Bus bar joint contact area vs cross-section area

[MMC2258]

I'm trying to understand how you can justify the joint of bus bars that are 2 different sizes. I saw in a Legrand busbar and distribution book (http://www.legrand.com/files/fck/File/pdf/Power_guide/EX29016.pdf) on page 26 that the contact area must be at least 5 times the cross-section area. I was just curious if anyone else has heard of that rule of thumb and if it's defined anywhere else?

 

[GoldDigger]

I'm trying to understand how you can justify the joint of bus bars that are 2 different sizes. I saw in a Legrand busbar and distribution book (http://www.legrand.com/files/fck/File/pdf/Power_guide/EX29016.pdf) on page 26 that the contact area must be at least 5 times the cross-section area. I was just curious if anyone else has heard of that rule of thumb and if it's defined anywhere else?

I think that the simple answer is that the contact area has to be at least 5 times the cross-section area of the smaller bus bar, since that area determines the spec limit for normal current through the joint.