Installing an AC Grounding System, Need Advice

You say a 7.5kva xfrmr. 230 volt primary 120 volt secondary? If it is indeed 120v out your electrician will bond one leg of the secondary to ground. This will become the grounded conductor and will terminate on the neutral bar of your new electrical panel. The electrician will bond the neutral bar to the panel enclosure with the supplied bonding screw. For the grounding, do you have a metallic water service, copper? This will be your primary ground electrode. Run a #6 awg min cu wire from the new elect panel neutral bar to a new water pipe clamp installed on the water pipe ahead of the water meter. Clean the water pipe with sand paper to a brite copper like new look. This #6 awg wire is called the grounding electrode conductor. While your electrician is at the meter location have him clean up your main service ground clamps and water meter jumper wire. Next ground to run is the secondary grounding electrode conductor. Not knowing where you live and the type of soil and moisture you have your electrician should know best. Drive two 10ft ground rods out side your house, at least 6ft apart at least 10ft 4" deep. Run a #6 awg min cu wire from the neutral bar of your new elect panel to these new ground rods.For these rods try to pick an area closest to your new panel. Your electrician will know the proper procedure for bonding the wire to the rods this can vary in areas due to local codes. Note, If the electrician feeds the 7.5kva xfrmr with romex run #10-2 w/grd awg cu min. Ground the case of the xfrm from your main house service panel. If the xfmr should have a short the fault current will travel back on this equipment grounding conductor and cause the breaker feeding the xfmr to trip open. With this xfmr you are creating a new seperately derived ac system. In the new panel board install a single pole 60 amp breaker for the main breaker. I recommend Square D QO panelboard and breakers.

Hope this will help.

Cincy bob, In your original post you said your were going to feed all your loads from one line. If you configure the secondary of the xfmr, and wire the new panel for 120/230 volt, then only use one line to neutral and not the other line to neutral for loads you will only be using half of the 7.5kva rating of the xfmr 3.75kva. Not knowing what your xfmr looks like, Most of the single phase xfmrs I work with have dual primary and secondary windings. From the description you gave in your last thread it sounds like you have at least dual secondary windings. To get 120/230v the two secondary windings are seriesed together. The center point being the common of the two windings, leads L1 and L2. L1 to L2 230v L1 to com 120v L2 to com 120v. If you want to get the full 7.5kva of the transformer you want to parallel the two secondary windings. Follow the data plate on the xfmr. My best recollection is tie [X1 to X3] line 1, [X2 to X4] line 2. Like I say check the data plate on the xfmr. With the two windings paralleled L1 to L2 will be 120Volt. Line 2 will tie to the neutral bar in the new panel. L1 will terminate on the single pole 60 amp breaker I discribed in my previous post. The panel you buy will be a main lug only type 120/240 single phase 3wire. install a jumper wire across L1 and L2 of the main lugs of the panel. Now you have a single phase 120V panel. Install breakers in any space you want. I dont care what you do for the secondary grounding electrode. But the primary grounding electrode shall be your copper incoming water line. Your existing electrical sevice uses this grounding electrode for it`s primary ground and the new 120V derived system shall bond to this grounding electrode via the grounding electrode conductor.

Cincy bob, I just read your second post again, about the transformer. The word "taps." Make sure you are getting a true isolation xfmr. Not a autotransformer.Autoxfmrs are not isolation xfmrs. Single phase autotransformers have taps. Isolation transformers have totally separate insolated primary and secondary windings. They can have AL or CU windings. For even better isolation you can buy a "Electrostatic Shield" type xfmr. The electrostatic shield is installed between the primary and secondary to attenuate some of the source line interference and common-mode noise. I guess what I am trying to say make sure you know exactly what you are getting...
Just curious What part of the country do you live in? North, Northeast, south, southeast, southwest? Curious about the soil and moisture in your area.
In my previous post I used the word "shall" in refering to the primary grounding electrode. I forgot to include NEC requires, at least my understanding of The National Electrical Code, and the electrical inspectors in my city.
Cincy bob, ask lots of questions of your electrician. Make sure exactly what you are getting and paying for.

