Step up transformer

[cutlerhammerman]

I’m having trouble wrapping my head around step up transformers. 150kva 208/480 step up xfmr feeding a 480volt 80 amp load, will 200 amp ocpd on primary side be sufficient? I know the maximum ocpd is 416 amps, is there a minimum ocpd for this application? Sorry, my only experience with step up is small buck n boost, and step downs.

 

[ptonsparky]

I’m having trouble wrapping my head around step up transformers. 150kva 208/480 step up xfmr feeding a 480volt 80 amp load, will 200 amp ocpd on primary side be sufficient? I know the maximum ocpd is 416 amps, is there a minimum ocpd for this application? Sorry, my only experience with step up is small buck n boost, and step downs.

The problem will be the initial inrush of the transformer when it is energized, even with no load on the secondary. Minimum would be whatever it takes to get past the inrush. I would lean towards the 400 amps and be done with it, not knowing other pertinent information.

eta. I did not check your math.

ETA: Checked your math. 416 Is the FLA, not max ocpd. Ocpd is close to 625.

 

[Sahib]

Check with the transformer manufacturer also.

 

[beanland]

Overcurrent Protection and Inrush

Overcurrent Protection and Inrush

I’m having trouble wrapping my head around step up transformers. 150kva 208/480 step up xfmr feeding a 480volt 80 amp load, will 200 amp ocpd on primary side be sufficient? I know the maximum ocpd is 416 amps, is there a minimum ocpd for this application? Sorry, my only experience with step up is small buck n boost, and step downs.

I agree with the others. A 150kVA transformer when energized at 208V has a 416A full load current. A 125% OCPD would be 600A (next size up). This is unlikely to have inrush problems. (You can go to 225% when there is secondary protection.) You can expect inrush to be 6-12x for 0.1 seconds (consult manufacturer because designs vary). If you have 80A @ 480V load, then a 100A 480V OCPD would be appropriate. The 100A OCPD provides the needed secondary 480V protection and prevents overloading the transformer. The 600A primary protection is for transformer faults and to avoid tripping on inrush.

 

[david luchini]

(You can go to 225% when there is secondary protection.)

I think you mean 250%, not 225%