Transformer Secondary OCPD

[wwatkin2]

150KVA, 208/120Y three phase transformer has parallel secondary conductors at 3/0 landing in a 400amp breaker. According to the code calculation 150,000/(208x1.732)=416.4.
Table 450.3(B) 125% (Currents of 9 amperes or more Primary and secondary protection)
1.25x416.4=520.5
Table 450.3(B) Note 1 one allows next size up standard rating.
Table 240.6(A) Next size up from 520 would be 600 or I can go down to 500 standard rating. Sticking with the 500 standard rating I divide by two 500/2=250 (parallel run)
Table 310.16-75 degree column 255 requires 250 kcmil.
What am I missing here? How come this print has the secondary conductors protected by a 400amp breaker? This is a service for two-Level 2 electrical vehicle charging stations which are considered continuous loads. The only thing I can think of is an exception to the 125% where the secondary conductors are allowed to be protected at 100% of the current rating (416.4A) which would then make sense moving down to a 400amp main breaker. The problem is I can't find any exception in regards to transformers or EVSI equipment that allows this.

 

[don_resqcapt19]

3/0 is a 200 amp rated conductor at 75°C and two 3/0s in parallel would be a 400 amp conductor.

There is no code issue with having parallel 3/0s come off the secondary terminals of a transformer and land on a 400 amp OCPD.

There is no requirement for the secondary conductors to have an ampacity equal to the secondary full load current. One set of 3/0s protected by a 200 amp OCPD would be code compliant.

There is no prohibition on the transformer secondary protection being less than 125% of the transformer rated current.

 

[Jpflex]

150KVA, 208/120Y three phase transformer has parallel secondary conductors at 3/0 landing in a 400amp breaker. According to the code calculation 150,000/(208x1.732)=416.4.

your math seems good here. I think maybe that the reason the secondary breaker is smaller than max transformer ampere output less than 416.4 i amperes is because the load or chargers load is rated at 400 i amperes or less or even under 400i amperes after applying 125% of the full load current assuming a continuous rated load.

 

[augie47]

Likely selected based on load and standard transformer sizes.
The next standard transformer below the 150kva would be a 112.5kva and a load approaching 400 amps would push that one to the limit.
In addition, if the projected load is not expected to exceed 400 amps, a larger panelboard (say 500, or 600) often is significantly more expensive.