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

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akyip

Well-known member
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?

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?
My photo attachments didn't come through the first time...

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?
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”.

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”.
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...

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

Did the specify a maximum or minimum? If not, how would you know what value to use?
Here's one example from Engineer Pro Guides Full Exam Question 79. The given induction motor is a Code G motor, which Table 430.251(B) does not apply for. So the solution uses Table 430.7 to calculate the max locked-rotor current based on the max KVA/HP ratio for Code G induction motors.

Table 430.251(B) is explicitly titled for polyphase Design B, C, and D motors.

Also, for what it's worth saying, there is a note stating that Table 430.251(B) is to be used only with sections 430.110, 440.12, 440.41, and 455.8(C). Notably, 430.110 in particular is the section for sizing disconnects for motors.

At least, all this is how I understand it.

I was always told that if the question calls out for Design (ie. B,C,D) always go to table 430.251(B)

Also, if the nominal voltage of the motor exceeds table 430.251(B) limit of 575V, use Table 430.7 like in the Engineering Pro example above.

Revisiting this topic with another example of using Table 430.7 instead of Table 430.251 for induction motor locked-rotor current. See attached from Complex Imaginary (set 4, question 76) Just a friendly FYI.