Disconnect sizing between 630 kW inverter and 75 kVA transformer

Hey All,

I'm trying to size protection between a 630 kW inverter and a 75 kVA transformer. I am tapping off the AC of the 630 kW inverter to a 75 kVA transformer, which feeds a sub panel for loads within the building. My question is do I size the disconnect for the transformer, or for the inverter?

Smart $

Esteemed Member
Well that's a somewhat loaded question...

You'd size for the transformer side of it, but depending on whether the transformer is primary only or primary and secondary protected and the load on the transformer, size (i.e. rating) can vary quite widely.


Staff member
The available short circuit current and incident energy at the disconnect would depend on the inverter, but it would be so low compared to that from the grid in the same situation that it probably has no effect on your choice of disconnect.



PE (Electrical), NABCEP certified
Austin, TX, USA
Electrical Engineer - Photovoltaic Systems
The available short circuit current and incident energy at the disconnect would depend on the inverter...
How so? A grid tied inverter cannot feed a short. Conductors from an inverter are sized so that the inverter cannot generate enough current to endanger them. OCPD is to protect the conductors from fault current supplied by the interconnection with the grid.
Last edited:


Staff member
There is a short, but non-zero time before the anti-islanding protection kicks in.
During that time the current will still be strictly limited.
We are assuming that the inverter in question is grid-interactuve.
In that case, the transformer is really being fed by both inverter and POCO, and the protection on the upstream, POCO, side of the connection point is an issue.
I should have been more clear about that.
If it is not a grid interactive inverter, we need to hear that from the OP.