Transformer Primary Conductors

[theshockdoc]

I'm having some problems understanding the sizing of the transformer primary conductor sizing. Not so much understanding the table 450.3(B) or matching a conductor to the OCPD. I realize that when protection is provided on the primary / secondary the primary protection may be set at 250% but is not required to be set at 250%. Here is the issue I'm having. Is the transformer primary considered a continous load? I've seen examples where the primary OCPD and conductors were calculated at 125% for "continous load", while this may be a "good practice" does the code require this?

If a transformer is sized to meet the demand of a panel and the panel size is calculated with the demand factors (or continous loads) then why would we factor in continous load again? Keeping this in mind - I've been looking at a couple of actual "one line" diagrams. Example 1 - a 150 KVA 3 phase transformer, primary protection 200 amp fuses feed with 250 kcmil Al. This is simply the transformer primary full load current rounded up the the next standard size OCPD. Example 2 - 500 KVA 3 phase transformer, primary protection 600 amp circuit breaker feed with parallel (2) 500 kcmil THWN Al. In this case the transformer primary full load current was actually rounded down.

Any clarification would be much appreciated. Industrial practice is nice but actual code references are better.

 

[don_resqcapt19]

There is no code requirement to install conductors and OCPDs that are greater than what is required to support the load, however with a transformer, there may be inrush tripping issues if you size the OCPD too small.