sizing of wire after distribution blocks

[ex protector]

Dear all,
I'm calculating a cabinet for UL508A. There is an fused (150A) incoming line 5x50 mm² (3x208V/N/PE) which is connected to terminals.
After the terminals I want to wire the power lines to single distribution blocks like CHDB from eaton or UDJ from nvent.

So my question is: Is it allowed to reduce the wire diameter from 50 mm² to 16 mm² or smaller (4 mm² are enough) to different MCBs (rating max. 20A).
The reduced wire between distribution and MCB is not fused. Is this okay?

Can anybody help me?

Many thanks beforehand for all comments.

Kind regards
Thomas

 

[Tony S]

Are you working to NEC or IEC?

 

[ex protector]

Working with NEC, backplanes need to be UL508A

 

[Julius Right]

NEC 2017 Art.240.21 Location in Circuit.(B) Feeder Taps
If the length of tapped conductor up to OCPD [overcurrent protection device- fuse, breaker] is as limited here and the conditions are as required you may connect the wire without other protection.

 

[ex protector]

Many thanks, this helps!