Secondary breaker size for 150kVA transformer

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mull982

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I'm looking a situation where we are replacing a 480-208/120V 75kVA transformer with a 150kVA unit. The existing panelboard on the transformer secondary is a 400A 208/120V panelboard with a 400A Main breaker. The question has come up weather or not we can keep the existing 400A panel or if we need to increase to a 600A panel with 600A Main.

The way I see it there are (3) things that need to be considered with the main:

1) Main breaker has to be equal to or greater than the total calculated load that will be fed from the panelboard (Not sure of NEC section?)
2) The Main breaker has to adhere to transformer secondary protection requirements of 450.3(A) which usually limits transformer secondary protection to 125%
3) The Main breaker has to adhere to transformer secondary cable protection requirements of 250.21(C)

Secondary FLA on 1 150kVA transformer is 416A and if you apply the 125% factor for 450.3(A) you get 520A and therefore being questioned as to weather or not 400A breaker is adequate.

To me based on the above requirements there is no reason why a 400A breaker cant be used as long as it is larger than the calculated load on the panel. So for example if the load calculations only show aprox 250A (based on requirements for load calcs) then there is not reason why this 400A breaker cant be used to supply this panel?

Also the 125% allowance for 450.3(A) is not a requirement but more of a maximum limitation? In other words there is nothing saying that you cant just use 100% of the transformer secondary FLA?

I wanted to see if others agree with me here? Have others seen this as a standard approach?
 
Most of the rules you list are not what the rules really say.

Lets start with the conductors. The NEC requires that the conductors be sized to handle the calculated loading. For some reason you chose a 150kVA transformer, why? What is your loading?

Second every panelboard requires a main protective device, which must protect the conductors feeding the panel, either at their beginning or at their end by using a tap rule.

Third, in a similar vein, choose and protect the conductors feeding the transformer.

Fourth consider how the protective devices you selected in steps 2 and 3 relate to the transformer protection requirements of article 450.
Does the primary side device protect the transformer at 125% of its primary and does it handle the transformer inrush/start up current? If yes, you are done and nothing else needs to be considered. If not, does the secondary protective device meet the 125% requirement?

No where does the NEC actually discuss transformer loading or sizing.

So, in the end, I agree with you that the existing 400A breaker can stay, if the conductors are adequate.
 
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