Electric Sub-Panel Installation

Cityboyfarmer · Jul 14, 2005

Hi Sparkkky!

Grounding seems to stir up more controversy than most subjects doesn't it? I'm curious, have you ever ohmed out the earth ground systems you installed to ensure it is 25 ohms or below per the code? I don't know of any inspector anywhere that has ever required that resistence be verified, even with single driven electrode systems. In my area, the AHD requires two rods be driven and calls it a day. The fact is, even though the two driven electrodes meet the code, in most cases, the resistence will still be 100 ohms or more, especially in sandy, dry or rocky soils. The bottom line is that the driven electrode system is the least effective grounding method, but it is the cheapest and hence the most widely used. Yes, I am guilty of this as well, especially when re-wiring an old house when expense to the home owner prevents better options.

So, what is the most effective grounding system? Some might say a metal underground water pipe, but widespread use of PEX & PVC piping have just about eliminated that. Article 250.81 lays out several preferred grounding systems. Note article 250.83 states that if none of the electrodes specified in 250-81 are available, then the electrician may resort to "made" electrodes. Well, most electricians go straight to the inferior methods, usually the driven rod as the primary choice. And in most cases, (all cases in my experience) the AHD says you have met the code.

For my money, the concrete encased electrode specified in article 250-81-c fits the bill perfectly. See attached photo. Now, which system would you really rather have? A 5/8" x 8' rod protecting your home, or a 200' x 12" concrete encased system?

Now, please understand that my intent is not to criticize anyone. My intent is to show that there are far better ways to ground your home and protect you, your family and your appliances from power surges & ground faults. Meeting the minimum standards passes inspections and moves you to the next phase of construction, but it is not the best protection available in many cases.

I hope I haven't bored you folks too terribly, as I find grounding discussions more stimulating than most folks. /forums/images/graemlins/laugh.gif

Cityboyfarmer · Jul 14, 2005

Another shot of the concrete encased electrode conductor in the poured footer. Note in the first picture before the pour, the electrode is in direct contact with the steel re-bar.

Inspector507 · Jul 14, 2005

sparkkky
Yep, we have discussed this a few times in the past. I will be the first to agree that different areas of the country/world see things differently. And I am not saying the way we look at it is the best either. We just follow the current edition of the NEC® as it is written.

 seems in the 2005 that may be a change 
Actually that was a huge change in the 2002 NEC®

Inspector507 · Jul 14, 2005

Cletrac,

 Running only 3 wires for a sub panel is against the national electric code. You must pull all 4 and the ground and neutral must not be bonded in the sub panel as they are in the main panel. Putting in a ground rod at the barn only will not draw enough current to trip a 15 amp breaker but it will draw enough current to kill you. 

You are "permitted" by the NEC® to run 3 wires to a SEPARATE BUILDING and install a made electrode there if no other metallic paths exist. Your GROUNDED conductor is used as the return path in this case and is permitted by the NEC®. Your electrode and the grounded conductor must be bonded together. This is one of the few cases you can do that. Your main service only has 3 wires, the neutral/grounded conductor is still the return path to ground.

Inspector507 · Jul 14, 2005

CityboyFarmer,

 I don't know of any inspector anywhere that has ever required that resistence be verified, even with single driven electrode systems. 

We actually own several Ground Resistance testers. Are they used much anymore? No. We found that less than .5% of the made elctrodes did not test lower than 25 ohms.

Other areas of the country/world with different soil conditions will measure differently.

sparkkky · Jul 14, 2005

seems in the 2005 that may be a change 
Actually that was a huge change in the 2002 NEC®

Missed that in CE class but I know that as late as last month per a talk with my local AHJ officer that they still require the 1999.

Funny that there is sometimes a double standard on what in the newer book is ok, like when the change said no more than one grounding conductor under one screw, but then that does not directly go aganist a prior code.

CBF, thats why I ask when I can what the inspector wants. The thing about rebar is it must be planed on from the first and doing mostly service with some new construction its not a choice.

In cites like Edmond, OK it is required to stub up rebar in the slab inspection.

Cityboyfarmer · Jul 14, 2005

(
We actually own several Ground Resistance testers. Are they used much anymore? No. We found that less than .5% of the made elctrodes did not test lower than 25 ohms.

Other areas of the country/world with different soil conditions will measure differently. )

It is good to hear that ground resistance is tested in some places. Have you ever gone back and retested after a year or two? I know that resistance can be decreased with rock salt and water, and that the electrode must be re-treated periodically. What about during drought cycles?

As you can see from my photos, the soil under my house is sandy. No way I would trust a driven rod alone to protect my home, plus it doesn't cost much more for 20' of bare copper conductor in a concrete footer. I put 50' of 1/0 bare copper in mine. Small price to pay for peace of mind IMO.

Cityboyfarmer · Jul 14, 2005

( CBF, thats why I ask when I can what the inspector wants. The thing about rebar is it must be planed on from the first and doing mostly service with some new construction its not a choice. )

Me too Sparky. When I would do re-wires, I would call the inspector for the area I was servicing and find out exactly what he was looking for. Makes life a lot easier that way.

You are right about the planning. Not too many general contractors ever think about bringing the electrician in when the footing is being dug. In most cases the electrical bids have not yet even been given in the foundation phase.

Inspector507 · Jul 14, 2005

I agree that the re-bar electrode is far superior in some areas of the country. AND it is now required in the 2005 NEC® if re-bar in the footings exist.

ZJ_HR · Jul 15, 2005

I'm curious about your way of making wiring and installation. Sorry for, for you maybe, stupid questions. I am electric engineer, living in Europe, and don't understand what are you talking about.
You have 220V, 60Hz single phase network. Why you need more then 3 conductors? Can somebody draw a diagram to explain me, please?

We have here either single or 3 phase network in houses, using Line, Neutral and Protective Earth conductors, or 3xLine+N+PE.
Neutral wires in installation are connected to the common busbar, and are connected to the neutral of utility overhead network, separated from ground. Neutral is grounded by the utilities.
PE is connected to the house grounding (either in basement on new biolt houses, or bar or pipe type grounding).
Switchboard circuit breakers switch just a phase conductor, and neutral is not switched.
In French or Italy are in use 2 or 4pole breakers, and they disconnect neutral, too.
Protection of insulation fail is provided by earth leakage switches/circuit breakers.
Are we that far in technology /forums/images/graemlins/confused.gif