Transformer Breaker 1

[timm333]

I need some help in sizing the breakers of transformers. We have following equipment.

1. 300 kVA Utility Transformer (12.47kV – 480V). (There is a 460V fire pump motor on this utility transformer)
2. Then a 480V transfer switch
3. Then a 150 kVA transformer (480V-208V)
4. Then a 208V panelboard

I can use the main breaker of the 208V panelboard as a secondary protection of 150 kVA transformer.

But how shall I provide the Primary protection of 150kVA transformer, and secondary protection of the 300kVA utility transformer?

Should I put junction boxes and put the breakers in them?

 

[davidbeach]

Have you checked the relevant portions of the NEC?

 

[timm333]

Yes I checked NEC. But I am wondering how to install the breakers. Should I put weatherproof junction boxes and install the breakers in these boxes?

 

[davidbeach]

Depends on the layout of what is located where. Generally the transformer is installed close enough to what ever it feeds to use a main breaker in the panel or switchboard. The primary can be as far way as desired. I weatherproof because it's outside, you can go as far as you want, as long as the secondary overcurrent is at the nearest practical point of entry into the building.

 

[timm333]

I have couple of more questions.

The sequence of equipment is as below:

1. 300 kVA transformer with a secondary current of 360A@480V.
2. Transfer switch with a service entrance type circuit breaker of 400A@480V.
3. 150kVA transformer with a primary current of 180A@480V and secondary current of 416A@208V
4. 208V panelboard with a main breaker of 600A. (Intermediate sizes of main breaker are not available. The main breaker is available is either 400A or 600A, so I had to use 600A.)

Here are the questions:

1. The transfer switch is right in between two transformers and it has a 400A service-entrance type circuit breaker. Can I use this 400A breaker in transfer switch as secondary protection of 300kVA transformer and primary protection of 150kVA transformer?
2. Can I use the 600A breaker in panelboard as the secondary protection of 150kVA transformer @ 208V?

 

[timm333]

Let me re-phrase what I said above:

1. Can the same circuit breaker be used as the secondary protection of an upstream transformer, and primary protection of a downstream transformer?
2. Can the rating of a breaker for secondary protection of a transformer be 110% of transformer full load current (instead of 125%)?
3. If transformer’s secondary full load current is 416A, then 416 x 1.25 = 520A, so next standard size is 550A. But 550A is not readily available size for the main breaker of panelboard. So can I use 600A breaker instead of 550A?

 

[davidbeach]

Yes, Yes, No.

 

[timm333]

Just one more question.

If I put 550A breaker in a weather-proof junction-box for the secondary protection of 150kVA transformer; still the main-breaker in the downstream panelboard has to be 600A, because 550A breaker is not available as the main breaker of panelboard.

So is it acceptable that the upstream breaker is 550A and downstream breaker is 600A?

 

[timm333]

In continuation to my last post, actually the table at the following link:

http://www.electrical-design-tutor.com/transformersizing.html

Shows that 600A breaker can be used on the secondary protection of 150kVA transformer @ 208V (instead of 550A).

So can I use 600A breaker for secondary protection of 150kVA transformer @ 208V (instead of 550A)?

 

[davidbeach]

What does the engineer that's going to sign the drawings have to say about all this?

 

[timm333]

Never mind, I just check the code again and the next standard size from 520 is 600A (not 550A). So I am good. Thanks for help...

 

[troque]

You may also want to look into the requirements in NEC Art. 695 for Fire pumps.

 

[timm333]

Ok