Transformer Sizing Requirements

[mull982]

It is my understanding that a transformer must be sized to meet the calculated load which it serves. For instance if a load is calculated to be 900kVA a 1000kVA transformer must be used and a lower sized transformer cant be used even if it is known that the load will never reach the calculated load? In other words the transformer rating must meet or exceed the calculated load?

Does this mean that the transformer base rating must be greater than calculated load or can one of the forced air ratings of a transformer be used to exceed the load even if base rating doesn't?

The only time I believe where a transformer doesn't need to meet or exceed calculated load is a utility owned transformer since they do not need to adhere to NEC?

 

[petersonra]

It is my understanding that a transformer must be sized to meet the calculated load which it serves. For instance if a load is calculated to be 900kVA a 1000kVA transformer must be used and a lower sized transformer cant be used even if it is known that the load will never reach the calculated load? In other words the transformer rating must meet or exceed the calculated load?

Does this mean that the transformer base rating must be greater than calculated load or can one of the forced air ratings of a transformer be used to exceed the load even if base rating doesn't?

The only time I believe where a transformer doesn't need to meet or exceed calculated load is a utility owned transformer since they do not need to adhere to NEC?

not a code requirement.

the service/feeder/branch circuit conductors have to be sized to the load.

 

[Smart $]

not a code requirement.

the service/feeder/branch circuit conductors have to be sized to the load.

Indirectly it is a Code requirement.

OCPD requirements are what truly force a transformer to be sized appropriately. Consider that non-specific purpose circuit OCPD's must be sized to a minimum 125% continuous plus noncontinuous, while the maximum of either the primary or secondary transformer protection OCPD is 125% of rated current.

 

[petersonra]

Indirectly it is a Code requirement.

OCPD requirements are what truly force a transformer to be sized appropriately. Consider that non-specific purpose circuit OCPD's must be sized to a minimum 125% continuous plus noncontinuous, while the maximum of either the primary or secondary transformer protection OCPD is 125% of rated current.

you can still have a 50A CB protecting the secondary of a transformer feeding a panelboard with a 100A main.

 

[Smart $]

you can still have a 50A CB protecting the secondary of a transformer feeding a panelboard with a 100A main.

True... but you cannot put more than 50A load on that panel board because of the 50A CB.

This brings up another caveat in that say the 50A CB is at 125% of rated current, which would be 40A. You could have a 50A noncontinuous load on that panelboard and be compliant under Code...!!!

 

[petersonra]

True... but you cannot put more than 50A load on that panel board because of the 50A CB.

This brings up another caveat in that say the 50A CB is at 125% of rated current, which would be 40A. You could have a 50A noncontinuous load on that panelboard and be compliant under Code...!!!

the point is that you can size the xfmr to any value you want, even if it won't work because it is undersized.

most calculated loads are well beyond what you are ever actually going to see though, so chances are if you calculated a 100A load you might see 50A now and then and probably less most of the time.

 

[iwire]

the point is that you can size the xfmr to any value you want, even if it won't work because it is undersized

No, smart explained why you cannot do that.

 

[david luchini]

You could certainly supply a 108kVA noncontinous load via a 75kVA transformer within the Code rules.