "separate structure".In my mind it was also the "big house" with the main disconnect and then outbuildings.But that does make sense.
The electrical coop and their linesman said I only need one ground rod at the pedestal.
I'm digging the trench from the power pole to the pedestal today and installing conduit. When the linesman comes out to do the inspection, I'm going to revisit the ground cable between pedestal and house issue. I will post back with his response.
"The electrical coop and their linesman said I only need one ground rod at the pedestal."That is good. Hope that you can get through the rock to get it in.Note Barry's comment on the exception to why a ground wire might not be needed. I knew that rule, but just did not think of the house being a separate structure.
> Where we're building is a lot of rock. I won't even be able to drive the ground rod straight down
How big are the rocks? If they're no more than about grapefruit size, you can sink a copper pipe ground rod hydraulically. A friend of mine used to do loads of them, 45 to 60 ft. deep, for TV transmitters. I've done one 30 ft. deep, and plan to do another.
-- J.S.
John,
Unfortunately, it's rock not rocks. We're in an area that was probably a nice beach x million years ago. We have vast strata of sandstone of varying hardness.
When digging the trench this weekend, I got into areas that were so hard I had to gnaw on them with a big Cat backhoe that had 9 rock teeth on the bucket. Of course, where the pedestal is going happens to be some of the hardest in the area. In order to get the conduit down low enough near the pedestal base, I had to dig a subtrench with a pick axe.
I dug a 2' deep trench with the backhoe adjacent to the pedestal spot and will bury the ground rod in it. 1/4 vertical and the rest horizontal.
"Grounding to rebar is a fairly hokey way of grounding.""http://www.ecmweb.com/mag/electric_code_changes_8/""#8. 250.50 GROUNDING ELECTRODE SYSTEMThe words “if available” have been replaced with “are present.” The effect is that a concrete-encased electrode (Ufer) is always required for new construction, because it is present. However, a new exception adds clarity that a concrete-encased electrode [250.52(A)(3)] isn't required for existing buildings or structures."http://www.eng-tips.com/viewthread.cfm?qid=123343&page=6"I also found out that the grounding electrode study is being done by the National Fire Protection Association research group, not by Underwriters Laboratories.I can assure you that the Ufer ground is an excellent ground in part because the concrete increases the contact area with the soil. If you have a 30 foot by 30 foot house with typical 16 inch wide footers, then bonding to rebars or a copper wire that goes all the way around the footer give you about 120 square feet of soil contact. Compare that to how back in the days of telegraphs that used the earth as the return wire the preferred electrode was a 3 foot by 4 foot metal plate giving 24 square feet of contact with the soil. If you drive a rod underneath the footer where the grounding electrode conductor hooks up then you will have a low inductance ground for any lightning current that is coming in over the service drop.A book on high performance grounding methods is Army Manual 5-690. I forget what the URL is but if you feed "army manual" and 5-690 into Google you should find it. This book cover things such as lightning rod systems, protection against heavy duty radio frequency interference, electromagnetic pulse protection, controlling signal leaks, and so forth. A rather heavy read but it has lots of drawings and diagrams to help you understand some concepts. It also has some tested methods of how to bond a waveguide or coaxial cable for which a listed grounding block is not available.""Talk to your local electrical inspector and bring some references with you such as the IEEE Green Book. There is a study that was done by Underwriters Laboratories and the Ufer ground was the 3rd best grounding electrode that can be used.The Ufer ground was first used in Arizona and was found in their soil conditions ( dry and sandy ) to be an excellent ground."
Thank you for the information and references.
First, I'm no where near an electrician, so someone correct me if I'm wrong...
That said, you said " i.e. a ground is a ground." - Not a good statement. An electrical system can have only a single ground. If you were grounded both at the pedestal and the house (if I understand you correctly), a difference in potential could develop. Somebody explained it to me something like this: If lightning were to strike the ground on a nearby hill (for example) electrical current would travel through the soil. If the house's electrical system had 2 grounds, at the moment that electrical current reached the first ground (but not the second) there could be a huge differences in potential between the 2 grounds and the ground system would be energized - not good!
Re the grounding method, here in NC we (obviously) have very different soil conditions, however we are required to have a metal ground rod (8' I think) driven into the ground and I'm fairly certain a ground to the reinforcing rod in a slab would never be accepted.. This ground rod must be driven within a specific minimum number of feet of the service entrance (meter base) (not sure of the number) and then in the main panel, which also has to be within a specific minimum number of feet of the service entrance the ground is bonded to the neutral. This is the only place where neutral/ground is bonded.
Sometimes 2 ground rods are driven right next to each other to get a better ground, but still only one ground conductor is connected to the 2 which are essentially connected in series.
Personally, if I were in your situation, I'd pay a licensed electrician to evaluate the situation and/or your installation unless you can get the elect co to give you a statement in writing. Although what the elect supplier says sounds right, I still wouldn't take his recommendations as gospel.
