Grounding of a 12.47/480 xfmr and switchboard

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AHE Tom

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I am looking for the correct NEC section regarding the grounding of an owner supplied 12.47/480 3ph pad mounted transformer and 480v switchboard system.
I am proposing it to be grounded/bonded as per 250.30, using the Exhibit 250.14 in the NEC Handbook, is this correct ?

If not what would be the correct section ?

Is this is correct, can the bonding conductor between the chassis of the switchboard and the case of the transformer be directly encased in the concrete ductbank between the transformer and the switchboard (this is what we have now).
The owner wants us to also provide an additional grounding conductors in the incoming conduits along with the bonding conductor in the ductbank. I feel this is incorret.

Thanks
Tom
 
I don't see any reason you would treat any differently then a 480 delta 208 transformer.

IMO you must run the bonding / grounding conductors within the raceways per 300.3(B), the encased conductor can remain but IMO can only be connected at one end as an grounding electrode not an bonding jumper.

Tell us more about the installation. :smile:
 
Within the secondary conduits from the xfmr to the switchboard there are the three phase conductors, and the neutral conductor, and then the bonding conductor in the encasement.

My question is, in an installation where the utility co. owns the pad mtd. xfmr, and they provide the incoming service conductors in the owner supplied raceway(s), I have never seen the utility co. install a bonding conductor in any of these raceways, why is this ? This is the exact same installation just that the equiment is owned by different parties.


Thanks again
Tom
 
My next question is is there anywhere inthe code that exactly defines how such a system should be hooked up.
 
AHE Tom said:
Within the secondary conduits from the xfmr to the switchboard there are the three phase conductors, and the neutral conductor, and then the bonding conductor in the encasement.

My question is, in an installation where the utility co. owns the pad mtd. xfmr, and they provide the incoming service conductors in the owner supplied raceway(s), I have never seen the utility co. install a bonding conductor in any of these raceways, why is this ? This is the exact same installation just that the equiment is owned by different parties.


Thanks again
Tom
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POCOs (power companies) are not covered by the NEC (90.2(B)(5))

They follow a different set of rules.
 
As John said, the NEC does not apply to the POCO, it would be the NESC.

With that said, there is actually no need to install an EGC between the transformer and the Main, the Neutral will wear two hats here, one being the role of a fault clearing conductor.

Just provide the Earthing and all associated bonding at the Main Switchboard and everything is fine.

Roger
 
AHE Tom said:
My next question is is there anywhere inthe code that exactly defines how such a system should be hooked up.
Click to expand...

Are you running a 480 Delta service or a 480 Wye service?

If Delta do you plan on making it a grounded system or ungrounded?

For us to help we need to know details. :smile:


As far as this.

This is the exact same installation just that the equipment is owned by different parties.
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That is pretty much it. The rules are very different depending on which side of the service disconnecting means your dealing with.

On the supply side of the service disconnect the neutral is used for grounding.

On the load side of the service disconnect it is rarely allowed to use the neutral as the grounding means.

Supply side covered by the NESC, load side by the NEC.
 
As has been mentioned, the rules change from the NESC for POCOs to the NEC for premises wiring (property privately owned).

With that said, a couple of code sections.
1. 300.3(B) requires that the conductors for a circuit or feeder all be installed within the raceway.

2. 250.24(A)(5) Requires the EGC and the Grounded conductor (neutral) to be separated (isolated).
 
I guess I should not have asked the util. co. question, as I knew that was the anser I was going to get from someone.

Anyway back to what we have:

The transformer is a Y secondary, and from what I understood, being a seperatly derived system, at the switchboard (main building disconnecting point) the neutrual and the grounding point are the same, and the neutral should not be isolaterd as this is not a sub panel, or sub swithcboard, is this not correct ?
With this said, then the conductor that runs from the case of the transformer to the switchboard is the bonding conductor, corrtet ? (looking at exhibit 250.14 in the code handbook).

Reading back in some other posts on a similar topic it was indicated that there should not be a paralled ground/bonding.
If a bonding conductor was run in each of the conduits (conductor sized on the total KCMIL servng the main switch) wouldn't this be considered as a parallel system ? Thus the bonding conductor was run separately.
 
Your only hope here is Section 250.102(E). If the equipment bonding jumper is longer than 6 feet you are out of luck and will have to run a bonding conductor from the transformer to the main disconnect. In my opinion, you can ground this service by using either Exhibit 250.13 or 250.14 in the NEC Handbook. IMO, Both options satisfy 250.30(A)(1) and 250.30(A)(3). But both options require either a system bonding jumper or an equipment bonding jumper run with the phase conductors in the same raceway.

If the transformer is located outside the building, you will also have to comply with 250.24(A)(2). See Exhibit 250.9 in the NEC Handbook. Note that the extra grounding electrode at the transformer is not bonded to the grounding electrode required at the disconnecting means except through the grounded conductor.
 
