Power transformers are rated in kVA and sized according to the calculated load that they are to be connected to. The basic formulas are:
kVA = E x I / 1000 for single-phase loads or
kVA = E x I x 1.73 / 1000 for 3-phase loads.
Let’s look at a couple of examples:
Suppose that we want to install a transformer with a 480V primary winding to supply a 240/120V, 100A single phase load. In this case:
kVA = 240V x 100A /1000
= 24000 VA / 1000
Now, let’s look at a transformer with a 480V Delta primary and a 208Y/120V secondary, feeding a 100A load. In this case:
kVA = 208V x 100A x1.73 / 1000
= 35984 VA / 1000
= 35.984 kVA or ~36 kVA
Note that the line to line voltage was used in the calculation rather than line to neutral voltage. We could have done the same calculation using the primary voltage since the kVA is the same on both the primary and the secondary sides. Also note that in our single phase example, it does not matter if the transformer is supplied by a 480V single phase source or by 2 legs of a 3-phase source. In both cases we are still energizing a single 480V primary winding in our transformer, thus we use the single-phase formula.
As a general rule of thumb, a transformer is selected that will supply at least 20% spare capacity to accommodate future growth. This figure may be increased based on specific facility requirements. Let’s look at an example. Suppose we need to size a transformer with a 480V, 3-Phase, 3-wire primary, and a 208Y/120V, 3-phase, 4-wire secondary, serving a panelboard that will be connected to a calculated load of 56 kVA.
Transformer size in kVA = 56 kVA * 1.2 = 67.2 kVA
In this case, a 75 kVA transformer would be chosen from the available standard ratings for this class of transformer. The industry standard ratings for a 480V – 208/120V transformers are: 3, 6, 9, 15, 30, 37.5, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1000 kVA.
While this calculation will allow us to properly size the transformer, in practice there are other considerations that must be taken into account when selecting a transformer. Of paramount importance is the type of load that will be connected. In years past, linear loads (resistive heating, induction motors, etc.) were the norm. In modern installations, electronic equipment often generates significant harmonic content, resulting in non-linear currents which cause significant losses and additional heating of the transformer coils and core. When non-linear loads are expected, a K-Rated transformer should be selected. The K-Rating allows the transformer to withstand the non-linear currents. K-Rating does not mitigate the problem but does help to protect the transformer from damage. Other considerations that should be taken into account are voltage ratings, taps, impedance rating, efficiency, temperature rise, and noise.