Voltage drop and OCPD

[sergiyny]

So he is the situation: I have 45KVA XFMR, the primary current is 54A x 1.25 = 68A -> 70A CB, the wire size is #4 AWG.
Now, what if the 45KVA XFMR is 450 ft away from its over current protection, the #4 AWG is no longer good due to the voltage drop, now I'd have to go with #2 AWG.
Now my question is, for such a long run do I still size my CB to be 70A or it should be sized based on that #2 AWG?

 

[Carultch]

So he is the situation: I have 45KVA XFMR, the primary current is 54A x 1.25 = 68A -> 70A CB, the wire size is #4 AWG.
Now, what if the 45KVA XFMR is 450 ft away from its over current protection, the #4 AWG is no longer good due to the voltage drop, now I'd have to go with #2 AWG.
Now my question is, for such a long run do I still size my CB to be 70A or it should be sized based on that #2 AWG?

The circuit breaker only needs to be sized to the load calculation. Some loads may have a maximum OCPD size. Long distance doesn't require you to use a larger OCPD.

One reason you might WANT to use a larger than necessary OCPD because of voltage drop, is to avoid having to upsize ground. As an example, both a 40A circuit and a 60A circuit require a #10 ground by default. If you need to size the current carrying conductors to #6 for voltage drop, you can strategically use a 60A breaker where a 40A one would suffice, such that #6 is the default size and the #10 ground could remain instead of being upsized to #8.

Another reason you might WANT to use a larger than necessary OCPD or disconnect, is to terminate larger conductors and/or have more bend radius within the enclosure. There are ways around this, such as transitioning locally to smaller wires, possibly in a separate enclosure.

 

[Smart $]

...
One reason you might WANT to use a larger than necessary OCPD because of voltage drop, is to avoid having to upsize ground. ...

The OCPD is protecting the transformer primary and 125% is the maximum primary only protection permitted. He can't upsize without changing to a primary and secondary protection scheme where primary is allowed to be protected up to 250% while the secondary protection is limited to 125%.