Leatherbark
Member
- Location
- Beckley, WV
I have been inspecting installations that use dual 200 HP, 460 volt (3 phase) motors. The motors are protected by a single OCPD adjustable (instantaneous) CB. The motor branch circuit conductors are 4/0 (ICEA cables) The feeder is a 350 KCMIL. According to 430.24 the feeder should be a 500 kcmil. Calculations are as follows
200 HP 460 volt FLC= 240x1.25=300amps+240amps for the other motor =540amp feeder ampacity. The owners are using a 350 MCM (ICEA cable) with an ICEA ampacity of 465 amperes. In an underground mine we use the ICEA ampacity charts for a 20 degree ambient. The cables that are used underground are 90 Degree insulated 0-2000 volt. The ICEA chart for this type of cable needed for the above motors is 540 amperes requiring a 500 KCMIL (MCM for us old guys). If I were to use 310.16 the size needed would be 750 KCMIL. Our regulations allow the ICEA chart to be used but I digress.
My question is. In 240.24 there is an exception No. 3 that I have trouble defining. The dual 200 hp motors above are not started simultaneously. There is some delay for the inrush of the first motor. What does Exception No. 3 mean?
Exception 3.
"Where the circuitry is interlocked so as to prevent operation of selected motors or other loads at the same time, the conductor ampacity shall be permitted based on the summation of currents of the motors and other loads to be operated at the same time that results in the highest total current."
Does this exception allow a smaller feeder for this application because the motors are not started together? It seems like it is saying that the feeder size will be determined by just adding the FLC of each motor without the 125% factored in if the motors start separately. These motors are not VFD controlled
Thank you all.
Bob
200 HP 460 volt FLC= 240x1.25=300amps+240amps for the other motor =540amp feeder ampacity. The owners are using a 350 MCM (ICEA cable) with an ICEA ampacity of 465 amperes. In an underground mine we use the ICEA ampacity charts for a 20 degree ambient. The cables that are used underground are 90 Degree insulated 0-2000 volt. The ICEA chart for this type of cable needed for the above motors is 540 amperes requiring a 500 KCMIL (MCM for us old guys). If I were to use 310.16 the size needed would be 750 KCMIL. Our regulations allow the ICEA chart to be used but I digress.
My question is. In 240.24 there is an exception No. 3 that I have trouble defining. The dual 200 hp motors above are not started simultaneously. There is some delay for the inrush of the first motor. What does Exception No. 3 mean?
Exception 3.
"Where the circuitry is interlocked so as to prevent operation of selected motors or other loads at the same time, the conductor ampacity shall be permitted based on the summation of currents of the motors and other loads to be operated at the same time that results in the highest total current."
Does this exception allow a smaller feeder for this application because the motors are not started together? It seems like it is saying that the feeder size will be determined by just adding the FLC of each motor without the 125% factored in if the motors start separately. These motors are not VFD controlled
Thank you all.
Bob