Locked Rotor Current code question - Spin-Up PE Sample Exam 3, question 3-49

akyip said:

Hey guys,

I have a quick question about locked rotor current using the NEC tables.

Spin-Up PE Exam 3 question 3-49 asks:

"What is the three-phase design C locked rotor current for a 75 HP, 460 V motor?"

The given solution is 543 A, using NEC Table 430.251(B), which is for maximum locked rotor current for sizing disconnects.

BUT, wouldn't it make more sense to use NEC Table 430.7(B) - Locked Rotor Indicating Code Letters to find the ACTUAL locked rotor current of a motor?

In my mind, using Tables 430.251(A) and (B) are mainly for a REFERENCE MAXIMUM locked rotor current for sizing disconnects for motors. Whereas Table 430.7(B) should be used for finding the ACTUAL locked rotor current of a motor... Am I wrong?

What are your thoughts on this?

Click to expand...

akyip said:

Hey guys,

I have a quick question about locked rotor current using the NEC tables.

Spin-Up PE Exam 3 question 3-49 asks:

"What is the three-phase design C locked rotor current for a 75 HP, 460 V motor?"

The given solution is 543 A, using NEC Table 430.251(B), which is for maximum locked rotor current for sizing disconnects.

BUT, wouldn't it make more sense to use NEC Table 430.7(B) - Locked Rotor Indicating Code Letters to find the ACTUAL locked rotor current of a motor?

In my mind, using Tables 430.251(A) and (B) are mainly for a REFERENCE MAXIMUM locked rotor current for sizing disconnects for motors. Whereas Table 430.7(B) should be used for finding the ACTUAL locked rotor current of a motor... Am I wrong?

What are your thoughts on this?

Click to expand...

DilutedAr18 said:

That table is only valuable for getting a possible range of KVA based on the design “letter” (in this case C), not Locked Rotor Current. Read the table column labels closely. In practical use, the LRC is used for sizing disconnects, so Table 430.251(A) or (B) would be the useful table. 

If you look back at question 2-34, that is the type of a question you could expect to use Table 430.7(B). They would need to ask for a maximum or minimum KVA or could spin it by giving you a number and have you find the letter to fit the “KVA/HP with Locked Rotor”. 

Click to expand...

akyip said:

I understand what you are saying about the possible KVA range, but I have seen some NEC questions/solutions from other practice exam problems in which they calculate the locked rotor current from the KVA off of Table 430.7 using I LR = S LR 3-ph / [sqrt(3) * V LL] or I LR = S LR 1-ph / V LN. That's why I'm bringing up my question...

Click to expand...

DilutedAr18 said:

Did the specify a maximum or minimum? If not, how would you know what value to use?

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