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Old 02-03-2009, 10:18 AM   #1
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Lightbulb How do i choose a circuit breaker for different loads?

Please help me with this question?

I have a four (4) three phase motors 15 Amperes each one, they must be protected each one by a circuit breaker, also i have a three phase transformer 150kVA also protected by a circuit breaker. These circuit breakers are connected to a 480V - 3PH bar inside a panel. So the question is how do i determine the feeder short circuit breaker of this loads (panel) and the Cb for each load. Also i need some help with dimensioning conductors for all loads and feeder?. If you can name the articles of NEC will be better.

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Old 02-03-2009, 11:24 AM   #2
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Quote:
Originally Posted by jennymb_18 View Post
Please help me with this question?

I have a four (4) three phase motors 15 Amperes each one, they must be protected each one by a circuit breaker, also i have a three phase transformer 150kVA also protected by a circuit breaker. These circuit breakers are connected to a 480V - 3PH bar inside a panel. So the question is how do i determine the feeder short circuit breaker of this loads (panel) and the Cb for each load. Also i need some help with dimensioning conductors for all loads and feeder?. If you can name the articles of NEC will be better.
I take it that because you are from Peru you are not familiar with the NEC, and not that you are some know-nothing handyman about to get himself killed...

Article 430: Motors
Article 310: Conductors
Article 240: Overcurrent Protection

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Old 02-03-2009, 01:11 PM   #3
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we do know the code, but i have some doubts about the way i dimensioned this panel. I dont understand why you answer in that way. If you know the answer just share that with us... and of course i know the articles you reference but i dont ask for this artcles i ask for the answer and the exact article referenced to you answer.

I dimensioned this panel in this way:

transformer:

150kVA .... In =181A .... CB: 3x500A (according to 430-152)

conductor dimensioned considering drop and ampacity

motor .... In = 15A.......CB:3x40A (according to 450-3 (a)(2)b)
conductor dimensioned considering drop and ampacity

(Main CB): 500+15+15+15+15= 560A ... CB:3x600A (according to 430-53)

conductor dimensioned considering drop and ampacity

Could you please confirm if this is correct?
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Old 02-24-2009, 02:18 AM   #4
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Quote:
Originally Posted by jennymb_18 View Post
Please help me with this question?

I have a four (4) three phase motors 15 Amperes each one, they must be protected each one by a circuit breaker, also i have a three phase transformer 150kVA also protected by a circuit breaker. These circuit breakers are connected to a 480V - 3PH bar inside a panel. So the question is how do i determine the feeder short circuit breaker of this loads (panel) and the Cb for each load. Also i need some help with dimensioning conductors for all loads and feeder?. If you can name the articles of NEC will be better.
First find full-load-amps (FLA) of your motor by looking up in 430.250 this is determined by the horse power of the motor.
The branch circuit conductor ampacity can be 125% of the FLA per 430.22.
The Branch overcurrent protection is determined by the table in 430.52. You can round up to the next larger size in 240.6
the feeder overcurrent protection is the largest size branch circuit overcurrent protection plus the FLA of all other motors per 430.62
If they are all the same size use one overcurrent device plus FLA of the other motors.
the feeder conductor size should have an ampacity greater than the feeder overcurrent protection

so 4 motors with a 15A FLA rating the conductors may have an ampacity of 18.75A (if using dual element time delay fuses) your branch circuit overcurrent protection can be 30A. your feeder overcurrent protection would be 30A+15A+15A+15A=75A and your feeder conductor ampacity needs to be 75A


I think that covers everything...

Last edited by collin.thomas; 02-24-2009 at 02:21 AM.
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Old 02-24-2009, 09:30 AM   #5
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jennymb,

What edition code book are you using? Article 430.152 for the transformer doesn't exist in 2005 NEC. Article 450 covers installation of all transformers (2005 NEC). With your overcurrent selection 250% you would need secondary protection of 125%. T450.3(B)

T450.3(B) 125% for primary protection, secondary protection not required. Use a 250A CB.

Can you post the HP rating of the motors?
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Last edited by Greg; 02-24-2009 at 09:33 AM.
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Old 03-26-2009, 01:23 PM   #6
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Quote:
T450.3(B) 125% for primary protection, secondary protection not required. Use a 250A CB.
i agree, would be nice to know prim VAC / sec VAC and %Z.

15A motors = _________HP ( i don't have a match, 5 hp @ 230 is 15.2)

motor branch circuit conductors 430.250 (make sure of termination temperature rating) 125% FLC

Branch OCPD 430.52(C1) (short circuit and ground fault) 430.52 for breaker or fuse

Feeder Conductors 125% of the largest and 100% of the rest 430.24

Feeder OCPD 430.62(A) use largest OCPD plus FLC of the rest (do not exceed, use next lowest if not standard size.)
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Old 03-28-2009, 01:18 PM   #7
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Just registered for this website 5 minutes ago. First time here.
And I am curious. "some know-nothing handyman about to get himself killed..."?
Why do you answer that way? Or how about the comment, "If you're gonna be stupid, you better be tough."? Sounds like a forum for narcissists.
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Old 03-28-2009, 01:52 PM   #8
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Quote:
Originally Posted by luby104 View Post
Just registered for this website 5 minutes ago. First time here.
And I am curious. "some know-nothing handyman about to get himself killed..."?
Why do you answer that way? Or how about the comment, "If you're gonna be stupid, you better be tough."? Sounds like a forum for narcissists.

