Installing an AC Grounding System, Need Advice

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

Jea48, thanks for all your input. It is really valuable to me as I lack personal expertise in this area. Here are some answers to your questions:

1. Soil and Moisture - As my user name implies, I live in the Cincinnati, Ohio area. Our soil in this area involves a lot of clay. We tend to have normal midwestern weather with a fair amount of precipitation yearound but occasional droughts in the summer months. Let me know if additional information would be helpful in this area.

2. Isolation Transformer - My isolation transfomer is an MGE Topaz isolation transformer that is described in the following web link:

MGE Topaz 100 Installation and User Manual

This web link will lauch a pdf file that contains the full user manual for my isolation transformer. The specifications for the unit are contained on page 11 of the document. My unit is model no. T-100H-7500. It is a hardwired 7.5kVA transformer (see pg. 12 for the specifications of the various model numbers).

The unit provides 140dB of common mode noise attenuation and 65dB of normal mode noise attenuation. I believe the transformer also provides the electrostatic shielding that you mentioned in your latest post.

By the way, my use of the word, "taps," simply shows my inexperience with the subject matter. I had no technical basis for the use of that term.

Based on the advice in your latest post, it sounds like I should use the transformer to step down the 240V primary voltage to 120V at the output stage.

It would be a lot of help to me if you would explain a bit more to me about the primary and secondary grounding electrodes in my system. If I understand your posts to date, you are suggesting that, to comply with code and safety standards, the grounding electrode on the primary stage of the isolation transformer must be tied back to the grounding system at my main electric panel in my house (i.e., the clamp that is attached to my water pipe service where the service enters my house).

When you refer to the secondary grounding electrode, are you referring to a second ground connection from the secondary side of the isolation transformer? Did I understand your first post in this thread to suggest that it is best to have a ground for this secondary side of the transformer that is independent of the water pipe ground that is used as the primary ground for my electric system? I have seen a lot of cautionary advice about the use of two separate grounds within an electric system. The cautionary advice seems to say that it is never acceptable to have two separate grounding systems for two parts of an electrical system. Can you clarify this part for me?

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.

Cincy: You really can't do a free, engineering project via an audio Forum. That's a bit much to expect, both financially and even more importantly, from a liability standpoint. You are very bright, curious, and well spoken, but, respectfully, and I'll buy the first beer - y'all are drifting into a real project. And, respectfully, if we don't know that ground rods come in 10 ft lengths, and that driving one takes a specialized tool, and is the ultimate in apprentice physical labor, and that there are a ton of NEC requirements to be met, well...

Saying that you may want to consider engaging a freelance (not an engineering company) registered electrical engineer to prepare several drawings on what you wish to do. You can verbalize your electrican through it, but, that is always poor practice. That includes an as-build of your entire home system. Expensive, but hand waving a Journeyman electrican will burn up a stupid amount of hours; and here in N. CA, the billing rate will be about $80/hour.

Not a shareholder, employee, sales rep of any mentioned product. The XIT system is a large, hollow ground rod packed w/ trick material. It is vented at the top to absorb ambient humidity, thusly maintaing a very low impedance with the contacting soil throughout the year despite weather conditions. It is used by Honeywell for their DCS (Distributed Control Systems) in their localized Remote Instrument Enclosures handling plant control of digital inputs/outputs and analog signals in the milliamp level. It is installed in at least one Chevron refinery in California.

It also is, possibly, stupid overkill for a residential app., but in this day and age when 6loons is reviewing new-car priced transports/DAC's, I'm lost...

If your curent system is in good condition, and has a measured reasonable impedance, I wouldn't loose sleep on this...