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Confusing on sizing MCB and Cable

Confusing on sizing MCB and Cable

Confusing on sizing MCB and Cable

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 ...

RE: Confusing on sizing MCB and Cable

Are you attempting to use the MCB as the only protection for the motor, because that is very difficult to get correct. To protect the motor from overload, the breaker usually ends up being too small to allow for inrush and starting current and nuisance trips. But if sized to allow for starting, you fall outside of the damage curve on overloads. That's why they make overload relays. So if you have an OLR, then the MCN need only provide the short circuit protection and can be sized differently. Here in North America for instance, the breaker can be 250% of the FLA when there is an OLR.

"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

Hi Jraef,

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?


RE: Confusing on sizing MCB and Cable

Your OL is too big for that motor. When selecting the OLR, you do NOT factor in the 125%, you set it for the nameplate FLA. The higher pick-up point for the OL curve is already factored in to the design by the OLR manufacturer. It will tell you this in the instructions.

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

Hi Jraef,

thanks and appreciate your helpful reply..

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