Transformer Sizing Requirements

[nhee2]

Are there any articles in the code which prescribe minimum transformer sizing?

Background - I was evaluating existing transformer for addition of new loads. In calculating transformer loads, I carried 125% of continuous loads. However, thinking further, I don't believe there is any requirement anywhere to size transformers for 125% of continuous, only conductors.

Does the NEC care how big a customer owned transformer is, provided it is properly protected?

 

[ActionDave]

No they do not. This is one of the last places the code holds true to it's stated purpose of not being a design manual.

 

[nhee2]

No they do not. This is one of the last places the code holds true to it's stated purpose of not being a design manual.

Thanks

 

[cppoly]

No they do not. This is one of the last places the code holds true to it's stated purpose of not being a design manual.

I always find myself coming back to this point a lot with the NEC. But I have to pushback a little on this, if a transformer is at risk of overheating because of not accounting for 125% of continuous loads, then wouldn't this be a safety issue? Or, it could be just entirely possible that transformers are simply fully rated for continuous current. Have you ever come across any transformers requiring 125% continuous load or has the standard always been 100% continuous plus 100% non-continuous loads?

 

[electrofelon]

I always find myself coming back to this point a lot with the NEC. But I have to pushback a little on this, if a transformer is at risk of overheating because of not accounting for 125% of continuous loads, then wouldn't this be a safety issue? Or, it could be just entirely possible that transformers are simply fully rated for continuous current. Have you ever come across any transformers requiring 125% continuous load or has the standard always been 100% continuous plus 100% non-continuous loads?

The continuous load derating ia something many people start applying to everything. I am not sure of the history and reasoning for it, but in the NEC it applies only to conductors and OCPD's.