It's generally good practice in sizing a transformer to not load it more than 80%. However, in looking through the code I cannot find a rule that specifically states this.

Is there such a rule in the Ontario Electrical Safety Code that states a dry type distribution transformer shall not be loaded more than 80%?????

Anthony Kerstens P.Eng.

 

Anthony: The code book is not an engineering guide. You will not find that information in the code. This information would have to come from the manufacturer. Also consider why not eventually load it to the max since the manufacturer states on the name plate that it is ok provided the ratings(kva, temp rise,impedance etc) are within limits. I agree that you should not load a transformer to more than 80% but that is only on the original installation. That will allow future loading (expansion). If you want to go over 100 % then the manufacturer will supply you with cooling fans $$ that fit on the cooling fins. This could take it up to maybe 133% of full load current.

Gord Graham P.E.C.

 

Actually, the code _is_ an engineering guide. It provides tersely defined rules governing the design and installation of electrical systems.

I was asking a very specific question about the electrical code because I am being forced to justifiy my position (by a hard-to-please
non-electrical type) and need something more than engineering common practice.

Anthony Kerstens P.Eng.

 

> Is there such a rule in the Ontario Electrical Safety Code that states a dry type distribution transformer shall not be loaded more than 80%?????

I don't recall one. Perhaps you are thinking of the 80% rule for 15 amp distribution circuits?

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Michael Griffin
London, Ont. Canada
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What the reference is probably to is Rule 26-256 of the Canadian Electrical Code. It states that the individual overcurrent device on the primary side cannot be rated at more then 125% of the rating of the transformer. Therefore, if you use overcurrent protection rated at 125%, the transformer current rating is now 80% lower then the rating of your overcurrent device.

Steve

 

More exactly, the code is a "Prescriptive Standard" where much of the engineering is done up front so that it need not be repeated on every job and need not be done on the fly in the field. In order to provide the most general fit the most possible circumstances it is "terse." I am not intimately familair with the Ontario code, but most codes allow engineering to the exact circumstances in many cases by a certified engineer. For example, the codes don't dictate exact features of a control panel design
because the panel is designed by an engineer. Codes also don't dictate the ampacity and sizes of a pcb trace becasue pcbs are designed by engineers, hence it is a "prescriptive standard"

 

I actually have a bookmark on that section of code. As a matter of math, yes, the FLA of the transformer would be 80% of the overcurrent device rating.

What I was looking for was something that said the calculated load in KVA should be no more than 80% of the KVA of the transformer. I prefer to only load transformers 80% of their capacity, and am looking for a written rule that states this.

I'm not sure I will find such a rule.

Anthony Kerstens P.Eng.

 

I'm not sure about Canada, but in the US, the transformer can be loaded to its max... You've already sized the conductors to allow this, here the only thing 80% applies to is the load on the circuit i.e. conductors. Again, here, the NEC is only a guide to the "minimum" requirements for a safe installation.