# 430.7 (b)



## raider1 (Jan 22, 2007)

When a motor is marked with a locked rotor indicating code letter here is what you do to convert the letter to the actual locked rotor curret.

First you need to take the HP of the motor and the find the Kva per HP from the Table using the Letter marked on the motor.

For Example let's use a 20 HP 460 volt 3-phase motor with a code letter G.

According to Table 430.7(B) the Kva per HP is 5.6 to 6.29 Kva so we will use the larger number for the calculations.

So to find the Locked Rotor Kva we would multiply 6.29 by 20 and get 125.8 Kva.

Now that we have the locked rotor Kva let's get the locked rotor current.

For this we will be using the formula LRA = 125.8 x 1000/ 1.732 x 480 or 125,800 / 831 So the LRA for this example would be 158 amperes.

Hope this helps

Chris


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## dadtodc (Oct 4, 2010)

This does help out tremendousl. So if I had a question that told me all of the information that you provided even with the letter designation, and all I had to do was size a conductor then I wouldnt need this extra step, but if Iwere tobe sizing the OCP, then I would need to have the correct KVA to do this right. I am not familure though withthe LRA formula. Where can I find this information in the code book. So once I get the letter designation I simply go with the highest value for the calculation, and multiply. then simply use the answer and multiply times 1000 to get the KVA? I am however a little lost on where the 1.732 came from. 

On a different not I have learned more from this site in 2 days than I ever learned from the test prep that a company put on for my department. hanks all!


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## raider1 (Jan 22, 2007)

dadtodc said:


> This does help out tremendousl. So if I had a question that told me all of the information that you provided even with the letter designation, and all I had to do was size a conductor then I wouldnt need this extra step, but if Iwere tobe sizing the OCP, then I would need to have the correct KVA to do this right.


This is only for locked rotor current, which is the amount of current that the motor will draw during start up. You don't size your conductors to the locked rotor current you size them based on the Table values in Table 430.247,248,249 and 430.250 based on the HP of the motor. Also the branch circuit short circuit and ground fault protection should also be based on the Table values and not the locked rotor current.



> I am not familure though withthe LRA formula. Where can I find this information in the code book.


The formula is not specifcally in the code book but is basic Ohm's law.



> So once I get the letter designation I simply go with the highest value for the calculation, and multiply. then simply use the answer and multiply times 1000 to get the KVA?


That will get you the VA not the KVA the KVA is Kilovoltamps where VA is just volt amps.



> I am however a little lost on where the 1.732 came from.


1.732 is the square root of 3. You must use the square root of 3 when doing 3 phase calculations. 



> On a different not I have learned more from this site in 2 days than I ever learned from the test prep that a company put on for my department. hanks all!


Welcome to the forum.:thumbsup:

Chris


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## dadtodc (Oct 4, 2010)

ok so why use 480? Was this supposed to be 460? Also if you dont have 3phase what then. If it is only 240v then what would be the multiplier.


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## jwjrw (Jan 14, 2010)

dadtodc said:


> ok so why use 480? Was this supposed to be 460? Also if you dont have 3phase what then. If it is only 240v then what would be the multiplier.



You just leave the x 1.73 out of the calc.


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## raider1 (Jan 22, 2007)

dadtodc said:


> ok so why use 480? Was this supposed to be 460?


I just used 480 volts as an example. I always use 480 as my nominal voltage for that system and not 460 but you could use that I suppose.



> Also if you dont have 3phase what then. If it is only 240v then what would be the multiplier.


If you don't have 3 phase then as jwjrw pointed out you would not use 1.732.

As for 240 single phase you would just divide the locked rotor VA by 240 volts.

Chris


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## Murphy (Dec 10, 2009)

dadtodc said:


> ok so why use 480? Was this supposed to be 460? Also if you dont have 3phase what then. If it is only 240v then what would be the multiplier.


 no multiplier used for single phase.. the square root of three multiplier is used because each phase is 120 degrees apart. therefore it is needed to make correct calculations


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## dadtodc (Oct 4, 2010)

Gotcha! Are there any good example problems to cut my teeth on? I am in it to win it now! Like I said I have learned more here than anywhere else!


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## dadtodc (Oct 4, 2010)

Hi all, I was looking at how to do this exact chart, and posted another question. Another member recommended this thread to be. After a little embarrassment I think I got it again. I did actually pass my test and now I am trying to really get this down. I had a few questions with the letter designation on it. Actually almost all. I totally drew a blank. I know I got them wrong. In the word problem it might have the letter designation, but you dont need it to do the problem. I keep forgetting when to use it.


