Medium Voltage underground feeders

[mshields]

For a 5KV or 15kV (anything utilizing shielded cable) underground distribution system, given that the cable is shielded, is a ground conductor required. If the answer is yes, table 250-122 is the applicable reference is it not. i.e. it is independent of voltage correct?

 

[charlie b]

I don't believe the shield is intended to perform the function of either a grounding electrode conductor (GEC) or an equipment grounding conductor (EGC). If the installation requires either of these wires, the fact that you are at 5kV or 15kV does not alter the situation. Table 250.122 would be the right source, if the installation requires an EGC. Table 250.66 would be the right source, if the installation requires a GEC. Voltage is irrelevant, as you correctly suggest.

There are installations that require neither. For example, if you bring a 4160V service into a building from the secondary of a 34kV-4160V transformer, with the main service panel inside the building rated at 4160V, and with the N-G bond located in the main service panel, then the service conductors would not include a ground wire of either the EGC or the GEC description. On the other hand, if you run a feeder from one building to another at 4160V, then you need to include an EGC sized per 250.122.

 

[mshields]

thanks

thanks

very much

 

[ron]

There are installations that require neither. For example, if you bring a 4160V service into a building from the secondary of a 34kV-4160V transformer, with the main service panel inside the building rated at 4160V, and with the N-G bond located in the main service panel, then the service conductors would not include a ground wire of either the EGC or the GEC description.

Charlie,
Is that because you would be bringing a grounded conductor in with the three phases and it essentially serves "double duty"?

 

[GoldDigger]

Charlie,
Is that because you would be bringing a grounded conductor in with the three phases and it essentially serves "double duty"?

The biggest difference is that the MV in Charlie's example would be part of the POCO service, and so no GEC/EGC is needed or even appropriate.
If POCO is supplying only a delta, there would be no neutral. It could be an ungrounded delta or a corner grounded delta as far as NEC is concerned. Any neutral loads would have to be fed from a customer transformer from there. As a result, the building wiring system GES and building wiring system EGC would connect at the SUSE panel. A neutral to ground bond at that point would be somewhat problematic, since there can be no neutral except downstream of a transformer which creates an SDS.
If POCO is delivering a 4-wire wye service, centerpoint grounded and capable of supplying line to neutral loads, (normally done at MV?) then POCO will have to supply the neutral conductor to provide a return connection for those loads. That conductor does not serve a grounding function for the NEC side, even though it is a grounded conductor.

 

[don_resqcapt19]

See 250.190(C)(2)

(2) Shielded Cables. The metallic insulation shield encircling the current carrying conductors shall be permitted to be used as an equipment grounding conductor, if it is rated for clearing time of ground fault current protective device operation without damaging the metallic shield. The metallic tape insulation shield and drain wire insulation shield shall not be used as an equipment grounding conductor for
solidly grounded systems.

 

[mbrooke]

In poco the shields act as combined ground/neutral conductors. Even when the MV is fed via an ungrounded source grounding and bonding of the cable still applies.

 

[don_resqcapt19]

In poco the shields act as combined ground/neutral conductors. Even when the MV is fed via an ungrounded source grounding and bonding of the cable still applies.

That would most likely be concentric neutral cable, not just shielded cable. The concentric neutral serves as both the shield and the grounded conductor. As I recall the size of the concentric neutral can be either the same size as the ungrounded conductor or 1/3 of that size. The full size one being used on single phase circuit and the 1/3 size one used on 3 phase circuits.

 

[mbrooke]

That would most likely be concentric neutral cable, not just shielded cable. The concentric neutral serves as both the shield and the grounded conductor. As I recall the size of the concentric neutral can be either the same size as the ungrounded conductor or 1/3 of that size. The full size one being used on single phase circuit and the 1/3 size one used on 3 phase circuits.

You are right, I was thinking of neutral concentric EPR and XLPE cables. As you mentioned the the 1/3 goes for 3 phase runs the full size on single phase spurs.

 

[Hv&Lv]

That would most likely be concentric neutral cable, not just shielded cable. The concentric neutral serves as both the shield and the grounded conductor. As I recall the size of the concentric neutral can be either the same size as the ungrounded conductor or 1/3 of that size. The full size one being used on single phase circuit and the 1/3 size one used on 3 phase circuits.

Correct. We generally use only full neutral CN cable, except for substation feeders. There we use 1/3 neutral. The cost of full neutral on the smaller CN cable is slightly higher, ( sometimes not) but the costs of stocking two different types for three phase of single phase installations, along with the aggravation aren't worth any potential savings.