Discussion in 'Electrical Forum discussion & Blog' started by Thatguy, Nov 20, 2008.

1. ThatguyHomeowner

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A bounty hunter like in "Raising Arizona"
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A resi single pole 15A breaker trips occasionally when powering up a 120v chop saw. The HO wants to test the breaker, since this is the second saw that has done this.

1. Somehow find out what breaker make and model you have, e.g. SQ-D QO-115.
2. Find the trip curve from the manu's website.
3. Read that twice the rated current, 30A, will trip this SQ-D breaker in 8 to 27 seconds.
4. Put enough hair dryers, toaster ovens and incand. lamps on this circuit so the total is 30A.
5. If the (SQ-D) breaker trips in less than 8 seconds or more than 27 seconds, it's bad.

Last edited: Nov 20, 2008
2. AlectricianDIY Senior Member

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Jun 15, 2007
Or, spend 5 bucks on a new breaker. If it trips, it wasn't the breaker.

4. ThatguyHomeowner

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so far, so good

Procedure, Rev. 1

Last edited: Nov 20, 2008
5. AlectricianDIY Senior Member

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Jun 15, 2007
I forget that not everyone lives in the city. You can't throw a rock here without hitting a Home Depot/Lowes etc.

6. jwelectricElectrical Contractor/Instructor

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No less than 100 questions with a four hour time limit. Must have a score of at least 70

7. ThatguyHomeowner

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Been there, done that. . .

8. hjModerator & Master PlumberStaff Member

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cb

Test the circuit to see what amperage it is carrying.

9. ThatguyHomeowner

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For a chopsaw, the whole startup event is probably over in one second, so you'd need a storage o'scope and even using a peak reading ammeter might be misleading.
That's why I was looking for an easier way to judge CB health.

From the person who complained of this problem, I think he already checked for no other loads on the same breaker.

10. jwelectricElectrical Contractor/Instructor

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One thing to remember is a saw is powered from a motor. A motor is nothing more than a coil of wire.
Coils of wire hate to have a change in current flow. When the coil has no current flowing it will try to keep current from flowing. When a coil has current flowing it will try to keep the current flowing.

A quick look at the Tables found in 430 of the NEC one can see that the locked rotor amperage is six times that of the full load amps.
In other words a 5hp 230 single phase motor has a FLA of 28 amps, Table 430.248 but that same motor has locked rotor amps of 168 amps Table 430.251(A).

When the motor is sitting still it has a locked rotor. This motor will draw 168 amps in order to start the rotor moving. This current does not die down quickly but takes a couple of seconds.

Here in lies the problem with the table saw. Install one size larger circuit and the problem is solved.

11. ThatguyHomeowner

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This was someones else's problem, but,
for my saw the DC resistance was 1.8Ω and with the Romex connecting cable the load on the breaker may have come to 2Ω.
120v/2Ω = 60A rms for at least a half-cycle, dying down to way less than the rated 15A or less, in a second or two.
This seems to be possible with a 15A breaker, with the right trip curve. And, the saw manufacturer is motivated to make his saw work with most run-of-the-mill 15A breakers. They also want to squeeze the max. performance out of these 15A.

That's what I'm getting at. If the breaker is tested, the HO can say this breaker is/is not meeting factory specs, and make troubleshooting decisions based on that.
It's easier to check a trip curve than puzzle over the starting transient current draw for a saw.

Just MHO.

12. jwelectricElectrical Contractor/Instructor

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Bless your little heart but it is not as simple as the voltage divided by the resistance of the motor.
The motor is a mass of windings of copper wire coils and has an inductive reactance to current flow.
If you were using the resistance of the motor then the 60 amps you would be drawing would be a constant draw not a momentary draw.
Simply by looking at the Tables in 430 I can find the full load amperage of any horse power motor.
Let’s say that the saw has a Â½ hp motor. That would be an amperage draw of 9.8 amps. 120 divided by 9.8 equals 12.24 ohms which is a difference of 10.24 ohms.
If the blade is locked down so the motor can’t turn then the motor will draw 58.8 amps. On a 120 volt circuit the resistance of the motor will drop to around 2 ohms.
To just ohm out a motor does not give you anything other than the resistance of the motor windings.

This is not the trip curve of the breaker. Attached is the trip curve of a twenty amp breaker courtesy of Mike Holt

It would be my guess and I must admit that I know nothing about the saw or the circuit but the problem is more than likely to be found in the saw. It could be that the saw is too big for the circuit it is on or the saw could be worked to hard. The list goes on and on.

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• 20 A CB trip curve 01.JPG
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Last edited: Nov 22, 2008
13. ThatguyHomeowner

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That Holt guy has done quite well for himself, and his family, I must say. I think he's a consultant for EC & M magazine, in addition to his books, tapes, videos, etc.

