Brent S in Cincinnati

My question is, even though I know it is acceptable in an enclosure to size the conductors to the load protection (motor protectors here) isn't there a minimum ratio ( 10:1 ) of the main fuse size to the current capacity of the conductors coming off of distribution blocks? I haven't been able to find this in the code. I usually build 1 and 2 door PLC panels with servos and vfds, so I'm trying to be careful with the 8 door vfd enclosure I'm putting together. I have a main disconnect for 3 phase 460VAC with 350 Amp fuses. This feeds a distribution block where the conductors step down to #1 AWG. From there I supply each of 4 distribution blocks, 1 for each pair of doors. From there I have #10 AWG feeding groups of 5 motor protectors through bus bars. In this setup the #10s are fed directly from 350 Amp fuses. It seems a little extreme to me. There are 154 Powerflex 40 vfds, mostly 1 hp. Each Powerflex is fed from its own Motor Protector, AB 140M-C2E-B40 for the 1hp vfds.

I have someone asking me about the drop length with respect to allowable current. It isn't an issue for the network I'm designing since it is all daisy chained on the trunk but I still need to answer the question. Page 1-16 in the Devicenet Media Planning manual has a chart of the allowable current for different drop lengths. Does anyone know why the current would depend on drop length? I understand why it matters for the trunk length with voltage drop on an 18 gauge conductor. I don't understand why a 6m drop would be limited to 0.75A or why the max for any drop is 3A. Thanks