Cable sizing is based on rated current. Forget the voltage and size the cable on the rated current. Use the nameplate value, or if you must adhere to the NEC, use the NEC table.
You probably don't have accurate information as regards to PF and efficiency. The manufacturer had that information when the motor full load current was determined.
You have not considered overloads and protection.
The protection is based on rated current and the motor may be overloaded to the limit allowed by the protection despite the applied voltage.
Calculating the motor current from the applied voltage and the HP is something that is done in the classroom.
In grade 7 I was taught that that Current was equal to (HP x 746)/Voltage. Later I learned that the efficiency and the power factor must also be considered.
Show your boss a copy of the Cowern Papers. Link
On .pdf pp. 65 and 66 Mr Cowern PE shows that the current at +5% Volts is equal to the current at rated voltage.
Again there is no justification for using a reduced value for current at 600 Volts.
It is sometimes done in the field as a rough sanity check with the understanding that by disregarding the PF and the efficiency we are introducing an error that may be from 10% to 20%.
There is no provision in the NEC or in the CEC to size motor feeders based on voltage in the way that you propose.
I have to wonder if the wrong size cable has been ordered and your boss is trying to justify its use.
There may be other factors causing a voltage drop in addition to the drop in the motor feeders.
Use the NEC.
Step 1> Look up the motor current in the appropriate table.
Step 2> Multiply the current indicated in the table by 1.25
Step 3> The minimum cable ampacity at a 75 degree C insulation rating shall be based on the result of step 2.
Step 4> Calculate the voltage drop, Maximum 5% total from supply to the motor, of which no more than 3% shall be allowed in the motor feeders.
Step 5> If the voltage drop exceeds the limits in step 4, increase the cable size as needed.
Note: The rating of motors at 575 Volts also recognize that the utility supplied voltage may vary by plus or minus 55.
By the way, as a quick sanity check, what size are the motor feeders based on the NEC tables, based on the nameplate rated current and based on your bosses method of determining the current.
Bill
--------------------
"Why not the best?"
Jimmy Carter
