motor FLC

Status
Not open for further replies.
electricalperson said:
well how did they do it and come up with those numbers when motor FLA is lower than the FLC in the tables
Click to expand...
To calculate motor current you need to know kW rating, power factor, and efficiency. I = kW*1000/(V*pf*effy*Sqrt(3))
Not all motors are the same in terms of PF and efficiency and I don't know how the NEC accounts for the differences. I imagine they take typical figures for PF and efficiency and maybe err on the conservative side.
I don't really think you can take the tables as absolute cast-in-stone values.
 
electricalperson said:
well how did they do it and come up with those numbers when motor FLA is lower than the FLC in the tables
Click to expand...



I don't know how it is all created.
I can remember when I was studying for a electrical exam.
My wife, who has her masters in math. She was able to get the answer in an other way, then I, and faster.
In the old days the examing board would have you answer a calculation in front of them. Because they know that a math matician can get the answer with out knowing any thing about electricity. It all has something to do with that pie = x stuff. Mike has the chart on the back of shirts you can purchase from this site.
 
buckofdurham said:
I don't know how it is all created.
I can remember when I was studying for a electrical exam.
My wife, who has her masters in math. She was able to get the answer in an other way, then I, and faster.
In the old days the examing board would have you answer a calculation in front of them. Because they know that a math matician can get the answer with out knowing any thing about electricity. It all has something to do with that pie = x stuff. Mike has the chart on the back of shirts you can purchase from this site.
Click to expand...
Mathematics is an important and necessary discipline in the field of engineering - including electrical engineering.
 
NEC motor currents were chosen so that any manufacturers motor could be substituted without having to change the wiring. I think that at one time there was an agreement that no motor manufacturer would exceed NEC values (this is part of the concept behind NEMA starter sizes).
 
The fla in the nec is not always exact. It is usually very close and good enough for me. I try to always use name plate. Two motors the same voltage,rpm, and hp from different manufactures sometimes have different fla. I think it is due to motor designed and other factors like slip.
 
shallo said:
The fla in the nec is not always exact. It is usually very close and good enough for me. I try to always use name plate. Two motors the same voltage,rpm, and hp from different manufactures sometimes have different fla. I think it is due to motor designed and other factors like slip.
Click to expand...
In general for short circuit and ground fault protection, switch sizing and conductor sizing, the NEC requires you to use the FLAs as shown in the code tables. 430.6
 
jim dungar said:
NEC motor currents were chosen so that any manufacturers motor could be substituted without having to change the wiring. I think that at one time there was an agreement that no motor manufacturer would exceed NEC values (this is part of the concept behind NEMA starter sizes).
Click to expand...

was it a number picked out of a hat that sounded good enough or was it all science?
 
electricalperson said:
was it a number picked out of a hat that sounded good enough or was it all science?
Click to expand...
The science was to find a value that was greater than that which would be found in the real world. Actually I believe NEMA determined a value that they would always be able to build a motor that was below it. Keep in mind the NEC values haven't changed in more than 30 years even though motors have become more efficient.
 
Status
Not open for further replies.
Top