Transformer load calculation

[walkdave]

Hi everybody, I'm new to the forum and have a pretty simple question.

I'm working on a design for a panel board that will feed single phase 240 and 120 volt loads. I have all the branch circuit calculations complete for the overcurrent protection but how do I determine the kVA rating of the transformer that will be used to power this panel?

I'm looking at using a single phase 480V to 240/120V transformer.

I can determine the individual voltage loads: (sum of all 240 loads) x 240 = VA.

But what is the correct way to size it when you are using both 240 and 120 volt loads?

By the way, these loads are pieces of equipment and not just wall receptacles.

Thanks and sorry if this is elementary.

- D

 

[charlie b]

If you can make a list of the loads, this is a straight-forward task. You take the sum of the 240 volt loads (i.e., the load values in units of amps), and multiply by 240. You take the sum of the 120 volt loads (i.e., the load values in units of amps), and multiply by 120. Then you add the two numbers. This gives you total KVA.

However, just to complicate matters a bit, you may need to include a factor of 125% on one or more of your loads. The largest motor gets an extra 25%, when you are adding up loads. So too does any "continuous load," a phrase defined as a load that will run for more than 3 hours at a time.

Then you do an engineering judgment as to which size transformer to select. It will depend on which one is the next higher standard size, and on how close you were to that next higher standard size, and whether you want to build in some room for growth.

For example, suppose the math problem above gives you a result of 28 KVA. The next higher standard size is 30 KVA. If you pick that size, you have no room for growth, so it may be best to go with a 45 KVA.

Welcome to the forum.