Actually you have several things backwards."An electrical system can have only a single ground."Actually it can and should have multipel "grounds". To start with the transformer secondary will get connected to the power companies grounding electrode which is commonly run down the side of the pole and connected to ground rod at the base or under the pole.Then the neutral is also connected to that centertap and connected to the grounding electrode system and run to the house where it is bondind to the house grounding electrode system which in most cases consists of more than one grounding electrode.And if there are any outbuildings there will be more grounding electrodes which are connected to the house grounding electrodes either by being bonded to the neutral at the outbuilding or to a separate equipment grounding conductor connected to the grounding electrode system at main panel."If lightning were to strike the ground on a nearby hill (for example) electrical current would travel through the soil. If the house's electrical system had 2 grounds, at the moment that electrical current reached the first ground (but not the second) there could be a huge differences in potential between the 2 grounds and the ground system would be energized - not good! "Actually the ground is a relatively poor conductor. But lightning strikes have huge currents and potentials. Junkhound can better tell you what happens in an actual strike. But the main concern in a home is that ANYTHING THAT IS METAL THAT CAN COULD BE ELECTRIFIED, EVEN ACCIDENTLY, HAS TO BE AT THE SAME GROUND REFERENCE. That is one of the reasons for multiple grounds with outbuildings."Re the grounding method, here in NC we (obviously) have very different soil conditions, however we are required to have a metal ground rod (8' I think) driven into the ground and I'm fairly certain a ground to the reinforcing rod in a slab would never be accepted.. This ground rod must be driven within a specific minimum number of feet of the service entrance (meter base) (not sure of the number) and then in the main panel, which also has to be within a specific minimum number of feet of the service entrance the ground is bonded to the neutral. This is the only place where neutral/ground is bonded.Sometimes 2 ground rods are driven right next to each other to get a better ground, but still only one ground conductor is connected to the 2 which are essentially connected in series. "The NEC reconnized a large number of different kinds of grounding electrodes, but in typical house construction the most common have been ground rods and metallic water pipes with at least 10 ft of ground contact. Others include concrete encased electrode (Ufer ground), ground ring, metal pipes, and plate electrodes.If a the water pipe is used as a ground electrode then a seconard electrode is required. That is often a ground rod.If a ground rod is used then either a second electrode is used or it has to be tested to show less than 25 ohms. That second electrode can metallic water pipe or it can be a second ground rod, but it can't be next to the first one. It has to be a MINIMUM of 6 ft away.Now the 2005 NEC has made the Ufer (concrete enclosed electrode) the primary electrode for new construction (I don't have a copy, but this is what I understand from comments on the 2005 code) and that it can be used alone. However, even if it used if you have a metallic water line it still has to be bonded to the ground electrode system.Now this is all based on the NEC and I have no idea which version of the NEC that NC has adopted, nor what admendments that they might have.But don't be surprised if they start requiring Ufer grounds in the near future.
I beleive that NC uses NEC 2005. Next time I have the electrician on site I'm gonna ask him about the grounding stuff. Should be in about a week.Matt
> If you were grounded both at the pedestal and the house (if I understand you correctly), a difference in potential could develop.
Not a problem at all. In fact, most residential systems are grounded at several points. The serving utility has a ground rod next to the pole where the transformer is, and the center tap of the transformer secondary, the neutral, is grounded there. You have a ground rod or two, and so do all your neighbors on that same transformer. All their ground rods and yours are bonded to the center tap of the transformer. There can be ground rods on the same transformer a couple hundred feet apart. Electrons take no notice of deeds and property lines. Any voltage difference that might exist is small, and the impedance to the ground rods is fairly high, so very little current flows.
-- J.S.
When I made the statement about a single earth ground I was speaking of the residential system as the customer owned portion; from the meter base to the main panel, sub panels, etc, etc. I was not talking about the utilities's equipment - although obviously I didn't state that very well. Like I said though, I'll be meeting with my electrician later this week and I'll pose the question. Also, later this week, I'm having another house "connected to the grid" and if I see the guys who bury the cable I'll ask them about grounding, although then again, what the utility does on their side of the meter is not my concern. Matt
> I was speaking of the residential system as the customer owned portion; ....
OK, but for the problem of lightning causing voltage gradients in the soil, electrons don't care who owns what.
If you plug in a long extension cord at a neighbor's house served by the same transformer, and run it over to your service entrance, you can check with a volt meter and find 120 volts between their neutral and each of your hots, and between their hot and your neutral. Your electrical system is wired in parallel with everybody else on that same transformer. That's why their old sewing machine makes snow on your TV set.
There is a continuous metal path between your ground rods and everybody else's on that transformer.
So, if lightning hits and causes a voltage gradient in the soil, the various ground rods and neutral/ground wires will short part of it out and conduct a bunch of current. If that turns out to be enough amps for enough time to cause some damage, several homes will likely be affected. But that seems to be rare enough that it's not an issue.
-- J.S.
He says the ground wire tied to the rebar in the foundation is not code. ..
If not to code then it is NOT good.... Go and ask the town/village electrical inspector!
Which begs the question what code the supplier might be thinking of. 250-50(c) of the 1999 NEC allows an Ufer ground to be the only ground electrode needed; no supplementary ground rod required.
Do you know how long the NEC as accepted Ufer grounds?BTW, I don't remember where I ran across this, but some city was trying to require people to install ufer grounds for service upgrades.
No I don't.
I have an old copy of the NEC Handbook around someplace; I'll see if i can find it, and see if it specifies an Ufer as one of the alternatives.
I haven't heard about the ufer requirement, but I have heard some noise about not counting the ground to the water pipe anymore, and requiring two grounding rods a certain distance apart. Me, I like that idea. I prefer that nothing that should not be energized to be fixed so that it can't be energized. Like plastic boxes over metal ones.I realize that properly done, it shouldn't happen. But if one option prevents it from being a possibility, I'm all for it.
But even at that the if you have metallic water pipe inside it still has to be bonded to the grounding electrode system.Too many places where water pipes come intact with electrical equipment. Thus the requirement.