Use of neutral to connect single pahse loads in Motor Control Center

[nanjangud]

We have installed a 3 phase, 3 wire, 480 V motor control center. This motor control center is fed from a 1600 kVA transformer (primary: delta and secondary: star), with a voltage ratio of 4160 V/480 V and we have a high resistance grounding system. Therefore the neutral bus is not run inside the MCC.
My query is as:
I have a single phase loads requiring 277 V supply. I would like to know whether I can form a neutral bus inside the MCC and provide 277 V from one of the phase and this neutral (which is a high resistance grounding)?
Is this permitted by NEC or any other standard?

Your early response will be appreciated.

Thank you in advance.

Sudheendra

 

[roger]

Are you saying the secondary side (the 480 wye) is impedance grounded?

If it is I must ask why?

Roger

 

[markstg]

We have installed a 3 phase, 3 wire, 480 V motor control center. This motor control center is fed from a 1600 kVA transformer (primary: delta and secondary: star), with a voltage ratio of 4160 V/480 V and we have a high resistance grounding system. Therefore the neutral bus is not run inside the MCC.
My query is as:
I have a single phase loads requiring 277 V supply. I would like to know whether I can form a neutral bus inside the MCC and provide 277 V from one of the phase and this neutral (which is a high resistance grounding)?
Is this permitted by NEC or any other standard?

Your early response will be appreciated.

Thank you in advance.

Sudheendra

The NEC does not allow Line to nuetral loads to be served from a HRG System.

 

[raider1]

The NEC does not allow Line to nuetral loads to be served from a HRG System.

Agreed.

Check out 250.20(B) for where a system must be grounded.

Chris

 

[Smart $]

The NEC does not allow Line to nuetral loads to be served from a HRG System.

250.36... (3) Line-to-Neutral loads are not served.

 

[don_resqcapt19]

Are you saying the secondary side (the 480 wye) is impedance grounded?

If it is I must ask why?

Roger

Why not? For industrial applicaitons it provides the beifits of both grounded and ungrounded systems without the disadvantage of either system. The only real disadvantage is brought out by the question in this thread. You can't supply line to neutral loads from this system.

 

[roger]

Why not?

Don, I didn't say there was a problem with it, I asked why especially since the OP says there are 277 V loads

The only real disadvantage is brought out by the question in this thread. You can't supply line to neutral loads from this system.

Which is a pretty major disadvantage as far as this thread is concerned and in fact it proves that it doesn't provide all of the benefits of a solidly grounded system.

Roger

 

[markstg]

Don, I didn't say there was a problem with it, I asked why especially since the OP says there are 277 V loads



Which is a pretty major disadvantage as far as this thread is concerned and in fact it proves that it doesn't provide all of the benefits of a solidly grounded system.

Roger

By having the HRG it keeps all my equipment up and running while there is a line to ground fault. Without a HRG, all, most, some of the loads will be lost on a line to ground fault. Many facilities like this feature so much, that if 277V loads need to be served, 480 to 480/277 transformers are installed to serve the 277V loads.

 

[don_resqcapt19]

Which is a pretty major disadvantage as far as this thread is concerned and in fact it proves that it doesn't provide all of the benefits of a solidly grounded system.

Roger

If this MCC supplies industrial process equipment that I want to keep running, I don't see the installation of a transformer to serve line to neutral loads as that big of a disatvantage.

 

[roger]

I don't see the installation of a transformer to serve line to neutral loads as that big of a disatvantage.

Nor do I except possibily for space but, if the 277 V loads were known to exist I still ask the question why the impedance grounding in the OP's case.

Room

 

[Electric-Light]

What is the 277v load? If it's lighting or small controls you can use a transformer like this

http://genet.geappliances.com/LightProducts/images/t04/0000000/r00502v-1.pdf

 

[markstg]

Nor do I except possibily for space but, if the 277 V loads were known to exist I still ask the question why the impedance grounding in the OP's case.

Room

You impedance ground so that on a Line to Ground Fault (the most common type of fault) the circuit breaker will not trip since you have limited the fault current to nominally 5 or 10 Amps with the impedance in the ground to nuetral circuit. Since the circuit breakers haven't tripped, my 3 phase loads keep operating since they continue to see the correct line to line voltage.

If there wasn't an impedance in the ground to nuetral circuit, a large fault current would occur on a line to ground fault and the circuit breaker would trip taking out all downstream equipment from that circuit breaker. This is a very good thing in industrial installations where tripping alot of pumps would cost many thousands of $ per day if allowed.

