Transformer protection

[kwired]

I thought transformer primary current for single phase was calculated at transformer 45KVA / primary voltage Then conductors selected at amperes x multiplyer according to code

Secondary conductors being calculated at transformer KVA / secondary voltage x multiplier

But it seems you are sizing conductors not based on total transformer ampere output but based on total ampacity of connected load(s)?

Which is correct? Thanks

And secondary

Can be either. Just because one has a 45 kVA transformer doesn't mean it must have 45 kVA of load connected to it. If you only have 20 kVA of load and want to size conductors and overcurrent protection accordingly that is allowed. You do have to keep in mind the possibility that lesser primary OCPD could have trouble holding when energizing though.

You could also have primary selected for maximum kVA rating and still use lesser conductors and OCPD on secondary if the load isn't there.

Secondary tap conductors mostly just need to comply with 240.21(C).

OCPD values in 450.3 are maximum values, you can go less.