Motor Circuit Study Question

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paac

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Location
Lapeer, MI
I have a study question regarding a motor circuit that I need some clarification on. Just trying to see if I'm figuring this out correctly. I apologize in advance for the length of this. Hopefully it is readable.

I'm in Michigan, so this will be based on 2005 code.

:cool:

Motor is 20HP, 230V, 3 phase, 48A FLC (nameplate), 1.0 SF, Design B. Will be using time delay, dual element fuses for overcurrent protection.

Disconnect:
Per 430.109 and .110, I need a disconnect rated not less than 115% of FLC. FLC according to Table 430.250 is 54A. 54 x 1.15 = 62.1A. So I need a disc rated not less than 70A, correct?

Branch Circuit SC and GF protection:
Table 430.52 Design B gives multiplier as 175% of FLC. 1.75 x 54 = 94.5.

430.52(C)(1) requires protective device NOT EXCEEDING values in table, but then states in ex #1 that you can use the next higher size if it doesn't correspond to standard sizes or ratings of fuses, etc.

240.6 gives 90 and 100 as standard sizes. I am assuming my answer would be 90, but could I use 100? Is this where hard start and easy start comes into play?

Motor Circuit Conductors:
430.22(A) requires 125% FLC. 1.25 x 54 = 67.5.

Per T310.16, I get #4 copper.

Overloads:
Motor has SF of 1.0 and gives no temp rise, so per 430.32(A)(1) I will use 115% of nameplate FLC for my overloads. 1.15 x 48 = 55.2.

Equipment Grounding Conductor:
In sizing the egc, 430.245 refers to part VI of art. 250. Using Table 250.122 and a fuse size of 90A, I come up with #8 copper.

Am I using the correct overcurrent device in my calculations (assuming 90A is the correct size)?

:cool:

Thanks in advance for any input.
 
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IMM_Doctor

Senior Member
USE NEC Table, Not nameplate

USE NEC Table, Not nameplate

Motor is 20HP, 230V, 3 phase, 48A FLC (nameplate)

See 430.6(A) (1)
This will refer you to tables 430.247 through 430.250

If someone were to replace that motor at a later date, it may have the same HP, but higher FLA than your nameplate.
 
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paac

Member
Location
Lapeer, MI
IMM_Doctor said:
Motor is 20HP, 230V, 3 phase, 48A FLC (nameplate)

See 430.6(A) (1)
This will refer you to tables 430.247 through 430.250

If someone were to replace that motor at a later date, it may have the same HP, but higher FLA than your nameplate.
Click to expand...

As I understand it, the nameplate FLA is used for overload calculation and tables 430.247 through 430.250 are used for calculations such as SC and GF protection, conductor size, disconnect, etc., which is what I used for those answers.

I know there is a difference between FLA and FLC. Perhaps I was using the terms incorrectly. Thanks for pointing it out. I'll have to go look it up, unless someone wants to give a short explanation between the two?

Thanks again.
 
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IMM_Doctor

Senior Member
Oops, I'm sorry. I saw "(nameplate)" in your opening line, and rushed off a response. After your reply, I looked deeper into your OP, and saw that you did use tables for minimum circuit calc's.

I
 
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paac

Member
Location
Lapeer, MI
Am I missing something? In the setup of the question, I indicated that the nameplate FLC was 48. According to my understanding of the following paragraph, I used the proper amperage.

430.32(A)(1) Separate Overload Device A separate overload device that is responsive to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:
SF >= 1.15 125%
TR <= 40 deg C 125%
All other motors 115%
Click to expand...

If this is incorrect, please explain why.

I'm thinking that I could have set this question up a little bit better. My apologies.

Thanks again.
 
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RonPecinaJr

Senior Member
Location
Rahway, NJ
Branch circuits up, feeders down.

So any questions on branch circuits the ampacity rounds up to the next higher size.

For feeder questions, round down.
 
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RonPecinaJr

Senior Member
Location
Rahway, NJ
paac said:
I have a study question regarding a motor circuit that I need some clarification on. Just trying to see if I'm figuring this out correctly. I apologize in advance for the length of this. Hopefully it is readable.

I'm in Michigan, so this will be based on 2005 code.

:cool:

Motor is 20HP, 230V, 3 phase, 48A FLC (nameplate), 1.0 SF, Design B. Will be using time delay, dual element fuses for overcurrent protection.

Disconnect:
Per 430.109 and .110, I need a disconnect rated not less than 115% of FLC. FLC according to Table 430.250 is 54A. 54 x 1.15 = 62.1A. So I need a disc rated not less than 70A, correct?

Correct, but there's no such thing as a 70 amp rated disconnect. Here you would use a 100 amp disconnect. Watch the wording in the question for something like this.

Branch Circuit SC and GF protection:
Table 430.52 Design B gives multiplier as 175% of FLC. 1.75 x 54 = 94.5.

430.52(C)(1) requires protective device NOT EXCEEDING values in table, but then states in ex #1 that you can use the next higher size if it doesn't correspond to standard sizes or ratings of fuses, etc.

240.6 gives 90 and 100 as standard sizes. I am assuming my answer would be 90, but could I use 100? Is this where hard start and easy start comes into play?

100 amp time delay, dual element fuses is the correct answer.

Motor Circuit Conductors:
430.22(A) requires 125% FLC. 1.25 x 54 = 67.5.

Per T310.16, I get #4 copper.

This is the correct answer until they specify a specific conductor insulation. You used the 60? column and that's okay because it less than 100 amps (see 110.4 (C)(1). For instance, if the question asked you to use THW insulation, then a #6 copper conductor would be the right answer.[/color]

Overloads:
Motor has SF of 1.0 and gives no temp rise, so per 430.32(A)(1) I will use 115% of nameplate FLC for my overloads. 1.15 x 48 = 55.2.

Correct.

Equipment Grounding Conductor:
In sizing the egc, 430.245 refers to part VI of art. 250. Using Table 250.122 and a fuse size of 90A, I come up with #8 copper.

Am I using the correct overcurrent device in my calculations (assuming 90A is the correct size)?

#8 copper is good as an EGC for circuits not exceeding 100 amps (T250.122). And shall never be larger than the supply conductors.

:cool:

Thanks in advance for any input.
Click to expand...


Hey, by the way, I'm preparing for the exam too. Except mine is tomorrow! :grin:
 
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