NEC 250.121 change

[jar546]

The 2011 NEC has a change in article 250.121 that reads:

An equipment grounding conductor shall not be used as a grounding electrode conductor.

Many want the entire section removed from the NEC but the CMP has held steady on this one with the following comments:

The panel re-affirms the position that the GEC and EGC’s have different functions and shall not be used as both. The substantiation for the proposal contains some of the reasons this proposal is not acceptable. This includes the fact that the two conductors are installed for different purposes, are sized differently and have different installation requirements. Here are a few illustrations of why the rule needs to remain in the NEC. (1) An equipment grounding conductor is required to be installed with the circuit conductors to the transformer enclosure. 250.30(A)(4) requires the grounding electrode to be “as near as practicable to, and preferably in the same area as the grounding electrode conductor connection”. These locations could and often are widely separated.

(2) A feeder is installed from a source in a building to a transformer that is located outdoors. 250.30© requires the grounding electrode connection to be at the transformer that is the source of a separately derived system. A combination grounding electrode conductor/equipment grounding conductor run to the source location would not be acceptable.

(3) A supply to a transformer-type separately derived system is installed from a sub-panel (feeder panelboard). Where would a combination grounding electrode conductor/equipment grounding conductor be connected? To the neutral terminal bar? A clear violation of 250.24(A)(5). To the equipment grounding terminal bar? The equipment grounding conductor supplying the panelboard may not be large enough.

Do any of ya'll ("yous guys" if you're from NJ/NY) have an issue with this section or a problem with ECs complaining about it?

 

[Gregg Harris]

The 2011 NEC has a change in article 250.121 that reads:Many want the entire section removed from the NEC but the CMP has held steady on this one with the following comments:

Do any of ya'll ("yous guys" if you're from NJ/NY) have an issue with this section or a problem with ECs complaining about it?

There should not be an issue, the substantiation is clear and CMP should continue to reject a change.

 

[gfretwell]

What is the grounding wire in a feeder to a detached building called?

It is certainly creating a connection between 2 ground electrodes. Is it an EGC or is it a grounding electrode jumper?

 

[ICE]

All grounding electrode conductors can be equipment grounding conductors but not equipment grounding conductors can be grounding electrode conductors.

 

[jar546]

Here was one of the arguments for removing 250.121 entirely.

What is the physics hazard reason that the same conductor should not function both as the GEC and the EGC? Please, if someone can, give me a single example of an installation, meeting the requirements of the 2008 NEC, and presenting a hazard of any kind. By all the evidence shown to me, 250.121 is an unjustified restriction to not allow a GEC to act as an EGC and vice versa. I have struggled with and pursued this problem for nearly two years now. This issue has been extensively discussed with many NEC experts such as, and including most of, CMP-5. Nobody... not anyone from a CMP, not any of the NECA Code reps, not any of many Los Angeles area Code reps (including the very conservative Chief Electrical Inspectors and plan checkers of both LA City and LA County)... nobody has come up with so much as a single example of an installation in full compliance with the 2008 NEC which presents a hazard, not one. The only examples given were ones where the 2008 NEC was not complied with.

I am not looking to get into a discussion about 250.30(A)(7) and/or 250.64(F) in general or 250.64(F)(3) specifically. I am not looking to point out any errors in installation. As an installation example: what conceivable good is it to be required to add a #8 copper EGC running in parallel with a #2 copper GEC in the same PVC underground conduit between a 480Y/277V service/distribution section and a 75 kVA, 3-Phase, 480V to 208Y/120V, transformer? This installation can easily be done in full compliance with the 2008 NEC using only the #2 conductor. The #2 GEC is far more than capable of carrying the rare-occasion ground-fault current from the primary circuit. A single conductor would always be the larger of the two required sizes, usually the GEC size.

There are an infinite number of ways to incorrectly install a GEC or EGC. They shall be installed legally in all circumstances. But there is no physics-hazard reason to not allow a correct installation to do both jobs. Nobody wants installations which violate Code sections; that is not the issue here.

 

[chris kennedy]

What is the grounding wire in a feeder to a detached building called?It is certainly creating a connection between 2 ground electrodes. Is it an EGC or is it a grounding electrode jumper?

Great point.

Thats said an EGC is permitted to be a metal raceway as per 250.118 which if used as a GEC would be a violation of 250.62.

 

[jwelectric]

What is the grounding wire in a feeder to a detached building called?It is certainly creating a connection between 2 ground electrodes. Is it an EGC or is it a grounding electrode jumper?

250.4(A)(1) is very clear of the purpose of the grounding electrode. If we understand the purpose of the grounding electrode then there is no problem understanding 250.121.

Let’s think about something for just a second. There is a 10 foot square metal storage building 100 feet away from the house that is supplied by #10 THW conductors in PVC pipe to a 4 circuit panel. So far everything sounds good, right?

Now let’s bring good ole Mother Nature into the picture and either have lightning strike that building or a high voltage overhead line break and hit this building. Just how is this high voltage going to dissipate if the EGC is used for the GEC?

Personally I would rather have all that energy go away at the building than at my house.

 

[gfretwell]

JW, I still do not understand the difference, You still have a direct connection between the grounding electrodes in both buildings, no matter what you want to call it.

