MCC Breaker Tripping - Troubleshooting Help

[bindem]

Just a retired commercial electrician but here is my 2¢. The numerous 100 HP 4 Pole plain Jane NEMA frame 480 volt motors that I installed & PM'ed had 124 amps FLA So a 100, 150'& 200 HP 480 Volt three phase motors would have a combined ampere of 558 amps if my old brain is correct. That is too high of a current for a 600 amp circuit breaker. If all three motors were on VFD'S with at least a 20 second ramp up time and maximum Hertz reduced somewhat ( if your system can deal with reduced running speed ) you should be able to get away with it until a larger circuit breaker along with larger wire are installed.

Can you expand on this? The breaker is 100% rated, so my understanding is that it can handle any value under 600A continuously.

 

[garbo]

Can you expand on this? The breaker is 100% rated, so my understanding is that it can handle any value under 600A continuously.

Most circuit breakers that close to motor FLA will cause tripping due to high inrush current. Forget what code article covers multiply motors on a single circuit but usually you have to mutually the motor with the highest ampere by 125% then FLA on remaining motors so here goes At 480 volts three phase plain Jane NEMA motor the 200 HP motor FLA = 248 Amos × 1.25 = 310 amps. 100 HP = 124 Amps & your 150 HP = 186 amps. 319 + 124 + 186 = 629 amps. For general starting loads I liked to go 175% of that #,. 628 × 1.75 = 1,099 amps so it appears you would need a 1,200 amp circuit breaker if you have to run all three motors at the same time. I disagree with the motor amps in the NEC . Of the thousands of motors that I wired in, troubleshoot, Pm"ed, measured ampere etc over 50 years never had one closer then maybe 94% of what the NEC values you should use. Even some of the 1950 era NEMA think they were called U frame less efficient motors that I came across still were lower then the NEC tables. If load was 100% resistive you might get away with a load at 600 amps on your circuit breaker . I installed 6 100 amp circuit breakers to run 100 KW load banks for several Diesel generators. We only had used 100 amp 600 volt circuit breakers and wanted them to order 150 amp circuit breakers. Two different brands due to two different panels. We ran them ( the 100 KW portable load banks ) for 1 to 2 hours a month and four hours once a year and we always ran them set for maximum 100 KW,load . which is 124 amps @ 480 volts. I was surprised the load banks never tripped one of these 100 amp circuit breakers.