To properly use a transformer, one must know its voltage and current ratings. Of course, from these ratings the power rating of the transformer can be calculated. Most transformers are also specified by their voltampere (apparent power) ratings.
Voltage Rating of Transformer
Manufacturers always specify the voltage rating of the primary and secondary windings. Operating the primary above rated voltage usually causes the transformer to overheat. The additional stress placed on the transformer insulation by the higher primary and secondary voltages can also be serious. Operating the primary below rated voltage does no harm, but this makes the secondary voltages lower than rated values.
The rated voltages of the secondaries are specified for fullload conditions with rated primary voltage. With no load, the secondary voltage is slightly higher than rated voltage (usually 5 to 10 percent higher).
There is considerable variation in the way in which manufacturers specify centertapped secondaries. For example, the secondary in Figure 1 may be specified in any of the following ways:
 40 V C.T.
 20 V020 V
 20 V each side of center
Figure 1. Centertapped Transformmer secondary specifications. The secondary is classified as either 40 V centertapped or 20 V each side of center.
Current Rating of Transformer
Manufacturers usually specify current ratings for secondary windings only. As long as the secondary current rating is not exceeded, the primary currentcarrying capacity cannot be exceeded.
Exceeding the current rating of a secondary causes its voltage to fall slightly below rated value. More serious than decreased voltage, however, is the increase in I^{2}R loss in the secondary. The increased I^{2}R loss causes the winding to overheat and eventually destroys the transformer.
Power Rating of Transformer
Some manufacturers specify a power rating (in watts) for their transformers. This is understood to be the power the transformer can deliver to a resistive load. Thus, the power rating is merely the product of the current rating and the voltage rating of the secondary (P = VI). For multiplesecondary transformers, the power rating is the sum of the powers available from the individual secondaries (P_{T} = P_{1} + P_{2} + etc.). The total power cannot be taken from a single secondary on a multiplesecondary transformer. The current rating of the individual secondaries must not be exceeded.
Voltampere (kVA) Rating of Transformer
The voltampere rating of a transformer is an apparent power rating. It is applicable to any type of load—resistive, reactive, or combination (impedance). The voltampere rating, like the power rating, is given for the total transformer instead of for individual secondaries. With a multiplesecondary transformer, the total voltampere rating cannot be taken from a single secondary.
A transformer can be loaded to its full voltampere rating and be delivering only a fraction of its power rating. Refer to Figure 2 for an example. Here the load is a motor which has a power factor (cosθ) of 0.6. It is connected to a transformer that is rated at 750 VA and has a secondary voltage of 120 V. The motor draws 6.25 A. The transformer provides 750 VA (120 VA x 6.25 A). However, the motor is drawing only 450 W (P = VI cosθ = 120 x 6.25 A x 0.6).
Figure 2. Voltampere versus power rating. The limiting rating is the voltampere rating.
As you can see, manufacturers use many ways to rate transformers. You will never overload a transformer or exceed any of its ratings if you observe two rules.

Never apply more than the rated voltage to the primary.

Never draw more than the rated current from any secondary.
Transformer Ratings FAQs
True or false. It is not possible to exceed the voltampere rating of a transformer without exceeding the power rating of the transformer.
True or false. The power rating of a transformer can be exceeded without exceeding either the primary voltage rating or the secondary current rating.
True or false. The voltampere rating of a transformer cannot be exceeded without exceeding either the primary voltage rating or the secondary current rating.
True or false. Secondary voltages are rated at full load current and rated primary voltage.
Answers:
 F
 F
 T
 T
Transformer Ratings Example
You need to operate a 240V device that draws 10 A and has a PF of 0.8 from a 120V source. Determine the minimum specification for a transformer needed to do the job.
Given.
Load Voltage = 240 V
Load Current = 10 A
Load Power Factor = 0.8
Source Voltage = 120 V
Find.
Transformer windings voltages Transformer power rating
Known.
V_{Pri} = V_{Source}
V_{Sec} = V_{Load}
P_{Apparent} = VA
Solution.
V_{Pri} = V_{Source }= 120 V
V_{Sec} = V_{Load }= 240 V
P_{Apparent} = V_{Load }x V_{Load }= 240 V x 10 A = 2400 VA = 2.4 kVA
Answer:
An ironcore transformer rated at 2.4 kVA (or higher) with a 120V primary and a 240V secondary.
The transformer in example 1 could also have a 120V primary and a 240V secondary rated at 10 A.