Gounding of System Neutral

[coloradog1]

We have a 5 MVA 46kV/4160V transformer which is impedance grounded with a resistor. The secondary conductors are parallel 500 kcmil.

How do you size the neutral conductor which feeds the grounded resistor?

If you could also tell me which section of the code gives the sizing for this installation that would be great.

Thank you

 

[dpc]

See NEC 250.186, but I don't think it is going to give you a sizing criteria. You should size it based on the resistor rating. So for a 400 A resistor, I would size the conductor for 400 A. Since the resistor is only rated for probably 10 sec, this will be very conservative. A smaller conductor could be used, but you would need to look at the short-time thermal rating of the conductor.

The conductor should be insulated for full line-to-line voltage, at least in my opinion.

 

[coloradog1]

Is there a minimum size which the conductor should be rated at?

250.190 sounds like the conductor should not be sized for less than 6 AWG copper.

Thanks

 

[umrpwr]

I was working on a project where a similar question came up regarding high resistance grounding of the same size transformers (5MVA) but lower voltages. I agree that conductor should be sized based on the current flow and short time thermal rating. In addition although a smaller conductor may work I typically would use some thing that is physically rigid enough to also afford some additional protection.

In reference a NEC section that addresses refer to 250.36. This however only applies to systems rated 480V to 1000V so that would not be applicable to your installation. It may offer some guideance though.

 

[dpc]

For 5 kV cable, you are not going to find anything much smaller than #2 AWG anyway. As I said, I would size for 400 A. This cable run is very short, so you may just want to use whatever you are using for the phase conductors.

 

[coloradog1]

In our case the resistor is 25 A. Are you saying it should be sized for 25A then? That would but us at a #8 or so.

 

[dpc]

If you can find a 5 kV shielded cable with a #8 AWG conductor, that would probably work OK. But I think you are going to end up with #2 because that is generally the smallest size available.