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## dadtodc (Oct 4, 2010)

Ok guys, I think I have my question figuered out not. Hopefully this thread will not be closed now. What really confuses me is the difference between 230.251 (b), and 430.7 (b). On my test I had several , and I didnt understand which table to use because all said either b,c,d.


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## Dennis Alwon (May 9, 2009)

430.51 is for sizing the disconnect while 430.7(B) is locked rotor amps which is used for sizing the overcurrent protection in fire pumps.


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## dadtodc (Oct 4, 2010)

How would I know if it were a fire pump though. This is the only part that has confused me so bad. I got the rest. I do appreciate all of the answers.


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## dadtodc (Oct 4, 2010)

dadtodc said:


> How would I know if it were a fire pump though. This is the only part that has confused me so bad. I got the rest. I do appreciate all of the answers.


 
Ok so the light bulb just went off! 

Where I was messing up was both tables said LRC. What confused me was how to know when to use B, C, D on 430.251 (b), or when to use B, C, D on 430.7 (b). While reading different threads in here I came across one that stressed the importance of Key Words. Which I thought I was good at spotting. So for anyone like me who just cant understand how everyone else on here just knows when here is a little tip. In the question it will specify DESIGN B, C, D, and you use 430.251 (B). If you need to use 430.7 (b) it will specify it by telling you the CODE Letter on the motor. 

Although I might be wrong. Anyone else agree or am I way off base?


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## Dennis Alwon (May 9, 2009)

430.251(B) is only for sizing the disconnect switch and controller. It says that in the title heading


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## dadtodc (Oct 4, 2010)

Dennis Alwon said:


> 430.251(B) is only for sizing the disconnect switch and controller. It says that in the title heading


 

Ok I am with you on why you use it, the problem I am having is when. Look at this question and tell me what stands out 
*Determine the minimum required conductor ampacity, the motor
overload protection, the branch-circuit short-circuit and ground-fault
protection, and the feeder protection, for three induction-type motors
on a 480-V, 3-phase feeder, as follows:
(a) One 25-hp, 460-V, 3-phase, squirrel-cage motor, nameplate​full-load current 32 A, Design B, Service Factor 1.15


Now my problem is that I wasnt sure when to use which chart. What I noticed though is that in some questions it clearly ask for a Design B , and in other questions it ask for a letter designation of B. I am asking if that is the difference when trying to decide.

*


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## Dennis Alwon (May 9, 2009)

Design letters and code letters are the same. I never knew them to be different. Not sure why they use the terms in the table like that.


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## dadtodc (Oct 4, 2010)

Okay.
So then the only reason to use 430.7 (b) would be for fire pumps. Even if it is listed the same way in the question above. Its only when it say B, C, D. I just dont know when to use. I am thinking I am the only one that doesnt know at this point.


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## HARRY304E (Sep 15, 2010)

Look at this.



> 430.7 Marking on Motors and Multimotor Equipment.
> (A) Usual Motor Applications. A motor shall be marked
> with the following information:





> (8) Code letter or locked-rotor amperes if an alternatingcurrent
> motor rated 1⁄2 hp or more. On polyphase woundrotor
> motors, the code letter shall be omitted.
> Informational Note: See 430.7(B).





> (9) Design letter for design B, C, or D motors.
> Informational Note: Motor design letter definitions are found
> in ANSI/NEMA MG 1-1993, Motors and Generators, Part 1,
> Definitions, and in IEEE 100-1996, Standard Dictionary of
> Electrical and Electronic Terms.





> (B) Locked-Rotor Indicating Code Letters. Code letters
> marked on motor nameplates to show motor input with
> locked rotor shall be in accordance with Table 430.7(B).
> The code letter indicating motor input with locked rotor
> ...


Now if you look at the 2011 NEC HANDBOOK on page 604. 

ARTICLE 430.7 (A) ((9) just after the read the commentary..



> The design letters referred to in 407.(7)(A) indicate a motor's speed/torque characteristic curve and are not to be confused with code letters ,used in table 430.7 (B).
> For technical accuracy,code letters should be referred to as "locked rotor indicating code letters,"which are explained in 430.7(B).
> 
> Design letters reflect characteristics inherent in motor design ,such as locked rotor current, slip rated load, and locked rotor and breakdown torque.



