Transformer inrush & Circuit Breaker selection

[FaradayFF]

Hi Guys,
Can I use an undersized breaker to protect a dry-type, one phase transformer if I'm only loading a fraction of the transformer. Say I've got a 100kVA transformer, 480V on high side, and utilize a 100Amp breaker. The loading on the transformer would be around 20Amps max. My concerns is the inrush current. Does the transformer get its max. inrush current value regardless of the load connected to it on the secondary?
The NEC allows to go up to 125% max. to select OCPD on the primary, but since I'm not going to load up the transformer to its nameplate rating, I'd like to use a smaller breaker(oversized xfmr situation).
Thanks,
EE

 

[ron]

The NEC establishes the maximum, so you can use smaller. If you use something smaller than approx primary FLA x1.25, it will trip on energization. I typically size it larger because I don't want to take a chance.

 

[jim dungar]

For the most part, transformer loading does not usually impact energization current. There will not be any output current until the magnetization has occurred.

 

[gar]

201005-1336 EDT

FaradayFF:

You can not reduce your breaker size. It is the physics of the circuit. You need to understand transformers and their possible initial inrush current. Newer transformers with more squarish saturation curves may be even worse than those of the past with standard transformer core material. Also a rather unloaded transformer may be left at a higher flux level at turn off.

See my scope image showing possible inrush current, P6, at http://beta-a2.com/EE-photos.html . This is not a new transformer.

.

 

[winnie]

I've used one transformer with a breaker about half FLA without difficulty; I believe this was a 30 KVA unit.

The secondary load will have only a second order effect on inrush; what has a much bigger effect is the impedance of the primary circuit, any residual core flux and the timing of the switch transition. If there is lots of impedance on the primary side, inrush is greatly reduced. If the primary closes right at voltage peak (where core flux is supposed to be low) then inrush is greatly reduced.

With a 100 KVA transformer at 480V, if I had a good reason to only use a 100A breaker, I would certainly give it a try. Most likely it will either hold or only trip on inrush some of the time. (An example of a good enough reason is transformer on hand and existing spare circuit.) But if I wanted to be sure it would work all of the time I would size the breaker and primary circuit normally for the transformer.

-Jon

 

[electrofelon]

I recently had a 25 kva 208-240 single phase transformer that had a 100A breaker on the 208 primary (83% of FLA) and it would trip about half the time. I would avoid going less than 125% if possible.

 

[retirede]

If the reason for undersizing the OCPD is because you have a smallish load and want to run smaller conductors on the secondary side, you may want to consider sizing the primary side OCPD normally and add an appropriately down-sized breaker on the secondary side.