My company was contracted to terminate 5kv and 15kv underground wire in pad mounted switchgear, 3 and 1 phase pad mount transformers and underground pull boxes. One of the transformers that was supplied for the project was a 5000 kva, 12470 Delta to 4160 Delta 3 phase pad mount. There was one (1) phase transformer feeding off of the 4160 side of the 5000 kva trans. The wire that was pulled from the 3 phase transformer to the single phase transformer was 5kv jacketed ribbon tape underground wire. There was also a #2 stranded bare cu. ground wire pulled in the same conduit. My question is, since the single phase transformer is 2400 to 240/120, do I need a dedicated nuetral wire pulled in along with the phase and the #2 ground wire? Is it legal per NEC to use the ground as the nuetral? If this not legal, I would apprecite anyone who could tell me where in the code book this could be found. Thanks
use of ground as a nnuetral
- Thread starter lineman
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Re: use of ground as a nnuetral
You said that you have a 5000 kva transformer
12.4kv delta to 4.16kv delta. You also say that the single phase transformer is 2400 volt to
120/240 volt. If the 5000 kva transformer has a delta secondary you do not have 2400 volts to ground. I suspect that the 4.16 kv is connected wye and not delta and is grounded.
Read 250.184
You said that you have a 5000 kva transformer
12.4kv delta to 4.16kv delta. You also say that the single phase transformer is 2400 volt to
120/240 volt. If the 5000 kva transformer has a delta secondary you do not have 2400 volts to ground. I suspect that the 4.16 kv is connected wye and not delta and is grounded.
Read 250.184
charlie
Senior Member
- Location
- Indianapolis
Re: use of ground as a nnuetral
Is this a 12470Y-4160/2400 volt transformer? Is this outside? Making the assumption this is true, the single phase 5kV cable is a shielded cable and needs a grounded conductor pulled in with it, not a grounding conductor. Therefore, the #2 stranded bare Cu. will be the grounded conductor and must be used to ground at the elbow terminator as well as the transformer case and the X0 bushing.
Since the single phase pad mounted transformer is outside (my assumption), the circuit will be run in the same manner as an electric utility would run the circuit.
Is this a 12470Y-4160/2400 volt transformer? Is this outside? Making the assumption this is true, the single phase 5kV cable is a shielded cable and needs a grounded conductor pulled in with it, not a grounding conductor. Therefore, the #2 stranded bare Cu. will be the grounded conductor and must be used to ground at the elbow terminator as well as the transformer case and the X0 bushing.
Since the single phase pad mounted transformer is outside (my assumption), the circuit will be run in the same manner as an electric utility would run the circuit.
charlie
Senior Member
- Location
- Indianapolis
Re: use of ground as a nnuetral
The answers I got from a private message were 1. No, the 5000 kva transformer is not 12470 wye, but 12470 delta, 4160 delta. 2. Yes, oil filled outdoor, pad mount.
Bennie, the cable assembly is (I think) a #2 Cu. 5kV shielded/jacketed solid dialectric (probably XLP) cable that needs a neutral or ground return.
Lineman, I assume your system voltage is 12470Y/7200 volts and is feeding the primary of a 12470 volt delta transformer. Even with a MGN (multiple grounded neutral) you will need a solid grounding path back to the source. The #2 bare Cu. will provide that return path in case of a ground fault. The #2 stranded bare Cu. will be the grounding conductor and must be used to ground at the elbow terminator as well as the transformer case and the X0 bushing.
Since the single phase pad mounted transformer is outside, the circuit will be run in the same manner as an electric utility would run the circuit except we would never call the #2 bare Cu. a grounding conductor.
The part I am confused about is the secondary of the 5000 kVA transformer, is it really a 4160 volt delta? If that is the case, the primary of the single phase transformer must be 4160 volts as well and you will need two 5kV cables run as well as the #2 bare grounding conductor. Of course, if you use concentric neutral primary, you can skip the extra grounding conductor.
The answers I got from a private message were 1. No, the 5000 kva transformer is not 12470 wye, but 12470 delta, 4160 delta. 2. Yes, oil filled outdoor, pad mount.
Bennie, the cable assembly is (I think) a #2 Cu. 5kV shielded/jacketed solid dialectric (probably XLP) cable that needs a neutral or ground return.
