OCPD for 25kV/480V Transformer

[Shahzad]

Hello Folks,

I need to feed a temporary chiller load. Unfortunately, I don't have direct 460V supply available, so I am exploring to get a 25kV/460V step down transformer to feed the load. I am wondering with following and would appreciate if anyone can provide some answer.

1-Do I need breaker protection at primary and secondary of transformer too, or only primary protection is enough?
2- As per NEC450.3, I guess, it will be unsupervised area, and impedance might be around 6%, so I need to have 600% rating of breaker or 300% rating of fuse to feed the primary of the transformer?
3- If I am using breaker, what setting I should use to set the breaker at? Long time should be 125% and Short time 600%. Is that acceptable to protect conductors?
4- I am sure rental chiller will have protection, so I would imagine the CB/ Fuse protection should be 125% since it will be less than 600V.
5- Cable sizing would be done as per NEC code, 125% primary and secondary. Correct?

 

[tortuga]

Question 2 On the 25kV side

1-Do I need breaker protection at primary and secondary of transformer too, or only primary protection is enough?

Primary and secondary per 450.3(A)

2- As per NEC450.3, I guess, it will be unsupervised area, and impedance might be around 6%, so I need to have 600% rating of breaker or 300% rating of fuse to feed the primary of the transformer?

Yes

3- If I am using breaker, what setting I should use to set the breaker at? Long time should be 125% and Short time 600%. Is that acceptable to protect conductors?

Not sure on this one I dont have much experience with 25000 volt breakers.

4- I am sure rental chiller will have protection, so I would imagine the CB/ Fuse protection should be 125% since it will be less than 600V.

Yes

5- Cable sizing would be done as per NEC code, 125% primary and secondary. Correct?

We would need the nameplate rating of transformer and the chiller to look at cable sizing.

 

[Carultch]

1-Do I need breaker protection at primary and secondary of transformer too, or only primary protection is enough?

To add onto the previous answer, there are certain qualifying topologies that enable your primary to indirectly protect the secondary conductors. It is most commonly the case that you don't have a qualifying topology, so in general, expect to have a secondary OCPD. "Qualifying topologies" is my term for this, rather than the NEC's term, so expect to explain this term if you choose to use it. See the first part of 240.21(C).

The qualifying topologies are:
Single phase 2-wire to 2-wire
Three phase delta to delta 3-wire on both sides.

The topologies that don't qualify are:
Anything with a connected center-tap
Anything with a wye topology on either side

In the qualifying topologies, any overcurrent on the secondary will be proportional on the corresponding primary winding.
In the topologies that don't qualify, there is a possibility that an overcurrent on the secondary gets re-distributed among the windings on the primary. The overcurrent will be in the blindspot of the primary OCPD, which is why secondary OCPD is also required.

As an example, suppose you had a 480V: 208V delta-to-delta transformer and a 100A breaker on the primary. It will effectively act as if it were a 230.7A OCPD on the secondary, calculated as 100A*480V/208V = 230.7A So all you need is 231A worth of secondary conductors, and that will be considered protected by the 100A primary. Given the same situation with a wye secondary, you'd need a secondary OCPD and conductors that follow 240.21(C). Likely a 225A breaker in this example.