grounding electrode for separately derived system

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mike abbott

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Question on how to effectively install a grounding electrode conductor on the secondary side of a step-down transformer:

We have a couple of 480//120/208V transformers to install in a new building; each trasformer is in a different part of the building. The primary feeders to each will be run in PVC underground. The building steel consists only of bar joists on masonry walls; these joists are not effectively bonded together. The main water service is about 150 feet away from each transformer.

The local inspector is requiring a separate ground wire to be installed outside of each conduit in the dirt, to be run from the secondary side of the transformer to the main incoming water line.

This sounds like overkill to me, but as I research the code it appears to be correct.

Any other suggestions?

Thanks
 
Re: grounding electrode for separately derived system

Perhaps it would be more practical to use the tap allowance from each system to a single GEC back to the water pipe. See section 250.30(A)(4).
 
Re: grounding electrode for separately derived system

The common grounding electrode conductor with taps wouldn't work in this case, only because the transformer are not near one another.
 
Re: grounding electrode for separately derived system

Mike why can't you use the provisions of 250.30(A)(2)(b) and 250.30(A)(3).

In my business we use a lot of what we call vertical risers and horizontal equalizers in large high-rise and large footprint buildings just for that purpose. I am confused why you cannot.
 
Re: grounding electrode for separately derived system

I assume that there is a properly sized equipment grounding conductor installed within the PVC conduit supplying the each transformer. I assume that the transfomrers are on the first floor. Since the building does not have effectively grounded metal frame members, each transformer can be grounded to Ground rods in close proximity to the transformer (at least 2 grnd rods if the resistance to ground exceeds 25 ohms). Separately derived systems should be grounded as close as posible the the transformer and location where the power is to be utilized. Section 250.52(7)of the 2005 NEC can be used.
 
Re: grounding electrode for separately derived system

Why not run a #4 outside, connect to a 20' rebar, bury it in the dirt and cover it with concrete?
UFER ground! aka concrete encased electrode.

edited to add:
You will also need to bond to the metal water pipe.

[ October 28, 2005, 11:44 AM: Message edited by: tshea ]
 
Re: grounding electrode for separately derived system

John wouldn't 250.58 apply and still require all electrodes to be bonded together to form a common electrode system? And if the EGC ran with the primary was sized via 250.66 for the derived conductors (secondary) it could serve as this common bond?
 
Re: grounding electrode for separately derived system

Since the metal water pipe is available, it must be used as the electrode regardless if it is 150' away. You can't use other permitted electrodes if either the water pipe or structural metal are available.
 
Re: grounding electrode for separately derived system

Section 250.58 refers to installations where supplied by more than one "service" not separately derived systems on the load side of the service. The Code Panel made it clear in their Report on Comments, Comment 5-112 Log #1126, NEC PO5 on section 250.58. There was a proposal to require the grounding electrode for separately derived systems to be bonded to the service grounding electrode, but it was rejected by the Code Panel.
 
Re: grounding electrode for separately derived system

I guess I have read into 250.50 to much. I was thinking that if the SDS was installed at the beginning (when the service was installed) then 250.50 would require us to also bring it's electrode into the grounding electrode system.

I don't see a problem with fault current as current will return to source but I do see a problem if raceways of both system were installed in close proximity to each other or to the same piece of equipment but isolated from each other, then there is a possibility that a ungrounded conductor of one system heating up the grounding of the other system without having a fault path.

Here is one example: Lets say a pump needs 480 volts but only 208 is available, a SDS system is installed to supply power only for the pump load. A flexible cord with a twist lock plug and receptacle (mounted on the wood building) are installed to allow quick change out. The pump is mounted on rubber mounts to prevent vibration, and there is other loads on the machine that is fed from the 208 service and the pump cord just happens to run across a sharp clamp or box edge on the 208 system that cuts into the cord and makes contact with one of the ungrounded phase conductors of the SDS pump circuit with "B" phase grounded. A fault won't occur and there will be a 480 volt potential between the grounding of the two systems unless there is a common bond between the two systems. If no water pipe exists for use as an electrode or bond as in 250.104(A)(4) or no metal building frame, where is it required for use to bond these two systems to provide this path in the NEC? 250.96 would seem to require this but it doesn't exactly indicate it is also for the bonding together of different systems?

I agree that the example would be very remote but it could happen?

[ October 28, 2005, 08:30 PM: Message edited by: hurk27 ]
 
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