Transformer OCPD per table 450.3

[ChargedUp]

Table 450.3 indicates MAXIMUM rating or setting of transformer primary/secondary OCPD. Is it acceptable for the OCPD on either side of the transformer be LESS than the listed MAXIMUMS in table 450.3 by a percentage of say 20%, plus or minus? Best practice would be to size to MAXIMUM, but any other technical implications or code implications to consider sizing to a smaller percentage than the table indicates? Practically speaking, it's my understanding start-up or in-rush current is something to consider, but it could be upwards of +/-6x FLA anyways, so a 20% smaller OCPD should not trip due to inrush? Thanks all.

 

[jim dungar]

I have rarely, if ever, sized my primary protective device to the maximum allowable value in table 450.3.
Since the last 2 iterations of the Department of Energy transformer efficency requirements, my preference is to find the inrush values for the transformer and compare them to the prospective protective device time current curve and then choosing the lowest Long Time pickup I can.

 

[petersonra]

Table 450.3 indicates MAXIMUM rating or setting of transformer primary/secondary OCPD. Is it acceptable for the OCPD on either side of the transformer be LESS than the listed MAXIMUMS in table 450.3 by a percentage of say 20%, plus or minus?

It is acceptable to the code.

Best practice would be to size to MAXIMUM, but any other technical implications or code implications to consider sizing to a smaller percentage than the table indicates?

I am not sure this is the "best" practice. There may be other considerations that make some other value a "better" choice.

Practically speaking, it's my understanding start-up or in-rush current is something to consider, but it could be upwards of +/-6x FLA anyways, so a 20% smaller OCPD should not trip due to inrush? Thanks all.

Where did you come up with 6X?

 

[ChargedUp]

Fair, 6x is a low end value, understood it can be larger. This specific application is utilizing a 25kVA step-up transformer, and sizing the primary OCPD (208V, 1ph) to 100A/2P and sizing the secondary OCPD (240V, 1ph) to 100A/2P.