Step down transformer protection

[ggunn]

I am assisting a colleague with a design incorporating a 480VAC inverter interconnecting with a 208V service through a step down transformer. What, if any, disco/OCPD does he need between the transformer and the inverter? The 208V side of the transformer is adequately protected from fault current coming from the service and the inverter has an integrated AC disco.

 

[Pharon]

See 690.9(F).

 

[ggunn]

See 690.9(F).

My code book (NEC2011) does not have a 690.9(F). The exception to 690.9(B) indicates to me that no OCPD is needed (the transformer is rated at greater than the maximum output current of the inverter); do you agree? I don't see any mention of the need for a disco, either.

 

[Pharon]

Ah, for some reason (B) got moved to (F) for the 2014 edition. The language did not change, though.

And yes, I think you are correct. As long as the transformer current rating is less than the inverter current, you should not need OCPD between the transformer and inverter.

I also don't see any language in 690 that would require a disconnect there, either.

 

[jaggedben]

...As long as the transformer current rating is not less than the inverter current, you should not need OCPD between the transformer and inverter.

Fixed that for you. I'm sure you just mistyped, but I thought for the record we should get it right.

 

[Pharon]

Yes, thank you.