Can the Protection of Primary side of a Transformer be over 200% of the rated current

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Samardas

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Electrical Engineer
Customer has a spare 200A breaker on the Main Switchboard. He wants to use his existing 75KVA, 480-208/120V transformer. Although the primary current for the transformer is about 90A, can he use his 200A CB that he already has in the MSB? Thanks.
 
Samardas said:
Customer has a spare 200A breaker on the Main Switchboard. He wants to use his existing 75KVA, 480-208/120V transformer. Although the primary current for the transformer is about 90A, can he use his 200A CB that he already has in the MSB? Thanks.
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Barring omitted pertinent details, yes... but will require secondary protection. See 450.3(B) and Table
 
Yes, but no cheating on the conductor size as we will be checking. The primary conductors will have to be sized to the 200 amp OCPD.
 
Can the Protection of Primary side of a Transformer be over 200% of the rated current

Thanks for your reply. What other info you were looking for? Also, yes the secondary will be protected as well. However, the sub-panel (also existing) to be connected is also 200A, which could have been bigger (225A), based on my calc.
 
Samardas said:
... What other info you were looking for? ...
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Not really looking for other info, but details are what make an installation Code compliant.

For example. do you know whether the Main Switchboard will be overloaded by adding loads to it? The transformer and subpanel by themselves present no load on the system. What you power with the subpanel does add load to the system. Just because there is a spare 200A breaker does not mean that the Main Switchboard will not be overloaded when adding loads.
 
Samardas said:
Thanks for your reply. What other info you were looking for? Also, yes the secondary will be protected as well. However, the sub-panel (also existing) to be connected is also 200A, which could have been bigger (225A), based on my calc.
Click to expand...

Actually the secondary breaker could be a 300 amp. 208 X 1.25 = 260 amps. Next size up is allowed here = 300 amp. Of course the conductors would also have to have an ampacity of 300 as well. But it is always an option.
But I guess you already have a 200 amp MB panel you want to use and that is fine as well.
 
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