Dear Fahmy,

I was given these thumb rules by one of my friend who is into business of making power transformers.

1. With air cooled copper conductors you can carry 1 to 1.5 Amperes per mm2 cross section of cable. for example if you need to carry 10 Amperes you must use at least 6.67 mm2 cross section or better on safer side 10 mm2.

2. With water or oil cooled also known as water/oil soaked conductors you can carry 10 to 15 Amperes per mm2 of effective conductor cross section. [ Valid for copper ].

-- Cross section selection depends on duty cycle as well you can work with smaller cross sections if the duty cycle is small.

-- When working with AC currents I prefer using stranded conductors rather than single conductor. [ Skin Effect ].

Regards,
Prahlad

Regards,
Prahlad Purohit