The rule of thumb is when the HP exceeds the voltage, go to a higher voltage.
But there are exceptions.
If you have a 4000 Volt system in place and are already using 4000 Volt motors, then strongly consider going to 4000 Volts.
If you do not presently have 4000 Volts available, it may be more reasonable to use a 460 Volt motor.
Your present maintenance crew may not be qualified to service 4000 Volt or 4160 Volt equipment.
When you compare the installed cost estimates of the two solutions the answer may be readily apparent.
Bill
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"Why not the best?"
Jimmy Carter
400HP is really at the breakpoint between voltage levels, so as Bill says you could choose either option and not be 'wrong' , but the decision may be made by other factors such as availability of an existing MV network. You almost certainly wouldn't put in a complete MV switchboard just for one motor.
If you already have the 4 kV infrastructure in place, , as others have said, you're in the lower end of the range for 4 kV motors. I've seen them as small as 150-200 HP, so you're on safe ground in your choice.
A 480 volt motor is going to have some BIG cables, very difficult to deal with for connections, etc., and is much more affected by voltage drop issues. On that subject, you will keep in mind that across the line starting of a motor in that size range is going to have significant effect on the system feeding it,
I have seen facilities that brought in a separate feed and a single outdoor metal-clad switchgear unit for a single motor just to avoid the voltage sag issues a low voltage motor would introduce into an existing system.
