Fuse Size Selection for (3) MV Transformers

[Sparky2791]

Greetings,
I am trying to determine how to size a set of primary fuses in a load break switch which will be feeding (3) loop fed 2500KVA transformers. It is considered an unsupervised location. NEC tells me maximum 300% of transformers rated current but that would be for a single transformer. I will be feeding 3 of them from the same switch. The utility company primary is 12,470V. I am likely getting into the need for a MV metal clad circuit breaker instead of fuses for proper coordination with the fuses in the transformer and with the utility company's upstream protection but I am still interested in how this is accomplished.
Thank You in advance for your interest and responses.

 

[beanland]

MV Fusing

MV Fusing

You are going to have problems using fuses. Three 2500kVA is 7500kVA or 347A @ 12.47kV. The largest standard MV fuse is 200A. You are going to need some kind of fault interrupter with relay, like a Cooper VFI or pole-mount recloser with relay. Then the problem is all coordination study.

I assume each 2500kVA transformer is internally fused? You would have to coordinate the relay settings with the transformer fuses and those need to be coordinated with the 3000A 480V breaker timing. This is not a trivial problem.

 

[Sparky2791]

If you read my entire message again you will see I already mentioned the need for a MV relay CB likely being required as you suggest. I further indicated there would need to be coordination with internal fuses as you suggest.
I do not appreciate the comment "This is not a trivial problem" If it was not trivial to me I would not have posted it.
I also disagree about your comment of a 200A fuse being the largest look at Bussman Part #15CLE-250E as an example.


So to make it more "trivial" how about if the transformers were only 750KVA That is only 104A @ 12.47KV Well under the standard 200A you indicate.

What I was seeking was assistance on how to size a single set of fuses feeding three transformers.

Is it total ampacity of transformers rated KVA x 300% = Next highest standard size fuse?

 

[Smart $]

...
Is it total ampacity of transformers rated KVA x 300% = Next highest standard size fuse?

No.

Each transformer must be protected at not more than 300% of its rated current. So for a set of fuses supplying three transformers, the rated current of the smallest transformer must be used.

 

[Sparky2791]

OK so as an example if I have (2) 750KVA and (1) 500KVA I size the fuses for the 500KVA and that protects all three?

My concern is in rush of all three transformers at the same time would be too much for the fuse sized for protection of the smallest (500KVA) alone. Perhaps this is a matter of specifying the correct type fuse to handle the in-rush?

 

[beanland]

Fusing

Fusing

I apologize if my reply irritated you, that was not intended. I have been in the utility industry for over 40 years and have specialized in protection for the past 15. I know of no installation where a utility would allow the installation of three 2500kVA 12.47kV 3-phase transformers in parallel without fuses on each transformer. But, this is not a utility installation, this is an NEC installation.

I agree with smart$ that, per NEC, each transformer must be protected at no more than 300% of full load current. Utilities may violate that 300% rule because they fall under NESC not NEC and follow industry standard practices, not the NEC. Three unfused transformers in parallel would have to be fused for the smallest transformer.

So, with three 2500kVA in parallel, you could fuse for 7500kVA (300%). But, you may have problems with inrush. With either fuses or a protective device, avoiding fuse damage or inrush tripping may be a challenge, but it can probably be accomplished.

I accept that fuses larger than 200A are available, my error. The challenge you may have is that the utility feeder might not coordinate with that large fuse. Common utility practice is to set the ground fault trip at 50% of the phase fault trip. Many utilities use 600A phase overcurrent pickup settings; in that case the ground fault trip would pickup at 300A. A 200A fuse starts to melt at 400A. For a phase-ground fault the utility ground element may operate faster than the fuse. But, this is just a hypothetical, not your specific case. You would need to work with the local utility to make sure it all coordinates.

 

[Sunny_92]

Greetings,
I am trying to determine how to size a set of primary fuses in a load break switch which will be feeding (3) loop fed 2500KVA transformers. It is considered an unsupervised location. NEC tells me maximum 300% of transformers rated current but that would be for a single transformer. I will be feeding 3 of them from the same switch. The utility company primary is 12,470V. I am likely getting into the need for a MV metal clad circuit breaker instead of fuses for proper coordination with the fuses in the transformer and with the utility company's upstream protection but I am still interested in how this is accomplished.
Thank You in advance for your interest and responses.

No.

Each transformer must be protected at not more than 300% of its rated current. So for a set of fuses supplying three transformers, the rated current of the smallest transformer must be used.

It looks like the OP is assuming each transformer will have primary side fuses, so I'm thinking Smart $'s statement may be misleading...

 

[Sparky2791]

It looks like the OP is assuming each transformer will have primary side fuses, so I'm thinking Smart $'s statement may be misleading...

That is correct.
Individual fusing on the primary side inside each transformer.
This is standard for pad mounted transformers (I think:?)

 

[Smart $]

That is correct.
Individual fusing on the primary side inside each transformer.
This is standard for pad mounted transformers (I think:?)

If each transformer is individually protected, you can size your fuses for the loop conductor.

 

[topgone]

Most of the time, we protect transformers with fuses that are sized to protect the transformers on short circuit faults. Overload protection is taken cared by the secondary protection.