grounding separately derived systems

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memyselfandI

memyselfandI

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According to art 250.30(a)(3), the GEC shall connect the grounded conductor of the derived system to the grounding electrode as stated in 250.30(a)(7). When you run an equipment ground with the primary you bond it to the casing of the trnsformer. From the secondary side, is it acceptable to run a wire from the casing through XO to a ground rod driven next to the transformer? Building steel and cold water lines are available, but they are not close to the transformer. This application has me concerned. Will the equipment ground be sufficient to satisfy the grounding of the derived system(which is the same size as the secondary), or does the re need to be a seperate wire for the derived system to groungd the secondary side of the transformer. IMO the ground rod is not tied the grounding electrode system and therefore is not a grounding electrode. 250.50 says that they all need to be bonded together to form the ground electrode. Thoughts would be greatly appreciated
 
okay let me try it this way.

480 3PH 3W 63KVA primary reducing voltage to a 230/400v secondary. This transformer feeds a machine. For thwe primaries they pulled a set of #4 CU and a #8 cu ground and bonded the #8 to the transformer can. On the secondary side they pulled 4-#4 CU and 1-#8 ground. They ran the ground from XO to the can of the transformer and then to a ground rod driven next to the transformer, the attached one of the #4 CU wires to XO as the grounded conductor. This is not what I consider a acceptable application if I interperet 250.30 correctly
 
memyselfandI said:
okay let me try it this way.

480 3PH 3W 63KVA primary reducing voltage to a 230/400v secondary. This transformer feeds a machine. For thwe primaries they pulled a set of #4 CU and a #8 cu ground and bonded the #8 to the transformer can. On the secondary side they pulled 4-#4 CU and 1-#8 ground. They ran the ground from XO to the can of the transformer and then to a ground rod driven next to the transformer, the attached one of the #4 CU wires to XO as the grounded conductor. This is not what I consider a acceptable application if I interperet 250.30 correctly
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Me either!
 
I agree the earth connection has to be made to a grounding electrode as defined by the code. A rod pounded into the dirt near the transformer not bonded to the building GES is not a code legal GE, IMO.
 
Sounds legal and legit to me provided the Xo to ground rod and MBJ are sized to 250.66. The ground rod is bonded to the GES via the EGC of the primary of the tranformer.
 
dereckbc said:
Sounds legal and legit to me provided the Xo to ground rod and MBJ are sized to 250.66. The ground rod is bonded to the GES via the EGC of the primary of the tranformer.
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then why drive a rod. If this is legit then the jumper should be sufficient. The code says to make the connection to the electode not a rod via an EGC.
 
memyselfandI said:
then why drive a rod?.
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Because of economics. It was easier and less expensive to drive a ground rod to satisfy 250.30(4)(3). The ground rod is bonded to the GES via the EGC in the primary. What is important is the GEC from the Xo to the ground rod and MBJ is the proper size. Also note the EGC in the primary has to be at least a #6 AWG to satisfy 250.53(E) for a supplemtal electrode.
 
dereckbc said:
Because of economics. It was easier and less expensive to drive a ground rod to satisfy 250.30(4)(3). The ground rod is bonded to the GES via the EGC in the primary. What is important is the GEC from the Xo to the ground rod and MBJ is the proper size. Also note the EGC in the primary has to be at least a #6 AWG to satisfy 250.53(E) for a supplemtal electrode.
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I apologize for not mentioning that this transformer is fed from a sub fed panel, which is sub fed from a sub fed, which is FED from the main service. Whew! My bad. All grounds and grounded conductors are separated in these panels. I couldn't find the references you gave me but I did find a 250.30A(4)(c)(3) which deals with multiple derived systems. This was not a multiple derived system, nor was there any exothermic welding. 250.53(E) for supplemental electrode connections says that it shall not be required to be larger than a #6 CU. They were not installing a supplemental ground, they were installing a ground rod for the purposed of using it as a grounding electrode to satisfy 250.30 A (3) and if that is the case it should be tied to the GES as it states in 250.50. If the panel they were feeding this transformer from was the main service, then I would not have a problem with them making the connection there because the GES connections are made at that point to the grounded conductor. I still believe that the intent was to ensure that the connection on the SDS is to be made at the grounding electrode and not on the ground bar of a sub fed panel. I once again apologize for not mentioning that, it seems that that would be important information that should be disclosed.
 
