An samun karshe, ko da ake kira I²R loss, yana faru a tafin autotransformer kamar yadda yake faru a wasu abubuwa masu transformer. Wannan samun karshe yana faru saboda matsayinta na karshe na copper a tafin. Idan karamin kai shiga tafin, zama aikin kimiyya ta faruwar hawa saboda wannan matsayi.
A cikin autotransformer, wanda ya yi amfani da tafi mai kowane don fun primary da secondary, samun karshe na copper tana faru. An samun karshe na copper ya ci gaba da tsarin:
P = I²R,
ida:
P itace samun karshe na watts (W),
I itace karamin kai wanda ya shiga tafin a amperes (A),
R itace matsayinta na tafin a ohms (Ω).
Saboda tafin da ake yi amfani da shi a kan primary da secondary yana bar karamin kai (jin karamin kai na primary da secondary load currents), mafi yawan karamin kai a kan tafin da ake yi amfani da shi itace mafi. Amma, saboda tsarin da kyau na autotransformer da kyau da kyakkyawar voltage transformation, samun karshe na copper itace mafi zama daga wadanda ke faru a equivalent two-winding transformer, ba mafi, saboda karamin kai mafi yana shiga baka daya na tafin da kuma ilimin karshe na tafin cece.
Amma, cin samun karshe na copper itace muhimmiyar abu na design. Wannan an samu ne ta amfani da karshe na matsayin daidai da kyau da kuma ci gaba da tsarin tafin. Yana da kyau a fuskantar hawa daidai don hasashen cewa transformer yana yi aiki a cikin limits na hawa daidai.