Yeah Welcome to the forum.....Sometimes questions that are posted that seem way to basic for an electrician are not always treated as kindly as they should be.

BUT unless I am missing something, or the MEAN post were removed this seems passable to me. not that I am any judge of KIND post!
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Old 03-28-2009, 06:43 PM   #9
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The line your quoting is a TAG line sometimes referred to as a signature line.
If your gonna be stupid you better be tough has a laughing face beside it.
It's a "funny"
It shows at the bottom of every post that person makes.
Look around quite a few people have tag lines. they're not meant to insult.

The other remark comes from the DIYers who come here pretending to be electricians. They start off firing questions about how would you do this, or how is this done in your area? The professionals hear see through that and a diyer sometimes becomes a target of the humor from this board.
We are a relaxed easy going sort of folk here. Pull up a chair, sit down ,have a beverage of your choosing, and watch the goings on.

Last edited by manchestersparky; 03-28-2009 at 06:48 PM.
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Old 03-28-2009, 06:57 PM   #10
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If you re-read Inphase's post you'll see you misread what he wrote.


Quote:
Originally Posted by luby104 View Post
Sounds like a forum for narcissists.
Actually this is a forum for electricians and electrical related trades. Not DIY'ers, not handymen, not GC's. There are plenty of other forums out there that fit that description.
This forum was created so that we all can discuss trade and electrical related topics and issues. This is made very clear on the sign up page, yet many folks conveniently choose to ignore this requirement.

If someone comes on here and lies about their connection to the trade, or simply ignores this requirement, just to ask a DIY or handyman question they are removed and asked to try one of the admin's other sites www.diychatroom.com or www.contractortalk.com .

It's not narcissism, it is dealing with folks who think the rules do not apply to them.

Whether this is the case with the OP we don't know. He made two posts and disappeared over a month and a half ago.
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Old 03-28-2009, 06:58 PM   #11
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Oh, and welcome to the forum.
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Old 03-29-2009, 10:18 AM   #12
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Default DIY's?

After looking around at other threads, wow! this is a great place to hang.
Lots of good stuff!! Lots of knowledgeable people here!

So how many of you in this thread are DIY's. Like when it comes to plumbing, carpentry, HVAC, etc.? And who do you go to when you have questions?

Somebody that does the trade for a living perhaps? And you're all calling that stupid??

If you don't want to answer a question, then don't answer. The judgmental
sarcastic comments are only put-downs and are not funny. Save them for your drinking buddies.

And who are the two dudes in the green t-shirts supposed to be?
Muggers or "The Village People" Electric Co.?
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Old 03-29-2009, 10:39 AM   #13
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Quote:
Originally Posted by luby104 View Post

If you don't want to answer a question, then don't answer.
That's not the point. DIY questions are simply not allowed on this site. Same as ECM, Mike Holt's, and many other "trade only" sites.

Whether or not someone is a DIY in another field is irrelevant.
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Old 03-29-2009, 10:41 AM   #14
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Quote:
Originally Posted by luby104 View Post
...The judgmental
sarcastic comments are only put-downs and are not funny. Save them for your drinking buddies.

And who are the two dudes in the green t-shirts supposed to be?
Muggers or "The Village People" Electric Co.?
Interesting comments from someone who has been here ONE day.

We hope you stay and enjoy the party.
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Old 03-29-2009, 11:11 AM   #15
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Quote:
Originally Posted by luby104 View Post
And who are the two dudes in the green t-shirts supposed to be?
Muggers or "The Village People" Electric Co.?
Those idiots?

Just a guy that is taking a photoshop class in night school, father of four, owner of a speciality electrical shop and most likely can smoke you in ALL aspects of the trade....Any questions someone here will bail you out.

Friendly enough for you.
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Old 03-29-2009, 11:57 AM   #16
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How 'bout we start over.
If the OP comes back fine. If not let's keep it on topic and civil.

Sound good?
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Old 03-29-2009, 12:35 PM   #17
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wowwwww lol.

so yea I love comin to these threads, makes me dust off my NEC codebook (circa 1997). That's still good right? lol...
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Old 03-29-2009, 12:53 PM   #18
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Darn! I just wrote this great big response and it didn't post!
Anyway, you are all better tradesmen than me.
That's why I'm here. I could not have answered the Peruvian's
question at all. But you guys did.
So. My bad. I'll stop wasting your time.
Thanks for the welcome
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Old 03-29-2009, 01:04 PM   #19
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Quote:
Originally Posted by luby104 View Post
Darn! I just wrote this great big response and it didn't post!
Anyway, you are all better tradesmen than me.
That's why I'm here. I could not have answered the Peruvian's
question at all. But you guys did.
So. My bad. I'll stop wasting your time.
Thanks for the welcome
You seem to have a tremendous attitude problem. Nobody put anybody down. Go back and re-read the posts. There is nothing in the world wrong with DIY, as long as it is informed and safe. Having said that, tradesmen, when talking amongst themselves, want a place to talk shop. A homeowner or handyman is not an electrician. This place is for electricians, be they apprentices, journeymen, masters, or plant electricians.

Speedy and I both contribute time for DIY questions at diychatroom.com. If you don't like that, go pound sand...
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Old 03-29-2009, 02:44 PM   #20
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You're right. I'm sorry. My mistake.

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