Sizing breaker for a 10hp motor

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BenjiF23

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Electrical Engineer
Hello all. I have a 10hp, 208/3 ph motor.

Using Table 430.250, we have FLC of 30.8A.

I am trying to size the breaker for this motor. Using Table 430.52, for OCP calculation:

30.8A*1.75 = 53.9A.

Now, T430.52/430.52(C)(1) says this is the maximum rating or setting, implying I would select the next size down, or a 50A breaker.

But 430.52(C)(1) Exception No.1 permits (no verbiage indicating it requires) us to use the next higher standard ampere size if the value determined in the above calculation doesn’t correspond to a standard size. This leads me to a 60A breaker.

Is this exception “mandatory”. In other words, do I select the 60A breaker because and only because 53.9A is not a standard size? Or is this exception there simply for design choices/flexibility.


Thank you in advance for your time.
 
Table 430.52 gives 175% for "Dual Element (Time Delay) Fuses," but then you mention breakers. That would typically be an inverse time breaker, for which Table 430.52 gives 250%.

As to the question, 430.52 just gives you the maximum allowing OCPD size. You could protect the motor with a 40A breaker if you wanted to and if it would start properly. But based on the 250% value and Exception 1, you are allowed up to 80A.

Cheers, Wayne
 
It has been my experience that folks generally go to the "next size up" or at least the high end of the 250% since the breaker is only serving as GFSC protection and they want to avoid nuisance tripping.
 
augie47 said:
It has been my experience that folks generally go to the "next size up" or at least the high end of the 250% since the breaker is only serving as GFSC protection and they want to avoid nuisance tripping.
Click to expand...

Agreed.

I want the full 120 psi in my air tank although I'm only going to be using 30 psi to winterize my camper.

JAP>
 
BenjiF23 said:
Hello all. I have a 10hp, 208/3 ph motor.

Using Table 430.250, we have FLC of 30.8A.

I am trying to size the breaker for this motor. Using Table 430.52, for OCP calculation:

30.8A*1.75 = 53.9A.

Now, T430.52/430.52(C)(1) says this is the maximum rating or setting, implying I would select the next size down, or a 50A breaker.

But 430.52(C)(1) Exception No.1 permits (no verbiage indicating it requires) us to use the next higher standard ampere size if the value determined in the above calculation doesn’t correspond to a standard size. This leads me to a 60A breaker.

Is this exception “mandatory”. In other words, do I select the 60A breaker because and only because 53.9A is not a standard size? Or is this exception there simply for design choices/flexibility.


Thank you in advance for your time.
Click to expand...


The ability to go up to 250% is there to allow you to size a breaker larger than the wire size in case the motor has trouble starting. In most cases that is not an issue and rarely do you need to use the 250% on smaller motors.

So you could have a #8 conductor protected by a 60 amp or up to 250% of the Flc and still be compliant. This is assuming there is motor overload protection. The overcurrent protective device is only there for ground fault and short circuit while the overload device will prevent the current from going higher than the conductor should carry
 
With this being a 3ph motor it's most likely in a commercial or industrial establishment.

With it being in in a commercial or industrial establishment it's more thank likely a pipe and wire install where individual conductors are being used.

If individual conductors such as THHN are being used, You'll probably find that the size of the wire to feed this motor is even smaller than #8.

Just thinking out loud.

JAP>
 
jap said:
With this being a 3ph motor it's most likely in a commercial or industrial establishment.

With it being in in a commercial or industrial establishment it's more thank likely a pipe and wire install where individual conductors are being used.

If individual conductors such as THHN are being used, You'll probably find that the size of the wire to feed this motor is even smaller than #8.

Just thinking out loud.

JAP>
Click to expand...


I don't see how. #10 is only good for 35 amps at 75C. 30.8 amp x 1.25 = 38.5 amps.
 
The calculator I just referenced must be wrong also.

Let me know.

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augie47 said:
That's an odd calculator not only that it missed the wire size but it also used a 1.15 multiplier for the heaters,
The SquareD slide rule motor chart has the correct wire and heater (30.9-32)
Click to expand...

No need to choke it.

Good to know.

Thanks,

JAP>
 
Dennis Alwon said:
The ability to go up to 250% is there to allow you to size a breaker larger than the wire size in case the motor has trouble starting. In most cases that is not an issue and rarely do you need to use the 250% on smaller motors.

So you could have a #8 conductor protected by a 60 amp or up to 250% of the Flc and still be compliant. This is assuming there is motor overload protection. The overcurrent protective device is only there for ground fault and short circuit while the overload device will prevent the current from going higher than the conductor should carry
Click to expand...
thank you. i was reading into the language too much when it says it "permits" you to go up a size. so i was looking for a scenario where you would do that since the 250% is already the maximum rating per T430.52.

Thank you all for your input
 
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