Transformer Secondary OCPD 450.3(A)

[Ozzy2054]

Lets say I have both primary and secondary protection, any location. When sizing the transformer secondary OCPD, my understanding is that if the circuit breaker is 80% rated, I must multiply the transformer secondary FLA by a maximum of 125%. However, if I specify a 100% rated circuit breaker, then I do not have to multiply the transformer secondary FLA at all.

For example:

-2500KVA Transformer 12.47kV to 480/277V 3Ph 4W D-Y feeds a switchboard
-Secondary FLA = 3007A
-For simplicity, ignore kva rating at 65/75 deg C temperature winding rise

If my switchboard has 80% rated main breaker, then it should be sized as 4000A (next size up rule)

If my switchboard has 100% rated main breaker (LV Power Circuit Breaker or 100% Rated Molded Case Circuit Breaker) then it should be sized as 3000A.

This has always been my understanding, as with lot of the engineers I talk to. However, I was recently told this is incorrect and that 100% rated only applies to continuous loads (i.e not transformer ocpd sizing) and that I need to multiply by 125% whether or not the breaker is 80% rated or 100% Rated.

What's everyone's thoughts on this?

 

[jim dungar]

First start with you calculated load. Decide if you will feed that load with a 100% or an 80% rated device.

Then see if the device you chose is within the 125% of the transformer output. For all intents the transformer is a 100% source.

 

[wwhitney]

For example:

-2500KVA Transformer 12.47kV to 480/277V 3Ph 4W D-Y feeds a switchboard
-Secondary FLA = 3007A
-For simplicity, ignore kva rating at 65/75 deg C temperature winding rise

If my switchboard has 80% rated main breaker, then it should be sized as 4000A (next size up rule)

That's correct if you want to allow your transformer to power a 3007A continuous load. But if you only need it to power a 3000A non-continuous load, you could use a 3000A main breaker. If you only need it to power a 800A continuous load, you could use a 1000A main breaker.

Cheers, Wayne