Best of luck

The feed from your existing service that will feed the 7.5 kva xfmr will be 2hots, 230v,+ 1 equipment grounding conductor. The equipment grounding conductor will bond, connect, to the case of the xfmr. This ground is for the xfmr only.
Think of this ground as a safety ground.

When you install a transformer of the size you are using you are creating basically a new service. That means the 120 volt secondary side of the xfmr must follow certain safety guidelines to meet the NEC and your local city codes. That is why you need to connect one of the 120 volt leads of the secondary of the xfmr to the neutral bar in the new electrical panel that will be installed. This wire will become the "grounded conductor", the neutral. From this neutral bar a #6awg cu ground wire will run to the main incoming water line ahead of the water meter. This wire is called the "grounding electrode conductor". The wire attaches to the water pipe with an approved water pipe ground clamp. The water pipe is now called the "grounding electrode". This is the primary "grounding electrode" and "grounding electrode conductor" I spoke of in my earlier post. The connection at the main water line is the common ground point that ties your new 120v service to your existing house service.They will both be on the same ground plane. Now back to the neutral bar in the new panel. The electrician will install a supplied bonding screw through the neutral bar into the panel enclosure. Now for the secondary grounding electrode conductor that will go outside to the second "ground electrode" you choose to use. This second ground also connects to the neutral bar in your new panel.The new panel will have two earth ground system connections. Primary,water line ground electrode, and secondary, outside new earth ground electrode. The neutral bar is a Star grounding point now. By the way in your area your water line is probably down 4 to 5 feet deep because of frost. Buried in moist clay the distance, length, from the house to the main trunk connection and beyond.
Back to that neutral bar in the new panel. This bar is where all the new branch circuit neutral wires and equipment grounding wires will terminate...Nec says the neutral shall be bonded to ground at the first point of attachment or disconnecting means and at no point there after. That is why sub panels have seperate equipment ground bars. This is not a sub panel. In this new panel the neutral wires and ground wires share the same connection bar, called the neutral bar. NEC also says the max resistance between the neutral, grouned conductor, and the equipment grounding conductor measured at the receptacle shall not exceed 3 ohms. Also If you can keep the new panel fairly close to the new branch circuit receptacles thats also agood thing.

I have tried to do a better job of explaining things. This is the simple part. Putting it all together in a neat and workman like manner is the tough part. Local codes can vary. Your electrician should know the code of your area.

Water method, better not let an electrical inspector catch you do it. The correct way is to drive the rod into the earth. Undisturbed earth. When using multiple rods they must be connected all together with an aproved wire to rod connecting means. Local codes very all over the place on the connecting means to be used.

Slv, thats the best way to do it. For those of you who want to do it yourself and do not have access to an electric hammer, you can use a steel fence post pole driver. It will get you within about 30" from the ground. dig around the rod about 6" deep, and you can drive another 6". That last 2' you will need to use a sledge hammer. Aim carefully.

When the rod is driven into the earth it gives the tightest friction fit possible. Using water as a lubricate you are boring a hole for the rod to travel through. As the water migrates into the soil and through evaporation,as time passes, the soil around the rod tends to shrink away from the rod. Does it make that big of a difference? ? It is a lot easier to install with water. And when you drive a rod dry you won`t know if you hit something important, like a Phone line, Gas line, Power line, Water line, Sewer line, you get the idea. The water method is safer. Course you could call for locations. But that won`t cover every thing either. If you do it your self call for locations.
8' distance between rods is better than 6'. NEC just says that is the min. The power company in my area requires 2-5/8 X 8' ground rods driven 8'-4" deep 8' apart 3' out from the foundation of the house for the electrical service grounding electrode. The general rule of thumb is the min distance apart should not be less than the length or the rod used.
How deep can a ground rod be driven? If you go to an Electrical Wholesale House you can buy ground rods with threaded ends. As you drive the rod you can use a couping and connect another and so on. If you live in an area where soil moisture is low this works well generally.