Transformers to step up then back down-grounding of higher voltage

[David Rogers]

We are discussing, in our office, the need of a ground wire when using a transformer to take 240v, step it up to 600v, run 1300 feet, then use a transformer to step the 600v down to 120/240, 3 wire service that will feed a residence. In the conduit that is carrying the 600v, do we need a ground wire or bond wire? It seems to me we establish a new ground at the 120/240v transform and no ground wire would be required, but we are disagreeing on that in our office.

Thanks

 

[charlie b]

I think you may be confusing two separate contexts of the word, "ground." The wires that bring power to the downstream transformer constitute a feeder, and as such they must include an Equipment Grounding Conductor. At the secondary of the transformer, you need to run a Grounding Electrode Conductor from the center of the windings to planet Earth. Start using those two terms, instead of the phrase "ground wire," and see if it resolves the disagreement.

 

[augie47]

If your 600 volt source (transformer) is not grounded, then, IMO, no equipemt ground would need to be installed, however, 250.21(B) would require ground detection.

 

[don_resqcapt19]

If your 600 volt source (transformer) is not grounded, then, IMO, no equipemt ground would need to be installed, however, 250.21(B) would require ground detection.

The only difference in the grounding and bonding rules for grounded and ungrounded systems is that the ungrounded system does not have a system or main bonding jumper.

 

[augie47]

The only difference in the grounding and bonding rules for grounded and ungrounded systems is that the ungrounded system does not have a system or main bonding jumper.

That said, do you see the need for an "equipment ground" to be run with the conductors to the second transformer ? It would serve as a bond, but with the 600v supply being ungrounded, I'm not sure i see the need.

 

[RB1]

With nonmetallic raceways between the transformers, only two wires are needed. A system bonding jumper could be installed at each transformer location per 250.30(A)(1) Exception No. 2. In fact, with a properly sized overcurrent device on the primary of the first transformer that is lockable in the open position, a disconnecting means in the 600 volt portion of the installation is not needed. A grounding electrode is required at each transformer where the transformers are located outdoors

 

[iwire]

That said, do you see the need for an "equipment ground" to be run with the conductors to the second transformer ? It would serve as a bond, but with the 600v supply being ungrounded, I'm not sure i see the need.

With an ungrounded system you still want to hold all the equipment at the same potential. In case you are touching two pieces of equipment at the same time when there is a problem.

Of course in this particular case you would need some freakishly long arms.

 

[texie]

With nonmetallic raceways between the transformers, only two wires are needed. A system bonding jumper could be installed at each transformer location per 250.30(A)(1) Exception No. 2. In fact, with a properly sized overcurrent device on the primary of the first transformer that is lockable in the open position, a disconnecting means in the 600 volt portion of the installation is not needed. A grounding electrode is required at each transformer where the transformers are located outdoors

I disagree. An EGC is required to be run regardless of the non metallic conduit to bond the metallic xformer enclosure and any other metallic items that could be energized by the feeder supplying the xformer.

 

[RB1]

Texie,

Your right. 250.30(A)(1) Exception No. 2 permits a system bonding jumper at both the source and the first disconnecting means where a parallel path with grounded conductor is not established. Therefore to use that exception there would have to be a disconnecting means before the step-down transformer. But you would only be required to have two conductors between the step-up transformer and the disconnecting means at the second transformer. Otherwise, what does the exception mean?

 

[don_resqcapt19]

That said, do you see the need for an "equipment ground" to be run with the conductors to the second transformer ? It would serve as a bond, but with the 600v supply being ungrounded, I'm not sure i see the need.

It is required by the code and is part of the fault clearing path when there are two ground faults at different locations.

 

[don_resqcapt19]

Texie,

Your right. 250.30(A)(1) Exception No. 2 permits a system bonding jumper at both the source and the first disconnecting means where a parallel path with grounded conductor is not established. Therefore to use that exception there would have to be a disconnecting means before the step-down transformer. But you would only be required to have two conductors between the step-up transformer and the disconnecting means at the second transformer. Otherwise, what does the exception mean?

We are talking about an ungrounded system and 250.30(A) only applies to grounded systems.

 

[RB1]

Don,

I am recommending grounding the 600 volt system. This avoids the need for ground detectors, and actually provides a method for eliminating the need for a supply-side bonding jumper (250.30(B)(2) 2011 NEC) between the first transformer and the disconnecting means for the second transformer. I just saved 1300 feet of wire and a ground detection system. What would you gain by operating this as an ungrounded system?

 

[don_resqcapt19]

Don,

I am recommending grounding the 600 volt system. This avoids the need for ground detectors, and actually provides a method for eliminating the need for a supply-side bonding jumper (250.30(B)(2) 2011 NEC) between the first transformer and the disconnecting means for the second transformer. I just saved 1300 feet of wire and a ground detection system. What would you gain by operating this as an ungrounded system?

I guess I missed that.

 

[texie]

Don,

I am recommending grounding the 600 volt system. This avoids the need for ground detectors, and actually provides a method for eliminating the need for a supply-side bonding jumper (250.30(B)(2) 2011 NEC) between the first transformer and the disconnecting means for the second transformer. I just saved 1300 feet of wire and a ground detection system. What would you gain by operating this as an ungrounded system?

Yes, I think your solution is a good way to go. As a practical matter the 600 volt needs to be grounded anyway and you still end up with a safe and compliant installation while saving all that wire. Good call.