Transformer secondary grounding cond. ?

[Copperclad]

Hi all,
We installed a 75kva 3ph. xfmr recently and the question was brought up about why we are allowed to use a smaller grounding cond. on the primary feed? It started me thinking that the grounding cond. should be the same size through out the system....any thoughts?

 

[infinity]

One the primary side you will have an EGC which is sized according to the OCPD ahead of the feeder. You would use 250.122 for sizing the EGC.

 

[Smart $]

The EGC is for carrying fault current. Fault current needs to return to its source. The size of the EGC must be capable of carrying enough fault current to trip (or blow) the overcurrent protective device which protects the conductors associated with the EGC. A step-down voltage transformer is also a step-up amperage transformer and the ratios of such are inverses of each other...

V_pri / V_sec = I_sec / I_pri​

Say for instance you have a 480V?208Y/120 transformer. Since the winding ratio is 4:1, your primary current will only 1/4 of the secondary current value.

Also, if the ratio was reversed, i.e. a step-up voltage transformer, the EGC on the primary would likely be larger tham the one required for the secondary.

 

[iwire]

As smart said the fault current has to go back to the source. On the primary side the fault current is heading back to the utility transformer and the smaller EGC running back with the primary feeder is more then adequate to trip the breaker or open the fuse protecting the feeder.

On the secondary side any fault current has to return to the that transformer over the larger bonding jumper / EGC.

 

[Copperclad]

ok

ok

Thanks all, that makes perferct sense.......