Transformer (Overhead 10-167kVA) Ground Wire Size

[joozu6]

My utility started using a #2 Stranded Copper for their loop ground on overhead distribution transformers (versus #6 solid copper previously). I can't figure out the reasoning to this. For a 167kVA 120V transformer the fault current is max 57,300 amps. With this size fault current the short-circuit ampacity of niether the #2 or the #6 would be big enough. However, the ground resistivity is much higher than the resistivity of the ground wire, so it seems like that would be the limiting factor. Therefore, a #6Cu wire would work just as well.

Can anyone enlighten me on the best ground wire size to use?

 

[cuky2000]

Without know the performance history of the #2 St Cu wire it is difficult to say much about the reason of the changes.

Using larger ground conductor will reduce the overall conductor impedance. However, this reduction may not have significant impact improving the surge arrester performance to protect the transformer.

There is a possible benefit increasing the conductor size to avoid fusing of the ground conductor in the event of short circuit for a duration take by the protective device(s) to clear the fault.

 

[rcw retired EE]

#2 stranded may be more flexible and less prone to damage than solid #6.
Could there be a concern about corrosion leading to oversizing wires for a corrosion allowance?
Is the #2 stranded easier to install or terminate?

 

[fsmyth]

Or maybe the utility just got a good deal on
a trailer-load of #2. Or somebody mis-ordered,
and now you got to use it up?
<als>

 

[joozu6]

Thanks for your posts.