If each conduit between the xfmr and the switchboard were to have a bonding jumper in it would this not be considered parallel paths for the bonding ? Is so then this should not be an acceptable means of bonding.



Thanks again
 
AHE Tom said:
The transformer is a Y secondary, and from what I understood, being a seperatly derived system, at the switchboard (main building disconnecting point) the neutrual and the grounding point are the same, and the neutral should not be isolaterd as this is not a sub panel, or sub swithcboard, is this not correct ?

Reading back in some other posts on a similar topic it was indicated that there should not be a paralled ground/bonding.
If a bonding conductor was run in each of the conduits (conductor sized on the total KCMIL servng the main switch) wouldn't this be considered as a parallel system ? Thus the bonding conductor was run separately.
Click to expand...

Assuming the installation is subject to the NEC:

250.30(A)(1) allows only one system bonding jumper (which connects the system grounded conductor to the equipment grounding conductors) to be installed. This SBJ can be installed at the xfmr secondary or at the switchboard. What this means is that if the neutral is bonded to the case and EGCs at the switchboard, then the X0 terminal at the xfmr should NOT be bonded to the case.

Since only one system bonding jumper is legal in this case, there are no parallel paths created by the equipment grounding conductors which should be pulled in the raceways along with the phase conductors and neutrals.
 
On the other hand, would exception 2 apply here and we could put an SBJ in both the xfmr and the switchboard, and rely on the grounded conductor to also serve as the EGC?

Did the OP state whether this was PVC or metal conduit?
 
AHE Tom said:
If each conduit between the xfmr and the switchboard were to have a bonding jumper in it would this not be considered parallel paths for the bonding ?
Click to expand...
Yes, they are in parallel with the neutral
AHE Tom said:
Is so then this should not be an acceptable means of bonding.
Click to expand...
Agreed, see post #6

Roger
 
This is the way I understand it also.

The entire reason for this post is to try to explain to a customer that what he wants done at this service is incorrect. He wants the E.C. to pull an indavidual ground in each incoming condut between the xfmr and the switchboards main circuitbreaker, along with the three phase conductors and the neutral conductor.

We have tried to explain to him that this is incorrect, but he will not believe us until we provide him with a code section indicating that this is incorrect.

So, after all this posting of information (I thank everyone so much for all the help) what section can I find this information in.


Tom
 
AHE Tom said:
He wants the E.C. to pull an indavidual ground in each incoming condut between the xfmr and the switchboards main circuitbreaker, along with the three phase conductors and the neutral conductor.
Click to expand...

This would be allowed in a owner supplied transformer but,... The main bonding jumper would have to be removed in the first ocpd from the transformer.
AHE Tom said:
We have tried to explain to him that this is incorrect, but he will not believe us until we provide him with a code section indicating that this is incorrect.
Click to expand...

Article 250.30 is the article. How you apply that section with the materials used is where the violation can start. IMO if the bonding jumper is intact, the GEC encased in your duct bank that is tied between the transformer and first ocpd is not a violation. The grounding conductor pulled with the grounded conductor would be. So if he wants the grounding conductor anyway, then remove the bonding jumper in the first ocpd.It'll confuse everyone down the road that works on it , and the inspector might not approve it this way.
 
is it me or does everyone have trouble with this?

is it me or does everyone have trouble with this?

this is one of the topics i see over and over because the confusion trying
to explaining bonding/grounding in sds systems primarily, can someone come up with an easy to understand graphic presentation to help remedy this
conondrum, did i spell that right?:grin:
 
RUWired said:
This would be allowed in a owner supplied transformer but,... The main bonding jumper would have to be removed in the first ocpd from the transformer.

...then remove the bonding jumper in the first ocpd.It'll confuse everyone down the road that works on it , and the inspector might not approve it this way.
Click to expand...
250.24(B) requires a main bonding jumper within the service disconnect enclosure. So regarding a service xfmr you cannot remove the MBJ in the first disconnect without it being a violation. This has been discussed recently:

http://forums.mikeholt.com/showthread.php?t=100003

In particular, posts #18-20.
 
AHE Tom said:
This is the way I understand it also.

The entire reason for this post is to try to explain to a customer that what he wants done at this service is incorrect. He wants the E.C. to pull an indavidual ground in each incoming condut between the xfmr and the switchboards main circuitbreaker, along with the three phase conductors and the neutral conductor.

We have tried to explain to him that this is incorrect, but he will not believe us until we provide him with a code section indicating that this is incorrect.

So, after all this posting of information (I thank everyone so much for all the help) what section can I find this information in.


Tom
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You will not find a code section which states this as incorrect. However, this installation falls under 250.24 (not 250.30) through 250.28. You will find that the only thing that you can compliantly connect such grounding conductors, at the xfmr end, is the transformer case and/or a grounding electrode (sizing will need to be correct for the puropose of serving as an EGC or GEC)... and in no case can these be bonded to the grounded conductor or its required grounding electrode if an outside xfmr.
 
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