Confused on NEC Table 450.3(B)

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liam913

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Electrical Engineer
In a scenario with a 15kVA delta-wye (480-208) 3PH transformer, I am getting confused on sizing the primary and secondary OCPDs.

The primary OCPD will be a 3P breaker on the 480V panel and the secondary OCPD will be the MCB on the 208V panel. For 15kVA, primary current is 18A and secondary is 42A.

According to NEC Table 450.3(B), for a transformer with current > 9A and primary/secondary OCP, the primary overcurrent protection should be sized MAXIMUM 250% and secondary MAXIMUM 125%.

I understand these are maximums, so I'm assuming these aren't a hard requirement. In plans I am reviewing, the primary OCPD is a 30A, 3P breaker and the secondary is a 50A MCB. Are these allowed?
 
With 125% secondary protection, the primary protection can be as high as 250% but is not required to be.
With 125% primary protection there is no requirement for secondary protection as far as transformer protection is concerned.
As noted on the Informational Notes Art 240 must be taken into consideration for conductor protection.



125% of your primary current is 22.5 amps and, since 25 amps is a standard size (240.6), a 30 would require you to size the secondary at 125% or 45 amps,.
 
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Agree the secondary is OK with a 50 amp (15000 @ 208 x 1.25) however, with the 125% secondary the primary can not be over 250% (there is no provision for next size up) 15000 /480X1,732=45 which is a standard size breaker.
 
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