Transformer 25' Tap rule

[brian john]

With the 25' tap rule for transformers.

The NEC states (roughly)

Have an ampacity that is at least the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the OCPD that protects the primary of the transformer.
Terminate in a single circuit breaker (or set of fuses) rated no more than the secondary conductor ampacity per 310.15 [Table 310.16].
Are protected from physical damage by being enclosed in a manner approved by the AHJ (such as within a raceway).
That first underlined item is throwing me. And I cannot not find consistency in the few online articles I have read or a complete working of the math .

Lets say standard dry type transformer 408 delta-208/120 wye voltage ratio would be 4 to 1 primary to secondary or .25 secondary to primary.

150 KVA transformer, primary OCP 225 Amps, taped with no 3 AWG THHN Copper terminating in a 100 amp FSS with 100 amp fuses.
One showed 480/120, another 120/480, another 480/208 another 208/480?

Is this 4x225=900/3=300 amps? or
Is this .25x225=56.25/3=10.75 amps or
Is this .44x225=99/3=33 amps or
Is this 2.31x225=519/3=173 amps.

 

[don_resqcapt19]

The voltage ratio would be 2.31 to 1 for that transfomer. The secondary conductor minimum size for the rule in 240.21(C)(6) would be 225/3(2.31) or 173.25 amps.

 

[brian john]

Don, Why 2.31 in lieu of the transformer ratio of 4/1?

 

[david luchini]

Don, Why 2.31 in lieu of the transformer ratio of 4/1?

The primary voltage is 480V phase-to-phase, the secondary voltage is 208V phase-to-phase. That's a 2.31:1 ratio.

Also consider that the primary rated current is 180.4A and the secondary rated current is 416.4A. That's a 1:2.31 ratio.