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Old 11-14-2008, 10:19 AM   #16
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Hitachi Miter saw trip 20A


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Originally Posted by micromind View Post
This is a very common problem.
The starting current of a typical capacitor-start motor (the kind with the 'hump' on top) is about 6 times the running current. The starting current of a typical universal motor (the kind with brushes; commonly used in chop saws, circular saws, vacuum cleaners, etc.) is about 10 times the running current. With either one of these motors, as the voltage drops, the starting current drops as well.
My chop saw dims the lights severely on startup but has never tripped the breaker. The blade is so big the startup inertia is hard to overcome.
I'd go with a different trip curve that withstands the saw startup but still protects the wiring.
Unless someone makes a "softstart" for these saws.

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Old 11-14-2008, 10:39 AM   #17
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Hitachi Miter saw trip 20A


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Originally Posted by micromind View Post
This is a very common problem.

If the panel is a Square D QO type, high-magnetic breakers are made just for this situation. The part number is QO120HM. They are usually a special order, and about the same price as a regular breaker. The magnetic trip unit on these breakers is quite a bit higher than a regular breaker. Other manufacturers might make a high-magnetic version of their breakers also, I don't know.

Rob
Thanks for the reply. I believe that is the problem with my dust collector. I'm using consumer grade breakers. Since my dust collector is on a dedicated circuit, it plugs into a GFCI which never trips.
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Old 11-14-2008, 10:47 AM   #18
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Hitachi Miter saw trip 20A


10 and 12 " miter/chop saws are generally 15 amp motors. On a typical 20 amp breaker they should start and operate normally. They would be worthless to the contractor/builder if they didn't. You have a problem with the saw motor in my opinion.... High magnetic breakers are a solution but are used for fixed motor loads in commercial and industrial per specification from the manufacturer or engineer.. I'm not saying they wouldn't get your saw started I'm just saying you should not need this solution in your situation. What good is your saw if you have to have it connected to a high magnetic all the time????
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Old 11-14-2008, 01:34 PM   #19
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Hitachi Miter saw trip 20A


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They would be worthless to the contractor/builder if they didn't. You have a problem with the saw motor in my opinion.... What good is your saw if you have to have it connected to a high magnetic all the time????
Speaking of which, I Googled

"class action" hitachi

and other key words, but a problem like this doesn't seem to rise to that level.

Maybe what saves the contractor is that he may use this saw at the end of a long, undersized, extension cable. That is really soft start, and shortens motor lifetime.

Last edited by Yoyizit; 11-14-2008 at 01:39 PM.
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Old 11-14-2008, 01:37 PM   #20
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Hitachi Miter saw trip 20A


So, I ran out to Home Depot at lunch. Oddly enough, they carry QO breakers but they will not work in my HOM box.
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Old 11-14-2008, 09:16 PM   #21
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Hitachi Miter saw trip 20A


What is the nameplate rating in Amps on the motor? Is the saw connected to an individual (read: DEDICATED) circuit?

According to table 430.52 you can upsize the breaker 250% for single phase motors to provide for sufficient starting current.

For example, IF the motor is rated at 15 Amps, the max breaker size could be 40 Amps. The calculation comes out to 37.5 Amps, and upsizing to the next higher standard rating is allowable. You can still use a #12 wire for such a scenario.

Personally, I'd use a 30 to see if it would hold before using a 40.
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Old 11-15-2008, 09:24 AM   #22
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Hitachi Miter saw trip 20A


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What is the nameplate rating in Amps on the motor? Is the saw connected to an individual (read: DEDICATED) circuit?

According to table 430.52 you can upsize the breaker 250% for single phase motors to provide for sufficient starting current.

For example, IF the motor is rated at 15 Amps, the max breaker size could be 40 Amps. The calculation comes out to 37.5 Amps, and upsizing to the next higher standard rating is allowable. You can still use a #12 wire for such a scenario.

Personally, I'd use a 30 to see if it would hold before using a 40.
I've never heard of this. It seems to assume that a motor either draws normal current, or peak current for a second or so, or is shorted and so trips the breaker quickly. And I guess they are right.

The dedicated circuit along with the known load must be how this table works.
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Old 11-15-2008, 12:43 PM   #23
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Hitachi Miter saw trip 20A


Mitre saws, drill presses, chop saws etc that are cord and plug are portable motor-operated appliances not motors. Since these are portable and not fixed (permanent installled) they are plugged into general purpose branch circuits that serve other loads besides motors or motor operated appliances. I understand the code to say you are not allowed to upsize the branch circuit breaker on 15 and 20 amp 125 volts nominal for a general purpose branch circuit. Please read 430.42(C) where it addresses motors or motor operated appliances 1 hp or less or 1hp or more that are cord and plug connected....the rating of the attachment plug and receptacle shall determine the rating of the branch circuit to which the motor or motor operated appliance may be connected in accordance with article 210.

