Sizing NGR Neutral Cable

[rockman7892]

Looking for guidance for approach to sizing neutral cable between zig zag transformer and NGR. An NGR with 1500A rating @ 5s and 50A continuous rating is being connected to neutral point of zig-zag transformer on 34.5kV system.


The NEC requires neutral cable between transformer and NGR to be rated at let-thru current rating of resistor.

Is typically considered based on the short time or continuous current rating of resistor? I’m assuming it’s based on maximum short time rating which in this case is 1500A?

Is the cable continuous ampacity typically matched to the NGR current rating or can a cable with smaller continuous rating be applied if cable short time damage curve is sufficient?

 

[9wearSuave]

I believe the code is referring to NGR let thru current, not continuous rating. Obviously you could plot different cables in a TCC curve and see that you’d probably be able to get by with a smaller cable but the code is code unfortunately.

 

[waross]

If your cable acts as a fuse, then you have lost your grounding.
The same may be said about a failure of the NGR, but we generally try to avoid adding failure points.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[rockman7892]

Is the grounding electrode conductor between the load side of resistor and grounding electrode also required to be rated and full let-thru rating of resistor? It would appear so since it will see the same current? Only difference is it does not need to be an insulated cable and can be bare copper?

Do I understand that the requirement for neutral cable per code is based on maximum let-thru of resistor and not any continuous rating?

 

[9wearSuave]

Do I understand that the requirement for neutral cable per code is based on maximum let-thru of resistor and not any continuous rating?

Correct

The equipment bonding jumper size for the NGR is not necessarily the same as the neutral conductor to the NGR. It is sized based off NEC 250.36 (G)