A current density of __________ is ilsed for large power transformers with forced circulation of oil or with water cooling coils?
A. 1.5 to 2.5 A/mm2
B. 3.5 to 4.5 A/mm2
C. 4.0 to 5.0 A/mm2
D. 5.4 to 6.2 A/mm2