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#82773 - 12/11/02 06:41 PM 250.30
Gwz Offline
Member

Registered: 04/29/02
Posts: 199
250.30(A)(6) does not address the requirement of 250.24(B).

Does that mean where the 480V 3Ø secondary of an exterior pad-mount SDS does have the Xo terminal grounded ( bonded ) to the transformer case and to a grounding electrode { is now actually an 3Ø 4W system} does not need the grounded conductor to be extended to the first disconnecting means within a building ?

Is an existing system and cannot determine if the grounding jumper as noted above, [ and maybe intended to be 250.30(5)] actually goes to the building. Only the 3Ø conductors are in the conduit leaving the transformer enclosurer, thus, is not run with the derived phase conductors.

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#82774 - 12/12/02 03:43 PM Re: 250.30
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
i am confused...

are you asking how an ungrounded SDS x-former is then grounded at the buildings service?

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#82775 - 12/12/02 06:18 PM Re: 250.30
Gwz Offline
Member

Registered: 04/29/02
Posts: 199
The 480V Xo terminal is grounded to the transformer case and to a Ground Rod at the transformer. The transformer is a Pad-mount about 30 feet from the building served. Only the 3 phase conductors are in the nonmetallic conduit to the building. Since this is an existing installation, (appears about 4 years old replacing an overhead supply), do not know if the grounding electrode conductor in the tranformer pad opening extends into the separate building, but if the GEC does extend into the building, it definitely is not run with the circuit conductors 250.30(A)(5).

If this were a Service, 250.24(B) would require the grounded conductor to be installed with the circuit conductors to the main disconnect(s).

Since this is a SDS, and the Xo terminal is grounded, 250.30(A)1st sentence tells us that it SHALL comply with (1) thru (6). I do not interpret a correlation to 250.24(B) in 250.30.

The question is: Since the Xo terminal is grounded at the transformer, does a grounded (circuit ?) conductor need to be run with the derived phase conductors to the first disconnecting means in the seprate building?

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#82776 - 12/12/02 07:02 PM Re: 250.30
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
i see the definitional quagmire.

i've beat this around the good book all night, no soap.

i'm a tad stuck on 250.20(b)(3).....

need sleep, will ch back in the am......

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#82777 - 12/12/02 07:32 PM Re: 250.30
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
99NEC250-2(d) …The earth shall not be used as the sole equipment grounding
conductor or fault current path.


A fault in the service-entrance equipment or downstream within the premises may easily cause hazardous touch/step voltage gradients, and non-operation of overcurrent devices to limit the shock hazard. {It’s quite distracting to be jolted touching an electrical enclosure standing on dry soil—with ~277V between two pieces of electrical gear thirty feet apart.}

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#82778 - 12/12/02 07:50 PM Re: 250.30
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
250.4(A)(3) requires an equipment grounding conductor for this circuit. 250.30(A)(1) requires a bonding jumper sized per Table 250.66 to connect the grounded conductor to the equipment grounding conductors. I don't see a rule that says you have to bring the grounded conductor itself into the building, but you must bring a fault clearing path into the building.
Don
_________________________
Don(resqcapt19)

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#82779 - 12/13/02 03:14 AM Re: 250.30
Gwz Offline
Member

Registered: 04/29/02
Posts: 199
The reason I've seen the existing SDS is that a contractor is installing 4160V from the primary side (feed-thru) terminals to a new 150KVA 208/120 system some 1000' away.

How would be a definite method to determine that the GEC at the existing SDS does, in-fact, continue to the electrical equipment in the existing building?

Another question: Is the drain wire on the 3 - 5KV - 1/C's to the new transformer a sufficient (size) grounding conductor to/at the new 150KVA ?

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#82780 - 12/13/02 05:19 AM Re: 250.30
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
There is no requirement to extend the grounding electrode conductor from the transformer, but you must have a fault clearing path back to XO. This could be, in some cases, the grounded conductor, or an equipment bonding conductor.
In most cases the drain wires are exactaly that and are not large enough to be used as the required equipment grounding conductor for the primary side of the transformer.
Don
_________________________
Don(resqcapt19)

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#82781 - 12/13/02 08:27 AM Re: 250.30
tdhorne Offline
Member

Registered: 03/22/01
Posts: 344
Loc: Maryland, USA
 Quote:
There is no requirement to extend the grounding electrode conductor from the transformer, but you must have a fault clearing path back to XO. This could be, in some cases, the grounded conductor, or an equipment bonding conductor.
In most cases the drain wires are exactaly that and are not large enough to be used as the required equipment grounding conductor for the primary side of the transformer.
Don


Don
Isn't the absence of a specific requirement to bring the neutral of a SDS to the disconnecting means a big hole in the NEC. If the SDS is grounded at the transformer then the lack of a grounded conductor to the disconnecting means creates a truly hazardous condition. If this transformer were owned by the utility instead of the customer the provisions of 250.24 (B) would require that conductor's installation in the raceway with the secondary conductors. I cannot see how the transformer's ownership can be allowed to obviate so basic a requirement.

Some few of these transformers can be reconfigured into delta secondary configuration. If that is possible in this case the transformer and the circuits it feeds could be readily changed to a corner grounded delta system by changing the connections and installing listed fuse shunts if applicable. The handbook commentary makes it clear that 480/277 wye connected systems are particularly prone to destructive burn downs from ground faults so if the user does not need the 277 voltage for their application there is no good reason to install the system as Wye connected.
--
Tom
_________________________
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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#82782 - 12/13/02 10:11 AM Re: 250.30
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
Tom,
While the code doesn't require the grounded conductor to be brought to the building, it does require an equipment bonding conductor to be run into the building unless all of the building equipment grounding conductors are brought to the transformer. The equipment bonding conductor is sized per Table 250.66 using the size of the transformer secondary conductors. This is the same size that is required by 250.24(B)(1)for a grounded service conductor. The rule for SDS is not as clear as for services, but I think that the code really requires the same thing.
Don
_________________________
Don(resqcapt19)

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