Transformer help please

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Brand New

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I was planning on putting in a 30 kva 480 D \ 208 W transformer in a shop that has a 208 three phase service. The transformer is to feed 2 three phase 480 motors. One motor is a 5 hp and the other is a 3 hp. After doing some reading about it I am getting worried about the inrush issue and tripping the breaker. Any thoughts as to worry or not? Any comments would be greatly appreciated.
 

sc57ford

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Location
South Carolina
You didn't state the protection scheme you have for this transformer, so I assume you have provided it according to Table 450.3(B) with the primary 208V fuses (or breaker) not greater than 250% (200A) and the secondary 480V fuses (or breaker) not greater than 125% (110A). Table 430-150 list the 5hp at 7.6A and the 3hp at 4.8A. Inrush on motors approximates 10X running load but only for a few cycles. If this is the only load you have on this transformer, everything should be fine.
 

Brand New

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I was planning on using primary only protection @ 125%. Do I need to use the primary and secondary method? Thanks for your help.
 

jim dungar

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Wisconsin
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PE (Retired) - Power Systems
If you use primary protection only @125% when running a transformer in reverse, you will almost be guaranteed of having nuisance tripping.

If you want to stick with only the primary protection you need to by a 208-480 step-up transformer instead.
 

winnie

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Location
Springfield, MA, USA
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Electric motor research
sc57ford, you may wish to double check your amp calculations.

Brand New,

I presume by inrush current you are worried about the transformer tripping the supply breaker when you energize the transformer. For what it is worth, I have had no problems feeding 30 KVA 208V to 480V transformers with a 50A breaker on the primary side, in three iterations of the same installation. This was in a lab situation where the transformers were regularly re-energized, sometimes several times per day. We never had a nuisance trip of the breaker on inrush.

The installation used OCPD of 60% the transformer primary rating, much less than you are considering. Oversized transformers were used because of availability, with the load considerably less than the 18KVA set by the 50A breaker. This was with two different transformers ( 1) a 208W to 480D, the installation that you are considering, and 2) a 208D to 480W, which provided a wye secondary for better grounding options ) and the installation was moved between two locations. The first location was very close to a 300KVA supply transformer (high available short circuit current, and presumably higher available inrush current). From this experience with this size transformer, I would not expect you to have problems with this aspect of your installation.

One significant issue which brings my experience more into line with Jim's comment above: in my application, the transformer was always energized with no load on the secondary. In many transformer application, when you energize the transformer there is always load connected on the secondary. Your application sounds more like mine; presumably your motors will be off, you will energize the transformer, and then turn the motors on.

No offence intended, but your questions suggest that you are not familiar with all of the requirements for your transformer installation. Remember that you need to provide over-current protection for the primary conductors. secondary conductors, primary windings, and secondary windings. Primary protection of 125% of the primary current rating is considered to protect the secondary winding, but not necessarily the secondary feeder conductors; look at articles 450 and 240. Additionally, you need to carefully consider your grounding and OCPD options when you use a delta secondary.

-Jon
 
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kingpb

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Location
SE USA as far as you can go
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Engineer, Registered
Brand New said:
I was planning on putting in a 30 kva 480 D \ 208 W transformer in a shop that has a 208 three phase service. The transformer is to feed 2 three phase 480 motors. One motor is a 5 hp and the other is a 3 hp. After doing some reading about it I am getting worried about the inrush issue and tripping the breaker. Any thoughts as to worry or not? Any comments would be greatly appreciated.

The following should be helpful:

View attachment 221
 
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