Breaker size for motor

[infinity]

If code allows you 150 amps breaker size for this particular motor, but i can use a 90 amp breaker, how do u determine that? I mean that's a big difference in amps and $$. As I see this as long as the conductors are protected, and the breaker size doesn't fall under the conductor amps, I'm ok??

It's tricky because you don't know if a smaller OCPD will allow the motor to start. There is really no answer other than maybe real world experience with this motor type. You can also do as kwired suggested and use a motor circuit calculator and see what that suggests.

 

[Hofebrand]

It's tricky because you don't know if a smaller OCPD will allow the motor to start. There is really no answer other than maybe real world experience with this motor type. You can also do as kwired suggested and use a motor circuit calculator and see what that suggests.

Awesome, yes that is the conclusion that I've come to. As one of you guys stated above, the Locked roter code calculations is only for non- thermally protected motors then?? As this motor we are discussing is thermally protected. Thank u guys so much for the knowledge, appreciate it

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[kwired]

So that brings me to my final question for you guys.. how the heck does a guy actually size this thing?? If code allows you 150 amps breaker size for this particular motor, but i can use a 90 amp breaker, how do u determine that? I mean that's a big difference in amps and $$. As I see this as long as the conductors are protected, and the breaker size doesn't fall under the conductor amps, I'm ok??

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But obviously a guy wants to protect the equipment and do the job right..

The circuit breaker is for short circuit and ground fault protection. Overload protection is inherently achieved via the motor overload protection. During SC/GF there is very high current but the 90, 100 or 125 amp breaker will still trip before any conductor insulation damage occurs.

During starting you may briefly see current levels that are pretty high yet only at SC/GF levels for just milliseconds at the most, if supply circuit impedance is low enough to allow that kind of current level. Those impedance levels from my experience are seldom that low for most single phase utility supply applications. But nonetheless NEC allows these larger sized breakers/fuses to allow trip free motor starting. The conductors will never see longer term overcurrent levels because of the motor overload protection.

 

[kwired]

Awesome, yes that is the conclusion that I've come to. As one of you guys stated above, the Locked roter code calculations is only for non- thermally protected motors then?? As this motor we are discussing is thermally protected. Thank u guys so much for the knowledge, appreciate it

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I never use those calculations. If anything they are for determining switch ratings when you have more than a single motor for load being supplied by the switch. Single motor - the HP rating of the switch generally is all you need.

As I mentioned a couple times already the amount of current that actually flows during motor starting is dependent on impedance between the source and the motor.

Say your 10 HP motor here is supplied by a 25 kVA transformer. It naturally will have a higher impedance than if it were supplied by a 75 or 100kVA transformer and will have higher current at starting even if everything else about the installation were the same. That motor wants to draw well over 100 amps at startup probably even over 300 for a short time if it can be delivered, but that 25kVA only being rated near 100 amps is possibly going to become a bigger impedance in the circuit than your motor circuit conductors are. Then comes conductor impedance - longer runs will lessen that starting current. Even if you oversize conductors for voltage drop at rated load, they still briefly draw a lot of current at startup and there will be significant VD during that brief time which will also lessen starting current. Also will likely decrease torque and acceleration time, so it effectively is a little bit of a soft starter by limiting voltage and current during starting. This all also holds true for three phase motors, but with three phase you have more places with higher capacity sources and with the third conductor also have lower current to begin with for same power output if at same voltage so there is some differences that may be rather common.

This is why I have seen some single phase 10 HP motors start with no tripping on a 50 amp breaker.

 

[Hofebrand]

I never use those calculations. If anything they are for determining switch ratings when you have more than a single motor for load being supplied by the switch. Single motor - the HP rating of the switch generally is all you need.

As I mentioned a couple times already the amount of current that actually flows during motor starting is dependent on impedance between the source and the motor.

Say your 10 HP motor here is supplied by a 25 kVA transformer. It naturally will have a higher impedance than if it were supplied by a 75 or 100kVA transformer and will have higher current at starting even if everything else about the installation were the same. That motor wants to draw well over 100 amps at startup probably even over 300 for a short time if it can be delivered, but that 25kVA only being rated near 100 amps is possibly going to become a bigger impedance in the circuit than your motor circuit conductors are. Then comes conductor impedance - longer runs will lessen that starting current. Even if you oversize conductors for voltage drop at rated load, they still briefly draw a lot of current at startup and there will be significant VD during that brief time which will also lessen starting current. Also will likely decrease torque and acceleration time, so it effectively is a little bit of a soft starter by limiting voltage and current during starting. This all also holds true for three phase motors, but with three phase you have more places with higher capacity sources and with the third conductor also have lower current to begin with for same power output if at same voltage so there is some differences that may be rather common.

This is why I have seen some single phase 10 HP motors start with no tripping on a 50 amp breaker.

That makes alot of sense, Thank you much for the info and the knowledge, God Bless

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[Tulsa Electrician]

Thanks for the info on red colored motors.

 

[Eddie702]

So that brings me to my final question for you guys.. how the heck does a guy actually size this thing?? If code allows you 150 amps breaker size for this particular motor, but i can use a 90 amp breaker, how do u determine that? I mean that's a big difference in amps and $$. As I see this as long as the conductors are protected, and the breaker size doesn't fall under the conductor amps, I'm ok??

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The breaker has to be selected that will allow the motor to start. Don't forget that you have thermal protection in the motor starter or in the motor that will shut the motor down on overload. The CB is more for short circuit protection. The 90 amp discussed sounds about right. Usually a breaker 50%-60% larger than the amp rating will start the motor unless there is something unusual.

 

[Hofebrand]

Thanks for the info on red colored motors.

No problem, yes red Weg motors very common on these grain handling systems.

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[Hofebrand]

The breaker has to be selected that will allow the motor to start. Don't forget that you have thermal protection in the motor starter or in the motor that will shut the motor down on overload. The CB is more for short circuit protection. The 90 amp discussed sounds about right. Usually a breaker 50%-60% larger than the amp rating will start the motor unless there is something unusual.

Thanks Eddie!

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[kwired]

Thanks for the info on red colored motors.

I think it is pretty standard for WEG and Leeson to have red colored farm duty motors. Baldor farm duty motors are usually tan from my observations.