It would be my understanding you would not be allowed under these provisions to upsize the breaker to get the motor started without tripping out.
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Old 11-15-2008, 05:15 PM   #24
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Hitachi Miter saw trip 20A


I don't know what the name plate on it says exactly, I will have to check when I am over there. I belive this is the model of saw at Lowes:

http://www.lowes.com/lowes/lkn?actio...FDH&lpage=none

They call it a 15 Amp.

The Hitachi web site ( http://www.hitachi-koki.com/powertoo...ch/c12lch.html )
Says the following specs about this saw:

15A / 120V
Power input 1,520 W
Max output 1,950 W

I appriciate the suggestions about changing out the breaker and such to solve the problem. It seems to me if the saw is working properly and the wiring / electrical is working properly that a 15A saw should not trip on a 20A circuit with no other load on it.

Maybe we are just extremely unlucky and we have had 2 defective Hitachi saws that both have the same defect (since 2 different hitachi saws of the same model cause breakers to trip on 2 different circuits).

Maybe I will see if there is a regular plug style outlet that will accept 10 gage wire so I could properly run the outlet on 30A. Would be a cheaper work around than buying yet another new saw.

Thanks
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Old 11-15-2008, 06:08 PM   #25
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Hitachi Miter saw trip 20A


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Originally Posted by Stubbie View Post
Mitre saws, drill presses, chop saws etc that are cord and plug are portable motor-operated appliances not motors
. This is laughable. Operating a motor without it being connected to something to perform some task is absurd. Whether it is portable or not is irrelevant. The OP stated this saw was in a workshop, on a 20 Amp circuit with nothing else connected. The use of a cord and plug satisfies disconnect requirements for these "motors"

Quote:
Since these are portable and not fixed (permanent installled) they are plugged into general purpose branch circuits that serve other loads besides motors or motor operated appliances. I understand the code to say you are not allowed to upsize the branch circuit breaker on 15 and 20 amp 125 volts nominal for a general purpose branch circuit.
Read my post again. I specifically stated the condition of an individual (dedicated) circuit, not a general purpose branch circuit. You are comparing apples to oranges here.

Quote:
Please read 430.42(C) where it addresses motors or motor operated appliances 1 hp or less or 1hp or more that are cord and plug connected....the rating of the attachment plug and receptacle shall determine the rating of the branch circuit to which the motor or motor operated appliance may be connected in accordance with article 210.
Again, I state we are not dealing with general purpose branch circuits in this particular discussion, so your point on this is not valid. The rules listed in subsection (C) which deals with cord and plug connection only apply to general purpose branch circuits, since that is the section which references those sub-sections.
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Old 11-16-2008, 03:20 AM   #26
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Hitachi Miter saw trip 20A


Kbsparky... what was all that about??

The post wasn't even directed at your reply other that to let the op know that he could not upsize the breaker on a general purpose branch circuit. He has a mitre saw that is cord and plug I didn't want him thinking he could upsize the breaker on general purpose branch circuits which frankly would be what of these would be plugged into most of the time don't you think??

Quote:
BTW

I know motors draw alot at start up. But having it on a 20A circuit with nothing else on it, and still seeing the breaker trip fairly often doesn't seem right.
Where in that are you sure he has it plugged into a dedicated 20 amp circuit??

Maybe you didn't take your medication today... my statement about motor operated appliances and motors was simply to state his mitre box is a motor operated appliance that was portable and not a motor as specifically mentioned in 430 and 422 of the NEC. I used the term portable only to highlight that it will get moved around and will almost certainly be plugged into general purpose branch circuits. I said nothing about them being treated differently as far as ocpd. However they can be treated differently as for the requirements of the disconnect in accordance with their horsepower. For the most part when 430 is talking about motors they are mostly considering commercial and industrial applications where motors are the only thing on the branch circuit and used to drive machines, conveyors etc...usually hardwired but not always.

Now what I think is... laughable.... is your advice that a cord and plug 15 amp power mitre saw tripping a 20 amp breaker would need to be addressed with a larger circuit breaker to get it to start when it is equipped with a 15 amp plug. I was born at night but not last night...

FYI... when I address someones post to disagree I will start it with the username of the person I disagree with if that is not the case then I am not addressing anyone in particular.

You know as well as I do that breaker upsizing for motors that will not start is almost always a commercial or industrial issue where high torque motors are required to move machinery or heavy loads such as loaded converor lines. I have never up-sized a breaker to start a workshop cord and plug power tool that uses a freakin 15 or 20 amp circuit...never.

These tools are started under no load so current inrush at start and the type motor used are designed to be handled by the branch circuit breaker that matches the rating of the plug on the tool. Common sense IMO. And the OP seems to understand this also.

If I made you feel I was disputing what you said in your thread in a personal manner I apologize.....

Last edited by Stubbie; 11-16-2008 at 05:21 PM.
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Old 11-16-2008, 06:29 AM   #27
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Hitachi Miter saw trip 20A


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Originally Posted by Stubbie View Post
You know as well as I do that breaker upsizing for motors that will not start is almost always a commercial or industrial issue where high torque motors are required to move machinery or heavy loads such as loaded converor lines. I have never up-sized a breaker to start a workshop cord and plug power tool that uses a freakin 15 or 20 amp circuit...never. Heck may be we should tell him to install a adjustable speed drive so he can soft start the mitre box motor.

