12.47KV Overhead Line

[Shoe]

I have a customer-owned 12.47KV overhead line ((3) #2 ASCR), which is primary-metered from the utility, that I need to bring underground to a exterior load interrupter switch to enter the facility and feed an indoor dry-type transformer.

I will be transitioning the overhead feeder to a pole-mounted airbreak switch to an undeground feed using EPR conductors.

Question: Does the EPR conductors require a concentric neutral to act as the grounded service conductor between the overhead line and the load interrupter switch? I plan to install a standard ground from the load-interrupter switch location to the interior of the facility but wanted opinions on how I needed to handle the conductors between the overhead line and load interrupter switch.

Thanks.

 

[Julius Right]

I don't know what electric code you intend to follow but I think it is not an EPR insulated conductor you are speaking about but a medium voltage cable of 15 kV rated[?] .The cable is provided with a shield or wire screen around the conductor insulation in order to confine the electric field radial in the insulation. You have to ground this shield-at least at one end- and you need to use cable terminating in order to seal the cable end[at both ends].You may employ the shield as a grounding conductor if your electric code permits this. In this case a circulating current will flow through the shield. This current will produce heat and a derating factor for the main current flowing in the conductor will be required.

 

[GoldDigger]

You may employ the shield as a grounding conductor if your electric code permits this. In this case a circulating current will flow through the shield.

Do you mean grounding conductor or grounded circuit conductor / neutral (or will both be the same thing in an installation at this voltage?).

 

[Julius Right]

Do you mean grounding conductor or grounded circuit conductor / neutral (or will both be the same thing in an installation at this voltage?).

Good question, GoldDigger. I think as Equipment Grounding Conductors as per NEC Art.250.118 Types of Equipment Grounding Conductors [for instance].

 

[GoldDigger]

Good question, GoldDigger. I think as Equipment Grounding Conductors as per NEC Art.250.118 Types of Equipment Grounding Conductors [for instance].

If it is just serving as an EGC I would not expect it to carry enough current to affect thermal derating

 

[Julius Right]

If it is just serving as an EGC I would not expect it to carry enough current to affect thermal derating

You are right. In the case of three-conductor cable or triplexed of less than 500 MCM copper conductor the derating can be neglected. Also it depends on grounding resistance at both ends of the cable of course.

 

[Shoe]

Thanks for the replies. I think my primary concern is whether or not the typical shield on a shielded cable would be sufficient coming from the overhead line to the pad-mounted switch or if a concentric neutral would need to be considered as it pertains to the purposes of providng a grounded conductor.

 

[Julius Right]

If it is a usual cable-MV cable ?you cannot use the shield as EGC. If it is an AC, MI or MC cable you may use it as EGC as per art.250.118(8),(9),(10).
In any case the minimum cross-section has to be as per art.250.122. In solidly grounding system only if the cable is provided with concentric neutral of suitable cross-section.