10HP motor wire size

[gene6]

Question, and my apologies if this is a motors 101 question please take it easy I am not a calc guy.
A 10 HP motor 208/3ph is being replaced, one of the conduit runs feeding it is stuffed with 9 CC so presumably the existing #8 cu needs a 70% derate 55 X .7 = 38.5A
Now the wire has to have an ampacity of 125% of the value in table 430.250.
I go to the table and a 208V 10HP motor is 30.8 A
30.8 X 1.25 = 38.5
But in the 200V column its 32.2A
32.2 X 1.25 = 40.25
So which column is correct?
There is not a 440V and a 480V column so why 200 and 208?
Thank you.

 

[Jraef]

There is not a 440V and a 480V column so why 200 and 208?

Motors are typically designed based on a "utilization voltage" standard, which is LESS than the associated distribution standard in order to accommodate expected voltage drop from the service point to the motor terminals. So for 480V distribution, motors are made to accept 460V, +-10%; 240V systems get 230V motors, +-10%, 208V systems get motors rated for 200V +-10%. You can expect rated motor performance within that +-10% window.

But for motors typically under 10hp, and because it is expensive to inventory 3 different voltage ratings of motors, motor manufacturers offer motors designed as "tri-voltage", meaning 208-230V / 460V, which compromises on the lower voltage rating by only guaranteeing performance at -5% instead of -10%. So your motor voltage, even after voltage drop with distance, is going to need to be no less than 198V. This is considered a reasonable compromise because 208V is typically only used in smaller commercial or light industrial facilities where the distances are less, and mostly for HVAC loads where the motor tends to be lightly loaded anyway. But if your application is critical and or your distance is long or the HP is greater than 10, you would need to use 200V motors.

You are expected to know what your motor actually says on it and use the appropriate data for conductor selection. If your motor nameplate says 208V, you use that column. If not, or you don't know, use the 200V column.

 

[gene6]

Motors are typically designed based on a "utilization voltage" standard, which is LESS than the associated distribution standard in order to accommodate expected voltage drop from the service point to the motor terminals. So for 480V distribution, motors are made to accept 460V, +-10%; 240V systems get 230V motors, +-10%, 208V systems get motors rated for 200V +-10%. You can expect rated motor performance within that +-10% window.

But for motors typically under 10hp, and because it is expensive to inventory 3 different voltage ratings of motors, motor manufacturers offer motors designed as "tri-voltage", meaning 208-230V / 460V, which compromises on the lower voltage rating by only guaranteeing performance at -5% instead of -10%. So your motor voltage, even after voltage drop with distance, is going to need to be no less than 198V. This is considered a reasonable compromise because 208V is typically only used in smaller commercial or light industrial facilities where the distances are less, and mostly for HVAC loads where the motor tends to be lightly loaded anyway. But if your application is critical and or your distance is long or the HP is greater than 10, you would need to use 200V motors.

You are expected to know what your motor actually says on it and use the appropriate data for conductor selection. If your motor nameplate says 208V, you use that column. If not, or you don't know, use the 200V column.

Thank you, yeah I did not get a good read of the nameplate other than 10HP and 215T, I called the local motor shop and they also recommended the 200V column for 208.
However management did not want to rip things apart so it stays on #8 for now.