These tools are started under no load so current inrush at start and the type motor used are designed to be handled by the branch circuit breaker that matches the rating of the plug on the tool. Common sense IMO. And the OP seems to understand this also.
Hi; I agree that upsizing is not the right answer, even thought it might work. Now if you told me I had to upside the breaker on our 3 phase 1.5HP monster bench grinder, that might be a different story (we run it on 240 and get it started by manually driving it with a drill to get it up and running, once moving it runs fine on 240). But for a regular plug in saw, I don't see why you should have to do anything like that.

I just thought maybe there was some piece to the puzzle that I was missing. I appricate the solutions offered. I think it is going to be time to try a third saw and see if it has the same problem, maybe 3's the lucky number on this one.

Thanks guy for the imput.
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Old 11-16-2008, 10:16 AM   #28
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Hitachi Miter saw trip 20A


An Internet search on this tells me this may very well be a design defect.

The cheapest solution may be to run the saw off an extension cord to add resistance to the circuit. There is a probably a table in your owner's manual listing cord sizes and lengths, vs. current draw.

This probably has a better chance of succeeding if your breaker holds for a second or so before tripping, meaning it's just a mild overload.

If you're comfortable with electricity:
along these same lines is a soft start resistor; 100' of #20 AWG gives you one ohm and should reduce the starting surge.
But, you'd need to rig up a way to switch this resistance out of the circuit when you need real power.

www.hosfelt.com and other places like it sell power resistors that can cobbled together to do this same function.

Lastly, someone may make an aftermarket device to overcome this problem, if there is enough of a demand for it.

The best is for Hitachi to recommend a breaker for you but then they would have to admit that there is a problem with these saws.

Last edited by Yoyizit; 11-16-2008 at 11:48 AM.
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Old 11-16-2008, 10:45 AM   #29
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Hitachi Miter saw trip 20A


Sq D makes a breaker for high inrush current that fits in the Homeline panel.

http://ecatalog.squared.com/catalog/.../17401013.html

You may need to go to an electrical supply house if the big box store cannot order one for you.
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Old 11-16-2008, 03:03 PM   #30
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Hitachi Miter saw trip 20A


Jamie

As kbsparky said you could upsize the breaker and it will probably work. Thing is it really is already upsized since flc of 15 amps is its maximum loaded running amps. So it should actually start on a 15 or a 20 amp branch circuit. You might see about contacting a rep from Hitachi.

Micromind and Jdwhites post to install a high magnetic would be your best bet IMO. The one you want is the HO120HM. These will give you thermal protection and a bit more time delay for the motor to start. It won't change a thing as for short circuit and ground fault protection (overcurrent). Neither will a larger breaker as Kbsparky stated other than it will increase your trip out curve on overload. If the motor has thermal protection internal to the motor this really doesn't pose a risk but if your branch circuit is general purpose and you operate something that relies on the circuit breaker to trip out on overload (wire getting hot) there is a little bit involved with that. Either way I wouldn't go throwing a 30 or 40 amp breaker on the circuit to start a mitre saw. You might go to a supply house and get a 25 and see what happens but a high magnetic is in my opinion the better route to take. Still though you shouldn't be needing to do either.

I would suggest suspecting a problem with the tool motor or supply voltage. In general a motor should start if the supply voltage does not drop below 85% of the motor rating during start up. Not knowing the multiplier the manufacturer uses for inrush current for your mitre box motor leaves us guessing what the inrush above nameplate current is but is generally around 4 to 6 times the tool rating. This can be measured with clamp on meters designed to capture the current inrush at the circuit breaker. And also voltage drop can be checked at start up. This would tell you a heck of a lot about where you problem exists. Residential circuit breakers are inverse time breakers. So as voltage drops current increases. If you have excessive voltage drop during start up inrush currents get larger than designed. This may be what is causing your trips. This could be a poor connection or damage to the branch circuit wiring between the tool and the circuit breaker or there may be other problems with the supply voltage. It would appear that this is not likely as you mention your not having any troubles with anything other this mitre saw. And I believe you have tried it on other branch circuits??? It is strange though that you had trouble with two different alike mitre saws. This sorta lends itself to a problem with voltage drop or low supply voltage under load.
Anyway you should not be having this trouble all things being correct IMO.

Just for grins here is a time line curve for inrush currents for a typical 20 amp breaker. The breaker should hold from 40 to 90 amps of inrush for over 1 second. The gray area shows the time frame that the breaker should trip at the given amps. At 100 amps the breaker will trip almost instantly as it sees this as pretty much an over current condition.
Attached Thumbnails
Hitachi Miter saw trip 20A-inrush-trip-curve.jpg  


Last edited by Stubbie; 11-16-2008 at 04:49 PM.
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