Sizing Fuse for Transformer Protection

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kvramesh

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I have the following system:

Transformer: 2.0/2.66MVA, 23KV-400V, Z=5.75%, 3ph, 50Hz, delta-wye (soild grounded)
Primary OCPD: Fused LBS close coupled to transformer
Secondary: 12ft - 4000Amp bus duct terminating in a LV Power circuit breaker

I need to size the primary side fuse for protecting the transformer as well as secondary side bus duct. Primary FLA = 67Amps

NEC 450.3A is little confusing to me. Not sure if it will be supervised or not...does the fuse to be sized 125% or 250% or 300% ? What should be the fuse size - 200amp or 225amp or something else?

Thanks in advance - RK
 
If I consider 125% of the rated current is required per NEC for primary and secondary of transformer protection, 125% of FLA of 2666kVA transformer is appr 82 amps. Does that mean I should go for 80E or anything else.

Any suggestions?
 
I am looking at 2005, table 450.3A. your Z is not more than 6%, so under the column "primary protection over 600 volts" the "Fuse Rating" shows 300% (or next size up per Note 1)

on the other hand, if you have a supervised location (read note 3), then you can still use 300% or next size up (per note 1).
 
imo, I read primary 300% fused (over 600V), secondary 125% breaker or fuse (600V or less), both use note 1 (next higher std size). Now, if this is a supervised location, then I read primary 250%, secondary not required.
 
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