Fusing Step Up Transformer

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Gene Shults

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Hello all,
Project I have is.
I'm wanting to install a step up transformer from 208 3 phase 4 wire feed to a step up transformer, 480 volts 3 phase 4 wire.
Step Up Transformer info: Primary 75kva, 208 3 phase Delta, Secondary is a 460 3 phase Wye.
Doing the math on transformer with it being a 75kva, primary side is 208 x 1.73 = 359.84, 75000 va / 359.84 = 208.4 amps.
Secondary side of trans. 480 x 1.73 = 830.4 , 75000 / 830.4= 90.3.
Ok if I have done my math correct then according to NEC Table 450.3 sizing of Over current protection I have to take my amps times 250% for the primary and 125 % for secondary.
Primary 208.4 x 250% = 521,
Secondary 90.3 x 125% = 112.875.
Am I correct????
If so on the primary side fusing is really high and does my incoming service have to match this?
Or do I have to fuse both primary and secondary sides of the transformer?
Can I just put a disconnect on the primary side and just fuse the Secondary side?
Thanks for all the help.
Gene
 
What are you supplying on the secondary side ? It's possible that primary only protection will suffice.

Also, be aware that your primary feeder will be a 3-wire feeder - not 4-wire as you indicated. The won't be any connection to the neutral on the primary side.
 
I'm supplying on the secondary side is two 30 amp 460 volt 3 phase welders.
I notice that the NEC 450.3, there are two methods of protecting the transformer. Method 1 is just the primary side at no more that 125% and Method 2 is both primary and secondary side with primary side at no more than 250% and secondary side no more than 125%.
What bothers me the most is if I protect both sides my primary side is going to be really high.
I'm going to have fused disconnects at both welders.
So can I just fuse the primary side of the transformer at 125% of the 208.4 and have my welders fused at 30 amps?
 
Sounds like you can just provide primary side protection and then connect your welder disconnects to the secondary side using the tap rules (Of course this is assuming you can comply with the tap rules based on your distance, etc.).
 
Gene,

It sounds like you are misinterpting NEC 450.(3)(B). The code does not require you to use a large primary OCPD.

In many cases the inrush (start up) current for a transformer can be so large that a primary device sized at 125% will operate on energization. To accomodate this problem the NEC allows you to upsize the primary OCPD if you supply a secondary OCPD sized at not more than 125%.

If you do not want to compare your estimated inrush current with the instantaneous rating of your primary OCPD, you should make provisions to install a secondary OCPD.
 
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