Georg {not "George"} Simon Ohm, as well as LÃ©on Charles ThÃ©venin, would disagree with some of what you said!

With your graph I have a generic test for 20A breakers, by loading them to 2x rated. It just takes more hair dryers, toaster ovens, etc.

"class action" with the name of the saw manu. but it didn't look promising as far as people suing for violation of "implied merchantibility".
There were some 10" table saws by the same people that did seem to have a problem with breakers tripping. The thing is with chop saws is that the blade is so big, the inertia may keep a high current flowing until enough back emf is generated to reduce the current.

Thank you for blessing my heart. Other people have had other wishes about my heart, or lack of it!

Last edited: Nov 22, 2008
14. jwelectricElectrical Contractor/Instructor

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And Mike is a deep Christian man who has raised his family in the spirit of the Lord.
Mike also has a battery of experts and electrical engineers that he calls on when making his programs.

Well it is easy to see that you are gaining your information from that editable site some think to be a true and exact fact, Wikipedia.
It would benefit you deeply to understand that George Simon Ohm worked with four elements of electricity, Voltage (E), Current (I), Power (P) and Resistance (R).
His entire wheel of 12 formulas contains only these four elements. We also know that there are a couple of other elements such as inductors and capacitors that Ohmâ€™s Law does not include in his wheel.
Do a little research into inductive reactance for yourself.

All your â€œgeneric testâ€ is going to give you is the total number of hair dryers and toaster ovens that one breaker will carry before it trips. What your â€œgeneric testâ€ will not give you is the trip curve of any type of breaker or fuse.

This in itself should be telling you something.

Again you have failed to touch base, The Counter Electromotive Force is at its highest when the motor first starts or the blade is locked to a complete stop. When the saw is running idle or under no load the Counter Electromotive Force is at its lowest. It is this Counter Electromotive Force that makes the resistance of the electric motor have varying resistance such as you test with your little ohm meter.

My prayers are that God not only bless your heart but also your entire body including your brain as you go forth in your endeavors into electrical theory.

15. ThatguyHomeowner

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Did you know that, for a 15A breaker, the wire is protecting the breaker at currents below 24A, since the IÂ²T rating for the breaker is more than for #14AWG (assuming it takes one second to melt #14 copper at 166A)?
I didn't know it either, till today. I take it to mean that #14 doesn't need protection below 24A.

BTW, Mr. Holt hasn't yet called me for advice; maybe he lost my number! (Do you see what I'm heading with this. . .?)
Hint: Wiki is not the best reference source but it's less trouble than scanning in and posting textbook pages.

Regarding God, I sometimes wonder about Her motives. . .

Last edited: Nov 22, 2008
16. jwelectricElectrical Contractor/Instructor

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Are you saying that a 15 amp breaker would let 166 amps of current pass for a full second?
Are you saying that #14 copper would reach approximately 2000 degrees Fahrenheit in one second at 166 amps?
More importantly are you implying that the plastic that the breaker is made of would withstand the heat to melt #14 conductor?

Not unless you are saying that Mr. Holt is way to smart to believe anything he reads on Wiki, is that what you are saying?

The big difference between the two is one is based on the continued editing by anyone who desires and the other is time proven printed text.

When I am discussing something with someone who brings up Wiki as their references I canâ€™t help but remember something I read once by Sidney Harris, Columnist. This might not be it word for word but it is real close.

â€œThere is nothing more dangerous than a person with a good mind who begins to reason, logically and coolly, but from insufficient premises; for his answers will always be valid, justified, rationalâ€”and wrong.â€

I use this statement in the classroom when I have a student that has spent hours reading something on Wiki and knows just enough to show just what they donâ€™t know.

17. ThatguyHomeowner

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Fair enough!

Speaking of religion and of the Internet, can you tell me how much a soul weighs?

Last edited: Nov 22, 2008
18. jwelectricElectrical Contractor/Instructor

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I'm still trying to find out how long it takes, whether 1 second or 10 seconds. None of the fusing current references that I can find list the melting, or clearing, time.
I'm hoping this helps.

Try this

No.
Lemme' try sumpin' else.

By your post I would say you spend way to much time over at Wiki when your time would be better spent elsewhere.

Speaking of religion and of the Internet, can you tell me how much a soul weighs?

More than the cross that my Savior carried plus the pain he wore that we all can have salvation. I already learned this so I didnâ€™t have to do any research.

19. ThatguyHomeowner

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When you try to blow the whistle on the corrupt Federal Agency that you worked for, you end up with a lot of free time and not a lot of money.
At least my attorney's fees were tax deductible.

20. jwelectricElectrical Contractor/Instructor

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North Carolina
Are they the one with the chop saw?

21. ThatguyHomeowner

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That's a good one.