Breaker size for motor

H

Hofebrand

Member
Location
Montana
Occupation
Electrician
Amperage on the motor is 40.9A. Hp10.
I will insert an image of the nameplate.
Conductor size would be Amps x 125%, that would come to 51.125 amps. So I would need a #4 Aluminum or #6 copper correct?? But the question is how do I size the breaker with the inrush on the motor? Isn't it Amps x 225% then? That would come to 92 Amps with inrush. Can I use a 100 amp breaker? My conductors will be protected since I sized them accordingly. Thanks.
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What voltage are you using? The nameplate says 10 HP, 230 volts, 1Ø. You need to look up the FLC from T430.248 and multiply that by 125% to get the minimum conductor size. For an inverse time CB you can go up to 250% of the FLC.
 
infinity said:
What voltage are you using? The nameplate says 10 HP, 230 volts, 1Ø. You need to look up the FLC from T430.248 and multiply that by 125% to get the minimum conductor size. For an inverse time CB you can go up to 250% of the FLC.
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I'm using 120/240v.

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In answer to your question, you can use the "next size up" breaker.
Note the numbers you are providing are based on nameplate and lower than NEC allows. As infinity notes, you need to use Tbale 430.248 for your calculations.
 
augie47 said:
In answer to your question, you can use the "next size up" breaker.
Note the numbers you are providing are based on nameplate and lower than NEC allows. As infinity notes, you need to use Tbale 430.248 for your calculations.
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Correct, I should have mentioned that you are permitted to round up to the next standard size. I this case at 250% I would round down.
 
infinity said:
Correct, I should have mentioned that you are permitted to round up to the next standard size. I this case at 250% I would round down.
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I deleted my above post, as it wasn't accurate. Sorry guys I don't have my code book on me, just good old fashioned Uglys. To size this motor I would take to 40.9 Amps x 250%? Is that correct?

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Hofebrand said:
I deleted my above post, as it wasn't accurate. Sorry guys I don't have my code book on me, just good old fashioned Uglys. To size this motor I would take to 40.9 Amps x 250%? Is that correct?

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To determine what size breaker i use.

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Hofebrand said:
I deleted my above post, as it wasn't accurate. Sorry guys I don't have my code book on me, just good old fashioned Uglys. To size this motor I would take to 40.9 Amps x 250%? Is that correct
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No you use the current value from the table which is 50 amps.
 
As I'm digging deeper here into the Locked roter code, this specific motor has a the Code H. If I do my calculations accordingly, I'm well into the 200 anp range for inrush current. 275ish. Darn that's a big breaker for this motor.

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Hofebrand said:
Thanks Infinity, so then the inrush current has nothing to do about it?

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Not for this application.
50*125%=62.5 amps conductor size
50*250%=125 amps circuit breaker*
I would use #6 AWG and a 125 amp breaker.

*Edit- fixed incorrect calculation
 
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Hofebrand said:
As I'm digging deeper here into the Locked roter code, this specific motor has a the Code H. If I do my calculations accordingly, I'm well into the 200 anp range for inrush current. 275ish. Darn that's a big breaker for this motor.

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Click to expand...
That 200 amp level only matters for instantaneous trip breakers or non time delay fuses. Thermal magnetic or time delay fuses will hold a pretty high current for short time during motor starting.

On top of that the actual starting current is limited by installation factors major ones being conductor size and length and source impedance. Not just the motor circuit conductors size and length but any feeders or service conductors factor in as well.

250% is what code says is maximum for thermal mag breakers but does allow you further increases if that won't allow a motor to start.

Real world applications - I have seen many 10 HP single phase motors start without tripping breaker with only a 50 amp breaker in some instnces seen 60 amp breakers in many instances. Others where there is less source to load impedance may need that 100 amp breaker that seems to be a fairly common selection though NEC will let you go with up to 125 amps initially.

I usually use a 90 amp breaker for most new installations. Square D selection calculators I have used usually recommend this.
 
kwired said:
I usually use a 90 amp breaker for most new installations. Square D selection calculators I have used usually recommend this.
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90 amps would probably work just fine. I think that even a 100 amp breaker is probably overkill for this motor. If you wanted to go near the maximum code allowable OCPD the 125* (although probably not necessary) could be used.

*Edit- fixed incorrect calculation
 
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kwired said:
That 200 amp level only matters for instantaneous trip breakers or non time delay fuses. Thermal magnetic or time delay fuses will hold a pretty high current for short time during motor starting.

On top of that the actual starting current is limited by installation factors major ones being conductor size and length and source impedance. Not just the motor circuit conductors size and length but any feeders or service conductors factor in as well.

250% is what code says is maximum for thermal mag breakers but does allow you further increases if that won't allow a motor to start.

Real world applications - I have seen many 10 HP single phase motors start without tripping breaker with only a 50 amp breaker in some instnces seen 60 amp breakers in many instances. Others where there is less source to load impedance may need that 100 amp breaker that seems to be a fairly common selection though NEC will let you go with up to 125 amps initially.

I usually use a 90 amp breaker for most new installations. Square D selection calculators I have used usually recommend this.
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@kwired, What you have stated is exactly what I have ran into as well. I am mostly familiar with these 10 hp motors, which are commonly used in agricultural systems. Older installs that i have seen used the minimum size breaker amperage and are still going strong. But like you stated with new installs, the guys that I talk to, with a 10hp motor, they would typically install a 90 to 100 amp breaker. I guess it is confusing when it comes to real word application and the code. But obviously a guy wants to protect the equipment and do the job right..

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infinity said:
90 amps would probably work just fine. I think that even a 100 amp breaker is probably overkill for this motor. If you wanted to go near the maximum code allowable OCPD the 150 (although probably not necessary) could be used.
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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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