Locked Rotor Current

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finhead

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Does anyone know why the locked rotor currents shown in Table 430.251(B) do not correlate with calculated locked rotor currents in Table 430.7(B)?

For example, the range of locked rotor currents for a 10 hp, 460 volt, design B, C, or D motor according to Table 430.7(B) is from 40 amps to 56 amps. Table 430.251(B) shows the locked rotor current for a 10 hp 460 volt motor to be 81 amps.

Perhaps the answer to this question will answer my second question. Why is Table 430.251(B) restricted to Sections 430.110, 440.12, 440.41, and 455.8(C)?

Thanks,
Brian Dolan
 
Tables 430.251(A) & (B) are NEMA design letters B,C & D and not motor starting current code letters. Look at the very bottom of the table for Design Letter A.

Table 430.7(B) is used for short circuit and ground fault protection.
 
dcspector said:
So which table would one use for LRA for rating ocpd on a fire pump?
Click to expand...

I have seen fire pumps without OCPD's, the feeling there is it is better to let the fire pump burn up and destroy itself rather than have an OCPD open the circuit and stop it from doing it's job. Having said that, if the fire pump burns up it will stop doing it's job also.
 
elohr46 said:
I have seen fire pumps without OCPD's, the feeling there is it is better to let the fire pump burn up and destroy itself rather than have an OCPD open the circuit and stop it from doing it's job. Having said that, if the fire pump burns up it will stop doing it's job also.
Click to expand...

True,
but if it is an overload, then the Motor Burnout would occur some time later.
The Motor would keep working for some time period.

Comments?
 
The ocpd is for short circuit only. If indeed it shut the pump down then short circuit protection allows one to fix the short and continue the pump. So I would use table 430.251 for LRA and apply that indefinetly for ocpd sizing. Correct? Yes.
 
dcspector said:
That is strange I am confused....sounds like you are talking to yourself.:-?
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It appears the moderators have choosen to remove my educational reference to my electrostatic motor. I need not complain though.

He that blows the coals in quarrels that he has nothing to do with, has no right to complain if the sparks fly in his face.

He that can have patience can have what he will.
 
Ben Franklin said:
It appears the moderators have choosen to remove my educational reference to my electrostatic motor. I need not complain though.

He that blows the coals in quarrels that he has nothing to do with, has no right to complain if the sparks fly in his face.

He that can have patience can have what he will.
Click to expand...

Ok............
 
dcspector said:
The ocpd is for short circuit only. If indeed it shut the pump down then short circuit protection allows one to fix the short and continue the pump. So I would use table 430.251 for LRA and apply that indefinetly for ocpd sizing. Correct? Yes.
Click to expand...

DC,
Reads like a proper method.
 
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