Grounded Conductor for 15KV Sevices

Status
Not open for further replies.

jwilson

Member
For a three phase 15kv underground service(customer installed) from the utility service pole, is a separate "grounded conductor" required by the NEC as it is for "less than 1000 volt" services? The cable shields are not approved as a grounded conductor.
 

Nick

Senior Member
Re: Grounded Conductor for 15KV Sevices

You say this is customer installed from a utility pole. Where is this system metered? The rules change depending on what side of the meter or service point we are talking about. How is the system you are connecting to configured? Is it ungrounded? Is it a common neutral muti grounded system?
250.24(B) specifically states it applies to grounded systems operating at less than 1000 volts. It does not apply in your case. Can you supply a little more info on the system you are dealing with?
 

jwilson

Member
Re: Grounded Conductor for 15KV Sevices

Originally posted by Nick:
You say this is customer installed from a utility pole. Where is this system metered? The rules change depending on what side of the meter or service point we are talking about. How is the system you are connecting to configured? Is it ungrounded? Is it a common neutral muti grounded system?
250.24(B) specifically states it applies to grounded systems operating at less than 1000 volts. It does not apply in your case. Can you supply a little more info on the system you are dealing with?
 

jwilson

Member
Re: Grounded Conductor for 15KV Sevices

The system is 12470 v 3phase 4 wire multigrounded neutral on utility side with the metering before the service pole. The load is 3 phase 3 wire (no single phase loads) feeding five substations with delta wound primary.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: Grounded Conductor for 15KV Sevices

I see no need for a grounded conductor, but you do need an EGC. The cable shield is often too small to serve as the EGC.
Don
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Grounded Conductor for 15KV Sevices

Don, I am no HV expert, but doesn't the shield act as protection from a flash-over in the event of insulation failure?
 

jwilson

Member
Re: Grounded Conductor for 15KV Sevices

I agree with Don that an EGC is needed, but I'm trying to logically work my way thru the NEC to present arguments to the designer, who relys on NEC 250.24(B, which states a grounded conductor is required for up to 1000 volts, thereby excluding a 15KV system.
Where does the Code require a grounded cond, or EGC for systems above 1000 volts?
 

bennie

Esteemed Member
Re: Grounded Conductor for 15KV Sevices

Systems of this voltage are engineering design issues, the NEC does not apply.

Consult the NESC. The requirements for UD systems are more stringent than the NEC.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: Grounded Conductor for 15KV Sevices

JW,
Look at 250.180 and 250.190. The shield may or may not be large enough to act as the EGC. The printed version of Southwire Power Cable Handbook has some extensive calculations that you can do to see if the ampacity of the shield is high enough to safely clear the fault. Unfortunately that section of the handbook is not available online.

Dereck,
The shield is intented to eliminate tracking and arcing discharges along the cable surface and to prevent a hazardous potential from accumulating on the surface of insulation. You can find some information in Southwire's Power Cable Handbook near the bottom of the page.

Bennie,
The NEC does cover this installation, but I agree it does not do so in much detail and that the NESC and engineering sources should also be consulted.

Don
 

bennie

Esteemed Member
Re: Grounded Conductor for 15KV Sevices

PGE, SoCal Edison, PP@L, and many other utility companies retain authority on feeders of this voltage. It is a legal liability concern.

The NESC requires a burial depth greater than the NEC. A cable buried under the NEC will be a liability for the owner, in the event of problem.

My statement about not being within the jurisdiction of the NEC, is correct in many areas.
Regardless of ownership.
 

bennie

Esteemed Member
Re: Grounded Conductor for 15KV Sevices

To get back to the original topic.

The MGN must be complete to each transformer. The MGN must connect to the tank and X-O of the secondary.

Multi ground neutral to be grounded with an electrode at transformer.

The semi-con shield on medium voltage cable is for containing the energy field produced by the magnetic field.

[ March 20, 2003, 02:03 PM: Message edited by: bennie ]
 

Nick

Senior Member
Re: Grounded Conductor for 15KV Sevices

PGE, SoCal Edison, PP@L, and many other utility companies retain authority on feeders of this voltage. It is a legal liability concern.
Bennie,
Can you explain this statement? I have been involved in a number of medium voltage campus distribution systems and am doing one right now. The utilities could care less what happens on the load side of the service gear.
 
Status
Not open for further replies.
Top