Is there a standard for Bus-Bar current rating calculation (copper)

[aju11]

 

Google search gives  ->  Current =1.2 x cross section

Is there any standard to prove this?

what are the considerations like length, thermal parameters while using factor 1.2?

[aju11]

 Also, current capacity of wire is calculated by thumb rule:  Current = 4 x cross section Or sometimes 6 x cross section.

Why factor of 4 or 6 for wire? and Why factor of 1.2 for Bus bar.

[strawberry]

acceptable power loss. ohms law
1mm^2 1m = 10W loss , 1mm^2 10m = 100W loss

[Berni]

It's not as simple as multiplying with a magic number. You want to use a table like this:
https://www.australwright.com.au/technical-data/advice/copper-busbar-rating/

For example a wide thin bus bar is going to have more cooling than a square crosssection and so be able to handle more current.

This is also the reason that wires can handle more current per crosssection area. The circumference of a wire does not change linearly with the crosssectional area. So this esentialy means a thin wire has more cooling capability per given crosssection and so can carry more current density

[strawberry]

For example a wide thin bus bar is going to have more cooling than a square crosssection and so be able to handle more current.

if vertical but horizontal could be worse than square
cooling affected by surface area and airflow
shouldn't be much difference in cooling effect between wire size if conductor temperature is less than ~50C or minimal difference between conductor and ambient temperature