1. Sizing Xfmr for Motor Loads
2. Formula –Motor LRC / Xfmr Secondary Current * Xfmr Impedance (As a Whole Number) = Voltage Drop
3. Assume a 40 HP Motor at 208V, and you have a 480V System. FLA of Motor is 120A, and assuming LRC is 6 times FLA, you would have approximately 720A.
4. If you based Xfmr Size on FLA, you would assume a 45 KVA Xfmr would suffice. Let’s look at the Voltage Drop.
5. Secondary Current of a 45 KVA Xfmr at 208V is 125A, and assume impedance is 5%.
6. 720//125 * 5 = 28.8% Voltage Drop
7. Let’s look at a 75 KVA Xfmr with 208A of Secondary Current.
8. 720/208 * 5 = 17.3% Voltage Drop
9. You would have to verify whether a 17% Voltage Drop would allow the motor to come up to speed. If not, you may need to go to 112.5 KVA Xfmr. Which is much bigger than most people would assume.
