Transformer Fusing - 3 Phase

[pegggu]

Hi,
Just a general question about primary & secondary transformer fusing.
If i have a 1500kVA transformer (13.8kV primary - 347/600V secondary)

If i were to calculate the primary fuse required it would be:
I = P/(V*1.732) = 1500kVA/(13800*1.732) = 62.76A correct?

If i were to calculate the secondary fuse requierd it would be:
I = 1500kVA/(600*1.732) = 1443.42A correct?

Where does the actual fusing of a transformer go? is it just in line with the incoming and outgoing feeders?

thanks in advance!

 

[augie47]

It is important to understand the overcurrent protection can be there to protect the transformer and must be there to protect the conductor.
Art 450 provides you with information for selecting the OCO for transformer protection which can consist of primary protection or secondary protection or both.
Once you have selected the protection required for the transformer you need to select the conductor accordingly and make sure you have met the Art 240 rules for providing overcurrent protection for the conductors.
Since the primary over-curerent device must protect the primary conductors it can be placed at any point from the transformer to the supply. The secondary over-current device must comply with 240.21(C)

 

[pegggu]

hi augie, thanks for the reply.
i was more curious from a calculation standpoint if that was correct.
as well as where the fusing is physically put inside the actual transformer

 

[augie47]

Your math is correct.
Table 450.3 would show how to handle it from there.
There are others here more qualified, but I can say most of the pad transformers I have seen have accessible primary fuses as part of the transformer. Some depend on the jacks on the primary. None that I have seen have built in secondary protection.

 

[Smart $]

i was more curious from a calculation standpoint if that was correct.

Your calculation is correct for determining rated current. Maximum OCPD rating will require additional calculation, which is simply multiplying your resulting values by the permitted percentages for the protection method utilized [450.3].

as well as where the fusing is physically put inside the actual transformer

Gus (augie) covered pad mounts. Dry-type usually have none inside the enclosure, but they're not commonly used for the voltage configuration mentioned. Rather obvious there will be none inside an oil-filled transformer enclosure (i.e. the winding housing).