high voltage winding shall carry a little current compared to lv side. so, arc energy to interrupt on hv side is little then on lv side. size of tapping switch required reduces.
so, hv side tapping switch is normally used.
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Exploring the Science of Electricity
Assuming that you have a 35MVA transformer with a Tap Changer or let us make it a little bit smaller transformer.
Assuming that you have a 1200KVA transformer or some of the transformer that you used in your plant that have the tap changer. Let do a little calculation.
Primary voltage for at the input, assume to be 11KV
Secondary Voltage at the output, assume to be 433V
Current for the primary (I) = KVA/(Vx1.732)
Current (I) = 1,200,000 / (11,000 x 1.732) = 62 Amp at 100% loading
Current for the Secondary (I) = KVA/(Vx1.732)
Current (I) = 1,200,000 / (433 x 1.732) = 1600 Amp at 100% loading
So that why the tap changer is always on the high voltage winding which carry a little current compared to low voltage side. The arc energy to interrupt on HV side is little then on LV side. size of tapping switch required reduces, as what Mr. Harry Potter had just explained.
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Transformer tap changer 
Re: Transformer tap changer