If we make the worst case scenario, the metal shed just has one or two rods driven into dry sand and the house has a very substantial GES, maybe even a gunite in ground pool. (the King Kong of Ufers).

There will be significant ground electrode type current in that feeder EGC if you have one of those events.

 

[jwelectric]

JW, I still do not understand the difference,

Not trying to sound rude toward you in any way but this statement is obvious.

You still have a direct connection between the grounding electrodes in both buildings,

You will have a direct connection from the main to the electrode but from the building the connection is indirect unless a new electrode system is established at the building. Any high current imposed on the remote building will have to travel all the way to the service before it has a ground connection which is very bad.

no matter what you want to call it.

I agree the name of the path means nothing but the result of the path is very different

If we make the worst case scenario, the metal shed just has one or two rods driven into dry sand and the house has a very substantial GES, maybe even a gunite in ground pool. (the King Kong of Ufers).

The equipotential bonding of a pool is not an electrode and should never be used as an electrode.

There will be significant ground electrode type current in that feeder EGC if you have one of those events.

But if you have the building properly connected to earth at the building the bulk of the current will dissipate at the grounding electrode and not travel back to the main. The utility has an electrode at every pole that has a transformer. This electrode connects to the high voltage grounded conductor as well as the secondary grounded conductor. If the grounded conductor of the high voltage could handle the current why would they waste time doing it at every pole? The answer lies in 250.4(A)(1) of the NEC. The wording found in the NESC which utility companies use for their installations is the very same as in the NEC.

 

[gfretwell]

Current will take all available paths. You might not call a pool a Ufer but you can't avoid it being one. There are a number of EGC paths from the service to the bonding grid and current will go there.

You are required to have a ground electrode at a detached building and that gets bonded to the service GES by the EGC in the feeder. You can play names with insulation colors and names but current will go where the wire takes it

 

[jwelectric]

This is true that current will take all paths but that is not the point. The point is that when we try to use the equipment grounding conductor of the feeders to connect the service disconnect electrodes to that remote building we have taken away one of the paths and the most important path, the one located where the energy originates.

250.4

(A) Grounded Systems.

(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by 1) lightning, 2) line surges, 3) or unintentional contact with higher-voltage lines and that will 4) stabilize the voltage to earth during normal operation.

In the event of 1) above you are saying that using the EGC of the feeders will be all that is needed to dissipate the energy caused by a lightning strike and that an electrode being established at the remote building does no good.

With my limited knowledge of current flow I would beg to differ with that line of thinking. A lighting strike is a cloud to earth, or the other way if it makes you feel better, event so a connection to earth at the point of the strike only makes sense. To force the energy to travel all the way back to the service disconnect in order to find its desired path makes no sense to me at all.

With the electrode established at the remote building most of the energy will dissipate at the building and very little will travel on the EGC to the service disconnect.

Without this established electrode and letting the EGC be the path to the earth connection then all the energy will travel to the service disconnect which is a very bad thing to happen.

If you research this rule you will find that it all started about using the primary EGC as the GEC for the secondary of transformers. Should we be in an industrial situation where the primary EGC would be a fraction of the size required for the secondary GEC we could end up with some pretty nasty results which have already been documented.

As to the pool bonding and the paths that current will flow take a look at 250.52(B)(1). Metal gas piping is not allowed to be used as an electrode. What happens when we use CSST piping in a propane tank such as the one I have at my home? The tank is 150 feet away from the house with copper tubing as the supply to the house. When I bond this CSST piping what will happen in the event of a lightning strike. Remember that current will take all paths available. By the way we have suffered a lightning strike that damaged the roof of our house. The tank is still intact and undamaged. Our roof is less than 4 years old. The gas tank has been in place for 18 years.

In the debate concerning the EGC being used as a GEC is also the resistance between 100 feet of #10 and a short #6. With the pool the same resistance comes into play as to the amount of current that will flow to the steel of the pool.

 

[ICE]

http://www.flickr.com/photos/97859466@N05/11386803745/[/URL]

What do you suppose becomes of a 10'x10' metal shed when this happens?

There is a one room country school house located in Illinois. It fell into disrepair during the great depression and a relative took it to his farm and converted it to a dwelling. Truth is, he stole it.

Several cousins and I would stay on uncle Shy's farm during the summer. During one helluva storm, lightning struck the ground a few feet from the schoolroom wall. The light was so bright that I could see the outline of the framing in the wall like an x-ray. The window was blown all over the room.

Years ago I was on the summit of an erupting volcano. Well not exactly on the summit...and not exactly me.....but it was years ago.

http://www.flickr.com/photos/97859466@N05/11386803325/

I'm guessing that the lava is the bigger issue...but hey now, it's your shed, not mine.

 

[chris kennedy]

Very cool pic. Look at the stringer coming off the tower on the left.

 

[gfretwell]

Lightning hit the mast of my weather station twice, once when I was in the driveway watching. The first time it fried the weather station and the serial port on the PC it was attached to. The second time (the one I saw) it only required a reboot. Good lightning protection really works.

The basis is a really good electrode.

I still bet it might have taken out equipment up and down the street because my electrode did not take the whole hit.

Current takes ALL paths.