Confusing on sizing MCB and Cable
Confusing on sizing MCB and Cable
(OP)
Hi Guys,
I am confuse while checking my contractor drawing.
Motor FLA = 47A
MCB 3P = 80A (Installed outdoor at 55degC inside enclosure, derating to 72A)
Power rating = 480V 3 phase 60Hz
Cable Run around 50 meter
Cable selected 35 mm2 (Current capacity in catalog 136 A)
While I doing the checking
Cable derating current is around 87A
With consider the rule of thumb
FLA (A) < MCB (A) < CABLE (A)
47A < 72A < 87A
All looking fine.. but if I look at motor load only 47A .. correct selection shall be 47A x 1.25 = 58.75A next jump is 63A MCB.
It is ok to use 80A breaker? Look like oversize. But the rule of thumb is fulfill... I am confuse ...
I am confuse while checking my contractor drawing.
Motor FLA = 47A
MCB 3P = 80A (Installed outdoor at 55degC inside enclosure, derating to 72A)
Power rating = 480V 3 phase 60Hz
Cable Run around 50 meter
Cable selected 35 mm2 (Current capacity in catalog 136 A)
While I doing the checking
Cable derating current is around 87A
With consider the rule of thumb
FLA (A) < MCB (A) < CABLE (A)
47A < 72A < 87A
All looking fine.. but if I look at motor load only 47A .. correct selection shall be 47A x 1.25 = 58.75A next jump is 63A MCB.
It is ok to use 80A breaker? Look like oversize. But the rule of thumb is fulfill... I am confuse ...





RE: Confusing on sizing MCB and Cable
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Confusing on sizing MCB and Cable
The Motor starter have 80A TP Contactor and OLR 63-80A.
We take IEC standard as guideline.. So if the motor start come with OLR. 1.25 times for FLA will be the MCB rating?
thanks.
RE: Confusing on sizing MCB and Cable
I can speak to IEC rules. But again, with a properly selected OLR, the breaker need only provide short circuit protection. So mostly you use the magnetic trip curve of the breaker.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Confusing on sizing MCB and Cable
thanks and appreciate your helpful reply..