So it looks like your question is talking about that quote above.


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## HARRY304E (Sep 15, 2010)

Also look at 430.52(C)(3) Exception No. 1




> Exception No. 1: Where the setting specified in Table 430.52
> is not suffıcient for the starting current of the motor, the setting
> of an instantaneous trip circuit breaker shall be permitted to
> be increased but shall in no case exceed 1300 percent of the
> ...


More on Design letters.
http://findarticles.com/p/articles/mi_qa3726/is_200208/ai_n9146414/


http://www.pdhonline.org/courses/e156/e156content.pdf


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## Dennis Alwon (May 9, 2009)

dadtodc said:


> Determine the minimum required conductor ampacity, the motor overload protection, the branch-circuit short-circuit and ground-fault protection, and the feeder protection, for three induction-type motors on a 480-V, 3-phase feeder, as follows:
> (a) One 25-hp, 460-V, 3-phase, squirrel-cage motor, nameplate full-load current 32 A, Design B, Service Factor 1.15



In this problem I would use Table 430.250 for the ampacity to determine the required conductor ampacity. 

The motor overload protection would be based on 430.32(A)(1) which is 125% for motors with a 1.15 service factor or greater.

The motor branch circuit and ground fault protection would be based on 430.52

For motor feeder I would use 430.62.

In this problem I do not see a need for the code letter. Normally one uses 115% for the disconnect size base on the FLA of the motor.

I am not sure when T.430.251 other then to determine the max LRA for motor disconnect. Generally this is on the nameplate. 

I guess the real question is when is locked rotor amps needed. I am not sure and the fire pump was something someone else had said but I cannot find that article.


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## dadtodc (Oct 4, 2010)

Now I am feeling a little less stupid. Hahaha. So they are different. In the bcd chart all you do is take the number and that's the answer. In the oTher you use the LRA. I a confused as to knpow when to use it when asked in a question. The only thing I can comeuppances with is to look food key spores such as DESIGN, and CODE LETTER.


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## HARRY304E (Sep 15, 2010)

dadtodc said:


> Now I am feeling a little less stupid. Hahaha. So they are different. In the bcd chart all you do is take the number and that's the answer. In the oTher you use the LRA. I a confused as to knpow when to use it when asked in a question. The only thing I can comeuppances with is to look food key spores such as DESIGN, and CODE LETTER.


Not sure what you mean maybe you could either edit your post or write a new one.



> look food key spores


 What does this mean?


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## dadtodc (Oct 4, 2010)

HARRY304E said:


> Not sure what you mean maybe you could either edit your post or write a new one.
> 
> What does this mean?


I am sorry for the last post. I was typing on my phone. I meant looking for key words. I should have proof read.


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## raider1 (Jan 22, 2007)

> I guess the real question is when is locked rotor amps needed. I am not sure and the fire pump was something someone else had said but I cannot find that article.


Dennis check out 695.4(B)(2), 695.5(B) and (C)(2).

Chris


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## Dennis Alwon (May 9, 2009)

raider1 said:


> Dennis check out 695.4(B)(2), 695.5(B) and (C)(2).
> 
> Chris


Thanks Chris I read past that because it is OC device- I was looking for disconnect. So is the fire pump the only place where the LRA would be used for sizing disconnects?


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## raider1 (Jan 22, 2007)

Dennis Alwon said:


> Thanks Chris I read past that because it is OC device- I was looking for disconnect. So is the fire pump the only place where the LRA would be used for sizing disconnects?


Dennis take a look at 430.110(C)(1) for motor disconnecting means for combination loads.

Chris


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## Dennis Alwon (May 9, 2009)

raider1 said:


> Dennis take a look at 430.110(C)(1) for motor disconnecting means for combination loads.
> 
> Chris


Thanks Chris. I guess I don't usually wire motors where more than one would use the same disconnect. Actually I don't usually wire motors. :thumbup:


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## jmsmith (Sep 10, 2011)

Dennis--
If I am reading all this right, your figures would be right, but overcurrent protection is not applied to an in-house fire pump, just short-circuit protection only. The thought here being that an overload relay shutting a sprinkler system down due to continuous operation of the pump would be a definite no-no in the fire marshall's book. There are still a few people out there that insist on using an induction motor for their plant dry pipe sprinkler systems.

Sent from my iPhone using ET Forum


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