Lineman, I assume your system voltage is 12470Y/7200 volts and is feeding the primary of a 12470 volt delta transformer. Even with a MGN (multiple grounded neutral) you will need a solid grounding path back to the source. The #2 bare Cu. will provide that return path in case of a ground fault. The #2 stranded bare Cu. will be the grounding conductor and must be used to ground at the elbow terminator as well as the transformer case and the X0 bushing.
Since the single phase pad mounted transformer is outside, the circuit will be run in the same manner as an electric utility would run the circuit except we would never call the #2 bare Cu. a grounding conductor.
The part I am confused about is the secondary of the 5000 kVA transformer, is it really a 4160 volt delta? If that is the case, the primary of the single phase transformer must be 4160 volts as well and you will need two 5kV cables run as well as the #2 bare grounding conductor. Of course, if you use concentric neutral primary, you can skip the extra grounding conductor.
charlie
Senior Member
- Location
- Indianapolis
Re: use of ground as a nnuetral
Again I got answers from a private message. The phase conductors range in size from 1000 mcm, 500 mcm, 350 mcm, and 1/0 mcm epr insulation. Ribbon tape shield, not concentric. Yes, the transformer is realy 4160 delta on the secondary side, and yes the single phase transformer is really a 2400 to 240/120. I would like to find in the code book where it states the type and size of the neutral to be used in this type of system.
Lineman, the system will not work! You need 2400 volts to power the primary of your transformer and you only have 4160. Also, I was thinking about the statement I made last night and realized I was not correct in my statement. "you will need two 5kV cables run as well as the #2 bare grounding conductor." is not correct. However, you will need two 5kV cables run to a 4160-240/120 volt transformer. A neutral or grounding from the 5000 kVA transformer is not required; however, grounding will be required at the single phase transformer.
Don't forget to ground the sheath of the primary cable and the semicon at the elbows.
Again I got answers from a private message. The phase conductors range in size from 1000 mcm, 500 mcm, 350 mcm, and 1/0 mcm epr insulation. Ribbon tape shield, not concentric. Yes, the transformer is realy 4160 delta on the secondary side, and yes the single phase transformer is really a 2400 to 240/120. I would like to find in the code book where it states the type and size of the neutral to be used in this type of system.
Lineman, the system will not work! You need 2400 volts to power the primary of your transformer and you only have 4160. Also, I was thinking about the statement I made last night and realized I was not correct in my statement. "you will need two 5kV cables run as well as the #2 bare grounding conductor." is not correct. However, you will need two 5kV cables run to a 4160-240/120 volt transformer. A neutral or grounding from the 5000 kVA transformer is not required; however, grounding will be required at the single phase transformer.
Don't forget to ground the sheath of the primary cable and the semicon at the elbows.
Re: use of ground as a nnuetral
I just made an interesting observation; The ANSI Standard C-57.12.26-1987 does not list a pad mounted transformer with a wye 4160/2400 volt secondary. The listing is only straight 4160. I would assume this is a delta connected secondary.
Although a floating wye would also qualify.
I just made an interesting observation; The ANSI Standard C-57.12.26-1987 does not list a pad mounted transformer with a wye 4160/2400 volt secondary. The listing is only straight 4160. I would assume this is a delta connected secondary.
Although a floating wye would also qualify.
charlie
Senior Member
- Location
- Indianapolis
Re: use of ground as a nnuetral
It is very difficult for me to think in the terms of a delta since almost everything we do is with wye configurations.
Bennie, that is very interesting since we are still converting a few patches of Indianapolis (Go Colts) from 4160 volts to 13.2kV. Some of our old primary customers are served with pole mounted transformer banks to convert the 13.2 kV back to 4160 volts for those customers; from the overhead banks, we furnish 4160Y/2400 volts.
It is very difficult for me to think in the terms of a delta since almost everything we do is with wye configurations.
Re: use of ground as a nnuetral
Charlie: The pole mounted transformers can be connected for 4160/2400 secondary. This, of course, is due to individual pots.
Apparently common core dry transformers (pad mounted) are only straight 4160.
I am curious now, to find out more about this situation. Oh well, back to school and the books
Charlie: The pole mounted transformers can be connected for 4160/2400 secondary. This, of course, is due to individual pots.
Apparently common core dry transformers (pad mounted) are only straight 4160.
I am curious now, to find out more about this situation. Oh well, back to school and the books
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