When the values of resistance are to be placed in a percentage format, both transformer reactance as well as level of equivalent resistance is to be placed in a percentage manner.
The percentage resistance is maintained by drop in resistance level by volts in comparison to a frequency that is taken at a rated voltage percentage level.
To answer the query as to why both these formats are expressed in percentage format – with both reactance and resistance expressing themselves in percentage mode, chances are there that both will have similar values. In case they are expressed in ohms, irrespective of secondary or primary areas, values found are different from each other.
Since current levels that will be flowed within the transformers will be dependent on impedances and percent reactance. This happens when secondary format is shorted in an accidental manner.
It is to be noted that only if transformer is minimal in size, the level of equivalent resistance is minute and amount of percent reactance can be replaced by impedance percent amount. Also, for getting a detailed view, you can surely make use of per unit values.
Hence, in most cases, these are expressed in percentage format.