480v Corner Grounded Service Drop

[frankbrown]

Scenario: An industrial contractor for a water plant in Kansas has a 480v corner grounded panel. The panel is fed from a 277/480v wye URD padmount transformer on the utility side. The URD padmount transformer has a primary meter assembly, which in turn eliminates NESC jurisdiction from the secondary side of the meter base. The service conductor that is fed from the secondary side of the meter base to the primary side of the industrial load?s 480v corner grounded panel is a 3 wire service conductor w/o a ground. The utility prefers that the service conductor have a ground added to the circuit up to the industrial panel for, if nothing else, a sound peace of mind. The industrial contractor does not want to pull a ground with this service conductor and mentioned that it is not required by NEC because the load is 480v w/o a neutral.



Assumption: My assumption is that the utility is not held accountable for anything past the meter base due to NESC Standards. However, the contractor must pull the fourth wire and ground the 277/480v feed transformer from the secondary side of the meter base to the 480v corner grounded industrial panel (in specific reference to Articles 408 & 409 of the 2005 NEC). From what I would assume, a second step-down transformer would then change the voltage from 277/480v Y to 480v Delta at the industrial panel, of which a ground would not be needed from that point on. The service conductor would require that the 4th wire be pulled in, instead of leaving the neutral ?floating?, and a bounded ground would have to be placed at the industrial panel for this 277/480v Y service conductor.

Thanks for the input ahead of time!!!

 

[jii]

Re: 480v Corner Grounded Service Drop

250.24(C) requires the grounded conductor to be taken to the service. 2005 NEC


250.24(C) Grounded Conductor Brought to Service Equipment. Where an ac system operating at less than 1000 volts is grounded at any point, the grounded conductor(s) shall be run to each service disconnecting means and shall be bonded to each disconnecting means enclosure. The grounded conductor(s) shall be installed in accordance with 250.24(C)(1) through (C)(3).

 

[thinfool]

Re: 480v Corner Grounded Service Drop

I had something similar some years ago. The AHJ had me pull the code sized grounding conductor from the grounded wye at the utility xfmr to the equipment ground in the customer's switchgear. The equipment ground was also grounded at the building entrance.

The specifics....3-500mcm line conductors and a #2 ground conductor.

 

[infinity]

Re: 480v Corner Grounded Service Drop

In the original post isn't the corner grounded phase conductor actually the grounded conductor? If so, why would another grounded conductor be needed?

 

[paul]

Re: 480v Corner Grounded Service Drop

Originally posted by infinity:
In the original post isn't the corner grounded phase conductor actually the grounded conductor? If so, why would another grounded conductor be needed?

Yes. And I'm wondering the same thing.

 

[don_resqcapt19]

Re: 480v Corner Grounded Service Drop

Frank,
This appears to be an SDS because of the primay metering. If so you must follow the rules in 250.30. This may require the installation of an EGC between the transformer and the panel. This assumes that XO is not terminated at the transformer and that one of the phase conductors is grounded at some point.
Don

 

[charlie]

Re: 480v Corner Grounded Service Drop

The meter is not service equipment and does nothing to the service point. The question is, where does the electric utility's ownership of the facilities end? We have several primary metered customers that do not control the primary or the transformers. We rent the facilities to them and maintain control.

Going further, assuming the X0 bushing of the 480Y/277 volt PMT can be lifted, it is not required to be taken. At that point a phase is permitted to be grounded and a 3? corner grounded delta service may be taken into the building. This applies whether it is an electric utility supplied service or a customer's SDS.

Edited to remove my opening stupid comment. :roll:

[ April 28, 2005, 06:51 AM: Message edited by: charlie ]

 

[mc5w]

Re: 480v Corner Grounded Service Drop

If the system is corner grounded and the service is single ended then you would install a bond from the grounded conductor to ground at both the transformer and the service switch. An equipment ground would create a parallel path for normal current.

If the service is double ended then you would have a single bonding jumper at the tie breaker or tie point and then use equipment grounding conductors to ground the transformers and service switches.

 

[jii]

Re: 480v Corner Grounded Service Drop

From what I read ,the system is a wye configuration. You cant turn it into a grounded delta, unless you change out the transformer. So you must pull the grounded conductor in. 250.24(c) .

Going further, assuming the X0 bushing of the 480Y/277 volt PMT can be lifted, it is not required to be taken. At that point a phase is permitted to be grounded and a 3? corner grounded delta service may be taken into the building.

I don?t understand this,could you explain

John.

 

[charlie]

Re: 480v Corner Grounded Service Drop

IF the X0 can be lifted and the neutral is not taken then there will only be three phases available to be used for the service. Once they are outside the transformer, you can not tell if it is a delta or a wye connection with any type of instrumentation. The code permits these three conductors to be used as a delta by using them as an ungrounded service or as a corner grounded service (or SDS).

 

[jii]

Re: 480v Corner Grounded Service Drop

Thanks, I had know idea you could do this with a pad mount delta/wye XFMR.


But unless you truley do change this into a delta, you must pull the grounded conductor.
cheers

 

[frankbrown]

Re: 480v Corner Grounded Service Drop

Thank you for the help fellow electrical workers. I have passed on all of your comments and expertise to the Utility Line Superintendent. I think his question has been answered, which is that the contractor DOES need to pull in the 4th (ground) wire. Much appreciated on my end for your help and guidance!

Frank Brown

 

[charlie]

Re: 480v Corner Grounded Service Drop

But unless you truly do change this into a delta, you must pull the grounded conductor.

You don't really change it into a delta but you are permitted to treat it as a delta if the neutral is not grounded or taken out as a circuit conductor. Most electric utilities will not use this option and they do ground the neutral. Once that is done, 250.24(B) in the NEC as well as the rules in the NESC require the neutral be taken and the minimum size is also in accordance with 250.24(B).