Moter Cal.

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Flight987

Senior Member
Going to submit this: This the question on the pratice test. Yes I have the answer key. Here we go.
Answer questions #3 through #13 with the following moter nameplate information: 230 volt's,three phase, AC
moter, service factor of 1.0, temperature rise rating of 40 degrees Celsius, rated for continous duty, design letter B, code letter R, full load current rating of 20 Amp's., and 7-1/2 horsepower.

Save that thought!

I did the first Ques. right.
#2 Asked
What is the Max. I.V.T.B. (inverse time breaker) to put on this ? It is a #10 Con.,okay,then if we take the
given amp's, from the Ques., or we take 430.250, it still comes to #10. Correct if I am wrong. The Key
Answ. ( Will call it AK from now on!). The AK say's 55 Amp's. I am 49 year's young and never saw a 55
Amp IVTB. 240.6a, you know where I am going with that. I would like some help on this one?
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The correct answer is larger than what is given in the answer key. You need the following sections to get the correct answer. 430.6(A)(1), Table 430.250, 430.52(read all of it) and 240.6
 
F

Flight987

Senior Member
Inv. Time Break.

Inv. Time Break.

430.52(c)(1) say's we can go higher to a 60. So is that right? If that is right,then here is the next Ques..
Don't trust there AK.
#5 What is the next standard INVB if the onne in Ques. #4 trip's on startup. AK say's 80. I'm thinking
that it is a 70. Need help on this one. If there AK was wrong on one thing, How do I know when they are right?
 
F

Flight987

Senior Member
Moter Disconnect Switch

Moter Disconnect Switch

For this given moter listed in my Ques. I'm having trouble get the size of the moter dissconnect. Can't figure
out how the code addresses this. Answer is 25 HP and Ref. is 430.7. What is the difference between design letter
B and the code letter R. Information of moter at first part of my Ques.
 
F

Flight987

Senior Member
Found 430.7B

Found 430.7B

Found 430.7B on the code letters (locked-rotor). Still need to know how to size the disscount.
 
F

Flight987

Senior Member
Mike talk's

Mike talk's

Mike Holt say's the round up rule doesn't apply to fuses on moter overload protection. I belive him but,
where is that in the code? That Ques. might be on the test.
 
J

jrohe

Senior Member
Location
Omaha, NE
Occupation
Professional Engineer
curtnobel said:
Going to submit this: This the question on the pratice test. Yes I have the answer key. Here we go.
Answer questions #3 through #13 with the following moter nameplate information: 230 volt's,three phase, AC
moter, service factor of 1.0, temperature rise rating of 40 degrees Celsius, rated for continous duty, design letter B, code letter R, full load current rating of 20 Amp's., and 7-1/2 horsepower.

Save that thought!

I did the first Ques. right.
#2 Asked
What is the Max. I.V.T.B. (inverse time breaker) to put on this ? It is a #10 Con.,okay,then if we take the
given amp's, from the Ques., or we take 430.250, it still comes to #10. Correct if I am wrong. The Key
Answ. ( Will call it AK from now on!). The AK say's 55 Amp's. I am 49 year's young and never saw a 55
Amp IVTB. 240.6a, you know where I am going with that. I would like some help on this one?
Click to expand...

Per Table 430.250, the FLC for a 7.5 horsepower, 230 volt motor is 22 amps. Per Table 250.52, the IVTB for a design B motor is permitted to be 250% of the motor's FLC. Therefore, the IVTB is permitted to be 22 amps x 250% = 55 amps. A 55 amp breaker is not a standard size (NEC 240.6(A)), so you are permitted to round up to the next standard size per 430.52(C)(1) Ex. 1. The correct answer is 60 amps.

curtnobel said:
430.52(c)(1) say's we can go higher to a 60. So is that right? If that is right,then here is the next Ques..
Don't trust there AK.
#5 What is the next standard INVB if the onne in Ques. #4 trip's on startup. AK say's 80. I'm thinking
that it is a 70. Need help on this one. If there AK was wrong on one thing, How do I know when they are right?
Click to expand...

Per 430.52(C)(1) Ex. 2c permits the IVTB to be a maximum of 400% for motors with an FLC of 100 amps or less. Therefore, the maximum rating the IVTB can have is 22 amps x 400% = 88 amps. This is the maximum rating and is not a standard OCPD size (NEC 240.6(A)) and you are not permitted to round this value up, so the correct answer the the next lower standard size = 80 amps.

curtnobel said:
For this given moter listed in my Ques. I'm having trouble get the size of the moter dissconnect. Can't figure
out how the code addresses this. Answer is 25 HP and Ref. is 430.7. What is the difference between design letter
B and the code letter R. Information of moter at first part of my Ques.
Click to expand...

Per 430.110(A), the motor disconnect is required to have an ampere rating of not less than 115 of the motor's FLC. Therefore, the minimum ampere rating of the disconnect = 22 amps x 115% = 25.3 amps. This is the minimum rating, so round up to a 30 amp disconnect.

curtnobel said:
Mike Holt say's the round up rule doesn't apply to fuses on moter overload protection. I belive him but,
where is that in the code? That Ques. might be on the test.
Click to expand...

The various sections of 430.32(A)(1) all say the rating of the overload device (whatever it is) can not exceed the values given in those sections. This is why Mike Holt has said (correctly) that rounding up to the next standard overload device rating is not permitted.
 
F

Flight987

Senior Member
This in

This in

The sentence in Mike's workbook goes like this : The next size up rule does not apply when fuses are used for
overload protexction. I Mike saying that cicurt breaker's can be sized up. How does 430.32 work with this.
 
F

Flight987

Senior Member
You still

You still

You still have moter Inf. to look at if you need. On table 430.52 Mike said on DVD that AC polyphase moters
were the same 3/0 moter's. I didn't look closenoff to see Design B moter heading. Design B moter heading doesn't
say 3/0 moter. Second from the top heading does. If they give a design on the moter use that and forget the 3/0?
How do I know what to use? The numbers are the same, but other than one number, but new code cycle that could change.
 
F

Flight987

Senior Member
Last Ques.

Last Ques.

Last ques. regarding this moter. Base the disconnect switch on the locked rotor amps of th moter.
I cut the Ques. short, but you get it. Need help with this. This maybe on test. AK is 25HP,REf. 430.7
 
Last edited:
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
curtnobel said:
You still have moter Inf. to look at if you need. On table 430.52 Mike said on DVD that AC polyphase moters
were the same 3/0 moter's. I didn't look closenoff to see Design B moter heading. Design B moter heading doesn't
say 3/0 moter. Second from the top heading does. If they give a design on the moter use that and forget the 3/0?
How do I know what to use? The numbers are the same, but other than one number, but new code cycle that could change.
Click to expand...
What is "moter Int."? What is a "3/0 moter". Also motor does not have an "e" in it.
 
F

Flight987

Senior Member
230 volts,3/0,acmoter,SF1.0, Temp. Rise 40 D.C.Con. Duty, D.L.-B,C.L. R, FLC 20, 71/2

230 volts,3/0,acmoter,SF1.0, Temp. Rise 40 D.C.Con. Duty, D.L.-B,C.L. R, FLC 20, 71/2

I need to rate the Dissconect by locked roter amp.s How do I go about this? How does table 430.7(B) apply?
I read through 430.110 and it is not very clear.
 
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