 

[markstg]

What is the 277v load? If it's lighting or small controls you can use a transformer like this

http://genet.geappliances.com/LightProducts/images/t04/0000000/r00502v-1.pdf

Usually the 277V loads are lighting. The product you point to is an AutoTransformer, and would not work on a HRG system since it is not isolated from the source. What you would normally see is a 480V 3pole CB, feeding a 480V delta primary/480V-277V Wye secondary standard transformer of KVA requied for the loads, where that transformer feeds a circuit breaker panelboard with 1Pole CB's.

 

[Electric-Light]

Usually the 277V loads are lighting. The product you point to is an AutoTransformer, and would not work on a HRG system since it is not isolated from the source. What you would normally see is a 480V 3pole CB, feeding a 480V delta primary/480V-277V Wye secondary standard transformer of KVA requied for the loads, where that transformer feeds a circuit breaker panelboard with 1Pole CB's.

Is the use of auto-transformer prohibited on a HRG? It maybe HRG, but not ungrounded. So, unless specifically prohibited, and the TS is just trying to run a few fluorescent fixtures(that's what I suspect, since HID ballasts are available in 480v)

If transformer wouldn't work out, Advance does make F54T5HO ballasts rated to work on 480v.

HRG is used in datacenters and such to protect generators and UPS from crowbar shorts caused by ground fault

 

[roger]

You impedance ground so that on a Line to Ground Fault (the most common type of fault) the circuit breaker will not trip since you have limited the fault current to nominally 5 or 10 Amps with the impedance in the ground to nuetral circuit. Since the circuit breakers haven't tripped, my 3 phase loads keep operating since they continue to see the correct line to line voltage.

If there wasn't an impedance in the ground to nuetral circuit, a large fault current would occur on a line to ground fault and the circuit breaker would trip taking out all downstream equipment from that circuit breaker. This is a very good thing in industrial installations where tripping alot of pumps would cost many thousands of $ per day if allowed.

Mark, you're missing the point of my question. I am very aware of the reasons for ungrounded and impedance grounded systems so you don't have to keep explaining it.

I am well versed in industrial and MV installations, they are large parts of my career, my question was aimed at why the need for the impedance grounded system in the OP's case.

Do you think just because an MCC is in the equation an impedance or ungrounded system is warranted?

We install many more MCC's fed from grounded supplies than we do from ungrounded or impedance grounded systems and we do a large amount of work in paper mills, older textile mills, manufacturing plants, etc....

Roger

 

[rcwilson]

In my opinion, high resistance grounding, when properly maintained, is safer than solid grounding at 480V/277.

If the facility has 1000 kVA of 480V motors and 25 kVA of 277 V lighting it is more economical to use HRG, run a 3-phase 3-wire feeder to all the MCC's and install one or two 480-480/277V transformers for the lighting and 277 V heating circuits.

The isolation transformers also knock down the 277V system short circuit current and usually reduce the arc flash levels on the 277 V system. The HRG does not reduce the calculated arc flash level on teh 480V system but it almost eliminates the arc from a ground fault and greatly minimizes the risk of a GF escalating into a phase-phase fault.

These advantages are in addition to the continuity of service a continuous production facility gets when using an HRG system. It's not for everyone but it is a good choice for many industrial plants.

If you have a 480V 3P-3W system and need 277V, install a 480V 3 pole beaker, a 480-480/277 V transformer and a 480/277 panel.

 

[markstg]

Mark, you're missing the point of my question. I am very aware of the reasons for ungrounded and impedance grounded systems so you don't have to keep explaining it.

I am well versed in industrial and MV installations, they are large parts of my career, my question was aimed at why the need for the impedance grounded system in the OP's case.

Do you think just because an MCC is in the equation an impedance or ungrounded system is warranted?

We install many more MCC's fed from grounded supplies than we do from ungrounded or impedance grounded systems and we do a large amount of work in paper mills, older textile mills, manufacturing plants, etc....

Roger

Sorry Roger....I did miss your point. Putting in an HRG is a design decision and I ASSumed because it was there that the facility felt the HRG was preferred over a solidly grounded system.

I do not think just because an MCC is in the equation that an HRG is warrented, and as you say, many installations would prefer to have a solidly grounded system, rather than having an HRG and accepting the negatives of an HRG installation.

I don't know why the op's facility designed an HRG over a solidly grounded system.