Transformer Breaker
Transformer Breaker
(OP)
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?
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?






RE: Transformer Breaker
RE: Transformer Breaker
RE: Transformer Breaker
RE: Transformer Breaker
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?
RE: Transformer Breaker
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?
RE: Transformer Breaker
RE: Transformer Breaker
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?
RE: Transformer Breaker
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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)?
RE: Transformer Breaker
RE: Transformer Breaker
RE: Transformer Breaker
RE: Transformer Breaker