Bering C Sparky
Lifetime Supporting Member
Hello all,
Well I am going to throw this out here just to get a general consensus of others thoughts of this subject.
We are going to do a Refrigeration Upgrade this Fall.
New Compressor Motors will be 300 HP 460v 3 phase.
This was supposed to be all planned out by the office already and contractors lined up to do the job. (Great No Worries for Me Right??)
Well today I get an email saying, "I think maybe we are going to have to increase the breaker and wire size for the new compressors, can you check this out and see what we need and if it will fit in our existing distribution"
Are you kidding me, "YOU THINK" here we go again, million dollar upgrades drawn out on ****tail napkins, being read through the ice in a scotch glass!!!
Anyway, since the only information I was given was the motors are 300 HP, I break out NFPA-70 turn to table 430.250.
300 hp motor, 3 phase, 460 volt = 361 FLA
Now I flip to 430.52 and the multiplier for Inverse Time Circuit Breaker is 250%.
361 x 2.50 = 902.5 amps
So 1000 amp breaker is needed for this application. (At least in my thinking)
Here is the kicker,
I look at an older 300 hp compressor we have on this vessel, it is on a 400 amp breaker.
I go over to another one of our ships that received this same Refer Upgrade a year ago and they put the Compressors on 500 amp breakers.
So where did someone come up with the sizing for these breakers?
I realize that they work and don't trip when the motor starts (they are on soft starts) but how did one come about calculating the size for them other than using 430 of the NEC?
I have found no exception or article that says you can/should use smaller breakers for motor circuits just because it is on a soft start.
Does anyone know of something I am missing here, or have an opinion on the subject.
Regards,
BCS
Well I am going to throw this out here just to get a general consensus of others thoughts of this subject.
We are going to do a Refrigeration Upgrade this Fall.
New Compressor Motors will be 300 HP 460v 3 phase.
This was supposed to be all planned out by the office already and contractors lined up to do the job. (Great No Worries for Me Right??)
Well today I get an email saying, "I think maybe we are going to have to increase the breaker and wire size for the new compressors, can you check this out and see what we need and if it will fit in our existing distribution"
Are you kidding me, "YOU THINK" here we go again, million dollar upgrades drawn out on ****tail napkins, being read through the ice in a scotch glass!!!
Anyway, since the only information I was given was the motors are 300 HP, I break out NFPA-70 turn to table 430.250.
300 hp motor, 3 phase, 460 volt = 361 FLA
Now I flip to 430.52 and the multiplier for Inverse Time Circuit Breaker is 250%.
361 x 2.50 = 902.5 amps
So 1000 amp breaker is needed for this application. (At least in my thinking)
Here is the kicker,
I look at an older 300 hp compressor we have on this vessel, it is on a 400 amp breaker.
I go over to another one of our ships that received this same Refer Upgrade a year ago and they put the Compressors on 500 amp breakers.
So where did someone come up with the sizing for these breakers?
I realize that they work and don't trip when the motor starts (they are on soft starts) but how did one come about calculating the size for them other than using 430 of the NEC?
I have found no exception or article that says you can/should use smaller breakers for motor circuits just because it is on a soft start.
Does anyone know of something I am missing here, or have an opinion on the subject.
Regards,
BCS