How to use 450.3(b)

[goodoboy]

I worked for a major MCC manufacturer and for 1kva 1ph 480v-120v CPT we would supple (2) 10a 600v ktk-r fuses for the primater and (1) 250v fnm 15a fuse for the sec.

Now I am even more confused. :?


Is my calcualtion I begin this thread correct or wrong?If wrong, please explain what I should do?

Thanks,

 

[goodoboy]

This is the way I have always thought of it:
Think of the Pri. protective device as actually providing short circuit protection should the transformer fail. It's the device that takes the transformer off line so that it doesn't affect the upstream distribution system. Sometimes the rating of the pri. protective device seems to be unreasonably high but it really isn't intended to provide overload protection for the transformer as that's the purpose of the secondary OCPD.

The transformer will be tied to the 8 amp load and the A/C unit load of 15amp.

The A/C unit is powered with 480V 3 phase, that same 480V is then tied to the 1kVA transformer, to provide 120V single phase 8 amp.

If I use 3.5A fuse on the primary, I am afraid startup and operation of the A/C unit will open the fuse and the 8 amp load not turn on.

Thanks

 

[Smart $]

Can you explaing this statement a bit more? How am I cutting it close with this 1kVA transformer? Is this becaus my fuse will be 3-3.5amps and could open during start-up of the secondary 8 amp load?

Thank you for the help

8A ? 120V : 2A ? 480V = 960VA of the rated 1000VA

Inrush current of inductive loads can be several to many times the nominal operating current. Fusing at even 3.5A is less than two times the nominal operating current.