Is it standard practice to have a single conductor serve double duty as both an equipment grounding conductor and a GEC? It seems like I would see that done frequently. Where does the code say I can do that......Wait I know the response: where does it say I CAN'T!
 
memyselfandi, do not know what else to tell you other than 250.30(4) defines what electrodes are permissible. I will not quote it word for word but you have 3 options.

1. To the nearest effectively grounded building steel
2. Water pipe within 5-feet of building entrance
3. Other electrode specified in 250.52

Well if you go to 250.52(5) a rod is permissible. It is considered bonded to the GES via the EGC of the transformer primary circuit no mater how many sub-panels it runs through to get there. This assumes the EGC of the transformer primary is adequate in size or an acceptable raceway to serve as a bonding jumper.

Like I said earlier I assume the electrician choose this option due to economics, and it is perfectly acceptable and safe.

The only thing I can see you might gig them for is they would have to use 2-rods if one did not satisfy the 25-ohm requirement.

There are plenty of threads covering this very subject if you need more confirmation. Like this:
http://forums.mikeholt.com/showthread.php?t=53312

One in particular is by John M. Calogerro. If you are not familiar with John, look at the cover of your NEC book for some light reading.

The only thing I can see you might gig tham for is to use 2-rods if a single rod does not meet the 25-ohm requirement
 
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I disagree that you can just drive another ground rod or rods and create a new grounding electrode system. When both building steel and metal water line are present no mater the distance. They are there and must be used before creating a new grounding electrode system. NEC 250.30(7) is very clear in my opinion.
 
dereckbc said:
memyselfandi, do not know what else to tell you other than 250.30(4) defines what electrodes are permissible.

250.30.(4) does not exist in the 2005 and does not define what electrodes are permissible. 250.30.(A)(4) deals with multiple seperately derived systems. Which is not what we are dealing with. 250.30(7) tells you what electrodes are permissible. 250.30(7) EXP 1 states that you can use the electrodes in 250.52 if the electrodes 250.30(7) are not available, which they are. Nowhere in the code does it say that it is permissible to use and EGC as a GEC for a seperately derived system. If it does , I need to see it otherwise IMO what they did is a violation.
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The code section is not clear in that the first part of 250.30(A)(7) requires that the SDS grounding electrode be "as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system". This makes it a judgement call as to the use of a driven rod next to a transformer where there is also building steel or metal water pipes within the building. I see no problem with the use of a "local" grounding electrode for this application. In reality this electrode serves no real purpose.
Don
 
memyselfandI said:
Nowhere in the code does it say that it is permissible to use and EGC as a GEC for a seperately derived system. If it does , I need to see it otherwise IMO what they did is a violation.
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The code is a permissive document, and any supplemental electrode can be bonded by an equipment or panel EGC anywhere along its length, it is a common practice. The code permits it because nowhere will you find that it "SHALL NOT" be permitted.
 
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The code may be permissive, but it is also very specific on what it is to be done and after all this I still see that there is NO reference in the code to allow a EGC to act as GEC. Of all the wording that comes with the NEC, if the boards had thought that this was an acceptable practice, then there would have been a reference to it in the code. To be as particular as to note how to terminate a wire on a receptacle, do you think that they would have mentioned this practice? I think so. As far as 250.30(A)(7) not being clear as to its definition of as near as practical to and preferably in the same area, you have to go right back to the first exception of that article to determine what is allowed and when. To drive a ground rod when there is a GES available and is near and practical to me is not economics but laziness.
 
To drive a ground rod when there is a GES available and is near and practical to me is not economics but laziness.
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No one said that the water pipe or steel was near or or practical. I have no objection because a GES installed for a SDS that is in the same building where the primary feeder is in serves no purpose. There is no technical reason to install this GES.
Don
 
Just a couple of things,
An SDS is its own system. An equipment grounding condutor must be supplied with the primary circuit to provide a lo impedance fault-currry path from the transformer case to the main service or source of supply. The overcurrent device on the "PRImary" of the transformer will then clear a short circuit or ground fault up to, and including, the transformer "primary windings."
That's the primary side.
The EGC connection to the transformer enclosure, plus the bonding jumper from the secondary does not constitute a electrical connection from the primary system to the secondary system. The EGC is not a systems conductor as the ungrounded and grounded conductors are.

The GEC is use to connect all grounding electrodes to the EGC and grounded conductor and the service, or at the source of a SDS. If all that is in place then I don't see a cause for concern.
 
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