Transformer Sizing Standard Practice

[engineerithink]

New engineer looking for some advice.

When interpreting the NEC, I have determined that it is okay to have a transformer rated at least 19.2KVA if it is only feeding a single phase 120/240V panel with a 100 amp main because the devices fed by the panel should be calculated to have a continuous load of no more than 80% of the main breaker. However, I have noticed that when adding devices such as lighting to a panel it often difficult to determine the existing load already on the panel because of poor documentation. Therefore, I don?t find it too hard to assume that someone could possibly add to much load to the panel.
Is it common practice to size the transformer in accordance with the amperage of the main breaker to prevent the transformer from exceeding its maximum rating? Or am I being too cautious?

Example

Transformer KVA = 240*100/(1000) = 24KVA

Thanks

 

[iwire]

I have determined that it is okay to have a transformer rated at least 19.2KVA if it is only feeding a single phase 120/240V panel with a 100 amp main because the devices fed by the panel should be calculated to have a continuous load of no more than 80% of the main breaker.

That is true but the panel could just as easily have non-continuous loading of up to 100 amps.

Is it common practice to size the transformer in accordance with the amperage of the main breaker to prevent the transformer from exceeding its maximum rating?

At least close to the rating.

As an example a very typical install for us is a 480-208Y/120 75 KVA transformer. The primary will have 125 amp OCP and the secondary would have 225 amp OCP.

The actual trans ratings are 90 and 208 amps.