三相交流三相平衡功率公式：P=1.732UIcosφ
其中
P—三相平衡功率
1.732—根號3
U—線電壓，一般是380伏，出來的電壓常常是400伏左右
I—線電流
cosφ—功率因數，是0到1之間的數值，電阻性負載為1，電動機一般為0.75到0.85，日光燈為0.5
在(zai)實用(yong)中(zhong)求出電(dian)(dian)(dian)流(liu)(liu)后一般(ban)是(shi)用(yong)電(dian)(dian)(dian)線(xian)(xian)電(dian)(dian)(dian)纜允許長期載(zai)流(liu)(liu)量表(biao)選擇導(dao)線(xian)(xian)截面(mian)積的大小。實用(yong)中(zhong)沒有用(yong)電(dian)(dian)(dian)阻的定義式(shi)推導(dao)線(xian)(xian)徑(jing)，因為(wei)導(dao)線(xian)(xian)中(zhong)的電(dian)(dian)(dian)流(liu)(liu)還有趨膚效應；另外R=ρl/s表(biao)示(shi)電(dian)(dian)(dian)阻的關系式(shi)。這(zhe)里的R只是(shi)直流(liu)(liu)電(dian)(dian)(dian)流(liu)(liu)在(zai)導(dao)體(ti)中(zhong)受到(dao)的阻礙作用(yong)。