Sizing a Circuit Breaker

[SparkOmatix]

What is the proper way to size a circuit breaker?

I have a squirrel cage motor with "X" full load amps. Does code say you will protect motors of this type...and perhaps between a certain horsepower...with a Circuit Breaker rated at "Y%" of the FLA? Thanks.

 

[sspeare]

Sizing of "motor short-circuit and ground-fault protection" is covered in Part IV of Article 430 in the 2008 National Electric Code.

 

[jraef]

Assuming he is in the USA... this is an international forum.

But if I may, sspear's point is still valid in a general sense. There are going to be local code related rules to sizing circuit breakers for motor circuits, you should really consult an Electrical Engineer with experience in your part of the world if you don't know this. If you are an EE and you don't know something this basic, you are in need of further training. I'm not trying to be condescending, but this is not a DIY forum, it is for Engineers and you identified yourself as an EE. But you are asking a question that should have been covered in EE-101 so I am assuming you are either not really an EE or you are an "EE/Computer Science" major and in need of additional exposure to power engineering...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[ShmoJoe]

jraef - "either not really an EE or you are an "EE/Computer Science" major"


What's the difference? I've worked with two EE/Comp Sci majors who didn't know much more than an apprentice level electrician. Is there really that big of a disconnect between a stand alone EE and the comp sci alternative?

 

[davidbeach]

Perhaps jraef should have said "either not really a power EE or ..." Far too basic a question for any engineer that has any need of the answer. Not supposed to practice outside your area of expertise and all that.

 

[jraef]

I was once a long time ago assigned an EE student intern to take on some rather mundane tasks during a period of heavy workload. I had her doing some basic schematic drafting of 120VAC motor control circuit designs that I would scratch out on napkins and such. She was actually more of a hindrance than a help because I had to keep showing her things I would have thought were basic electrical drafting 101 stuff (she was about to graduate). After a few weeks I was doing a performance review for her and she said she was struggling to understand all of my series-parallel RC networks. "What?" I said?. She was interpreting my N.O. contact symbols as capacitors and N.C. symbols as varistors. I was shocked and said "Aren't you a 4th year EE student?" She responded "Well yes, but I'm EE/CS, so I don't know much about this high voltage stuff. Please don't tell the boss, I really need this job." High voltage to her was 120VAC. She said they didn't cover "power" in any of her classes. The way the degree programs were at that time, and probably still are in many cases, her sheepskin would have said "Bachelors Degree in Electrical Engineering" without qualification, but she was totally unqualified to work in any branch of EE that dealt with "high voltages" of 120VAC and above.

I'm just sayin...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[ScottyUK]

Now jraef, that's because of those odd-looking symbols you guys use for contacts. You couldn't mistake an IEC symbol for a capacitor. maybe a drawbridge, but definitely not a capacitor!


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

LOL, true!


"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electricpete]

SparkOmatix - I think you'll find there's a ton of knowledge here, and people mostly helpful, but sometimes back away when they sense someone might be in over their head.

I suggest you look around the site, perhaps take a look here:
thread248-288260

Post back here if you have more specific questions. Perhaps it would be valuable to give very specific information about an example motor and supply system as a starting point, clarify which country (I assume code means NEC, but....).

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(2B)+(2B)' ?

 

[ShmoJoe]

jraef - thanks for the story and explanation. Made me laugh. I'm not a PE, but I have enough field experience to spot BS is when it's being spewed. Your story mirrored my experiences with the two CS/EE majors I mentioned earlier. I don't know why these individuals felt compelled to casually omit their true level of training - guess they felt as though it undermined their on the job status. Very frustrating for others though who have to follow their work, knowing that their safety is being placed in someone's hands who really has no practical clue as to how things are done in the real world.