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- Engineer
Exactly, and 430.62 is giving you a MAXIMUM OCPD size. 430.63 is giving you a maximum OCPD for conductors sized in accordance with 430.24 for Motors and Other loads.
Feeder Calculation Largest Motor Multi Branch Circuit
- Thread starter fifty60
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Exactly, and 430.62 is giving you a MAXIMUM OCPD size. 430.63 is giving you a maximum OCPD for conductors sized in accordance with 430.24 for Motors and Other loads.
petersonra
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- Semi-retired engineer
here's a screwy question for you.
you have 2 ten HP motors on the same feeder.
14X2.5 = 35A.
So 35A is the maximum OCPD that can provide SC protection to the motors.
What allows you to feed both motor controllers with the same 35A feeder breaker?
and what do you size the conductor ampacity at?
there are no downstream CBs. so the feeder has to be sized to the ampacity of the conductors, right?
you have 2 ten HP motors on the same feeder.
14X2.5 = 35A.
So 35A is the maximum OCPD that can provide SC protection to the motors.
What allows you to feed both motor controllers with the same 35A feeder breaker?
and what do you size the conductor ampacity at?
there are no downstream CBs. so the feeder has to be sized to the ampacity of the conductors, right?
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I disagree. You are choosing to ignore "based on the maximum permitted value." If your view was correct, they could just leave the part in the brackets out. Its superfluous.
Consider two feeders with identical motors. One had a 70A, 20A and 20A branch c/b...and the other a 60A, 20A and 20A branch c/b. The feeder conductors would be the same size, the full load current on both feeders would be the same size, the motor overloads on each motor would be the same size. Why would one require a smaller feeder c/b?
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I'm not sure where your going with this, but let's start with the most obvious...
If there are no downstream CBs, then you have a branch circuit, not a feeder.
14x2.5 + 14 = 49, so 45A is maximum OCPD that can provide SC protection to the feeder. Each motor would need its own branch circuit SC protection.
The feeder conductors would need an ampacity of at least 14x1.25 + 14 = 32A, or #10AWG.
Each branch circuit could be supplied by a #14AWG from the 35A branch OCPD.
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- Engineer
If you really wanted to consider two motors on the same branch circuit, lets consider two 15HP motors instead.
The max OCPD would be 21x2.5 = 52.5 => or a 60A breaker. This would fall under 450.53(B). (That section also requires that it be determined that the branch OCPD will not open under the most severe normal operating conditions.)
The conductor ampacity would have to be 21x1.25 + 21 = 47.3, or #8. So the branch OCPD can be larger than the ampacity of the conductors.
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petersonra
Senior Member
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- Northern illinois
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- Semi-retired engineer
i am ignoring it because it is explanatory in nature and not an allowance to use a larger value than is actually installed.
it isn't superfluous, it is explanatory.
its because the code says so. No good reason.
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petersonra
Senior Member
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- Northern illinois
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- Semi-retired engineer
I still don't see where you are allowed to go up to the next std size CB.
450.53(B) is handy when you have two motors of the same size, and you are convinced that if you start them up simultaneously they won't trip, or you make it so they can't start simultaneously.
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According to 90.5(C), explanatory material is included in Fine Print Notes. In addition, Brackets containing references to other NFPA documents are for informational purposes.
Since the bracketed text in 430.62 is neither a Fine Print Note nor a reference to another NFPA document, I would have to believe that it is not explanatory material, but part of the Code text.
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- Connecticut
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- Engineer
450.53(B) says "IF the branch circuit short-circuit and ground-fault protective device is selected not to exceed 430.52 for the smallest rated motor..."
430.52 says the protective device (inverse c/b) shall not exceed 250% of the motor FLA, exception 1 lets you go up to the next standard rating.
petersonra
Senior Member
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- Northern illinois
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- Semi-retired engineer
yea. I was still thinking of 430.62.
These multi-motor things make my head spin sometimes.
makes me glad for type F motor starters.
Sorry for the headache's guys. So let me see if I understand. 215.3 is giving the MINIMUM feeder size that you can have, which is 125% of all the continuous loads motor or non motor. 430.62 and 430.63 is giving you the maximum size OCPD. So if I am using a D curve breaker sized at 125% of the largest motor load, and add all of the other loads together...and this is less than the value obtained by the method in 215.3, then I HAVE to use the value in 215.3. I can never have a feeder OCPD larger than 430.62-3 UNLESS that value is less than 215.3, then I have to use the rating obtained in 215.3.
petersonra
Senior Member
- Location
- Northern illinois
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- Semi-retired engineer
you can have whatever size OCPD you want if you make the wires big enough.
this whole exercise is about making the OCPD as small as possible for the minimum wire size required.
They can be whatever size, but they must be at least as big as the rating obtained in 215.3. 430.62-3 allows you to raise the size of the breaker whilst not raising the size of the conductor, and still be able to maintain the minimum conductor size pointed to by 430.62. Does that sound correct?
There is no situation where I can size teh motor OCPD close to the motor (like 125%) and then be able to use a feeder OCPD under the rating obtained from 215.3? Taking the motor only OCPD sized at 125% is going to be less than taking all of the loads multiplied by 125%...
There is no situation where I can size teh motor OCPD close to the motor (like 125%) and then be able to use a feeder OCPD under the rating obtained from 215.3? Taking the motor only OCPD sized at 125% is going to be less than taking all of the loads multiplied by 125%...
Can I use 430.63 to size my OCPD smaller than what is designated by 215.3? 125% of the largest motor plus all of the loads is smaller than 125% of all the loads.
It seems by the explanations here is that I can get my minimum feeder conductor size, which is 125% of largest motor, 100% of other motors, and 125% of everything else and then use an OCPD for that conductor in accordance with 430.63. If the OCPD rating is less than the ampacity of the above conductor then I can use it. But this is where I get confused...
215.3 is giving the MINIMUM i can use...does whatever I calculate in 430.63 have to be above this, or I am forced to default to the 215.3 value?
The advantage of 430.63, with my current understanding, is that you can size the OCPD larger than the ampacity of the feeder conductor within the confines of 430.63.
It seems by the explanations here is that I can get my minimum feeder conductor size, which is 125% of largest motor, 100% of other motors, and 125% of everything else and then use an OCPD for that conductor in accordance with 430.63. If the OCPD rating is less than the ampacity of the above conductor then I can use it. But this is where I get confused...
215.3 is giving the MINIMUM i can use...does whatever I calculate in 430.63 have to be above this, or I am forced to default to the 215.3 value?
The advantage of 430.63, with my current understanding, is that you can size the OCPD larger than the ampacity of the feeder conductor within the confines of 430.63.
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