Conductor Size to Motor Disconnect and Motor

[Flapjack]

I have a cable from a VFD to a local motor disconnect that is routed underground in a duct bank of 6. Motor is 60 HP @ 460 V.
77 amps * 1.25 = 96.25 amps. If the cable wasn't routed in the duct bank and was above ground, the minimum cable size per Table 310.15(B)(16) is 3 AWG at 75C.
With 6 electrical ducts, the cable size is 1 AWG per Table B.310.15(B)(2)(7).

Since the cable from the local motor disconnect to the motor is entirely above ground, would this be considered a continuation of the cable in the electrical duct and 1 AWG would be used, or would it be considered separate and a 3 AWG would be used?

 

[Jraef]

The higher size when part of a duct with the other conductors has to do with mutual heating of the conductors in the duct. Once they are separate that is no longer necessary.

Separate issue; I hope that is shielded VFD cable in that duct with other conductors. Otherwise you are asking for trouble.

 

[Flapjack]

So if I called out for 1 AWG from the local disconnect to the motor, then the cable is increased in size from the minimum size that has sufficient ampacity for the intended installation and 250.122(B) would apply.

Yes, it is VFD cable.

 

[Jraef]

So if I called out for 1 AWG from the local disconnect to the motor, then the cable is increased in size from the minimum size that has sufficient ampacity for the intended installation and 250.122(B) would apply.

Yes, it is VFD cable.

I'm not clear on your wording, it's late in the day and the mental fog is settling in. But it sounds as though you are asking if it is OK to over size the conductors all the way to the motor by using the size required for the duct bank potion. If that's what you are asking, then yes, it certainly is never a code problem to over size conductors. It might be an issue in terminating them in the motor peckerhead, so keep that in mind. Nobody wants to be that electrician who has to make up those connections with over sized cable that is too stiff to maneuver in a box designed for smaller cables...

 

[Julius Right]

In my opinion, there are two different cables. You have no splices but terminals in boxes.
You may oversize it if you want it but you don't need it.
NEC art 310.15 (A) (2) Selection of Ampacity. and 310.60(A)(1) Selection of Ampacity
is referring to same cable passing in different type locations.