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Old 11-16-2008, 04:14 PM   #31
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Hitachi Miter saw trip 20A


Toss the saw in the trash. It is obvious at this point that it is defective. You may go through all the trouble of making the circuit hold, only to have the saw fail anyway.

Or... cut the plug off, and replace it with a 30 A twist lock, and put it on its own circuit. Waste of time if you ask me...

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Old 11-16-2008, 07:39 PM   #32
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Hitachi Miter saw trip 20A


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Originally Posted by InPhase277 View Post
Toss the saw in the trash. It is obvious at this point that it is defective. You may go through all the trouble of making the circuit hold, only to have the saw fail anyway.

Or... cut the plug off, and replace it with a 30 A twist lock, and put it on its own circuit. Waste of time if you ask me...
I applogize this thread got so long winded. The problem with the breakers tripping got to be a mystery after we did toss the first saw in the trash and the current saw is having the same problem with tripping breakers.

Jamie
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Old 11-17-2008, 10:03 AM   #33
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Hitachi Miter saw trip 20A


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Originally Posted by Stubbie View Post
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.
HI
Yes, you are correct, this is the second saw. First one had the same problem and was trashed. It does not do it ever time you make a cut, but it happens pretty often. I have an electrical supply house close by that I bet has specialty breakers. Otherwise, I my sub panel that I pulled this week, I might just try and swap out the panel, I think my is a different brand. I am going to gage the wire myself also make sure there wasn't a mistake. If 14gage was used by mistake, I wonder if the extra resistance could be contributing to the fault.
I have 10 gage around also that I can try if necessary. I may drag the saw out of the workshop and try it from a different service panel and see what happens. I have the brand new CH panel here, and some 20A outlets with brand new wiring, that I just did, so I know it is perfect. I should see if it works here on a known good 20A and a good breaker panel.
Thanks again everyone for the trouble shooting asssistance you have given me a number of things to explore.
Jamie
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Old 11-17-2008, 04:38 PM   #34
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Hitachi Miter saw trip 20A


I would say that you have plenty of supply voltage available. You don't need to upgrade your panel, nor increase the size of the supply wire.

Consider this:

These power tools are manufactured at the largest size that still allows for a "15 Amp" cord and plug, per UL standards. Any larger, and it would have to be fitted with a 20 Amp plug, or rated for a 240 Volt circuit. This does not make for good marketing, since most applications have only "15 Amp" outlets readily available. Better sales volume this way.

Most of the time, these saws are used on jobsites. They are usually fed with rather long extension cords. This has the effect of limiting the inrush current upon starting the saw, and therefore they operate just fine under these circumstances.

Now you have a nice subpanel in your shop, with a full sized circuit conductor (12 gauge, 20 Amp circuit) on a relatively short run. This allows your saw to draw more amps when starting, since more is available to it. As a result, you are experiencing nuisance tripping of the supply circuit breaker. Motors can draw 6 times or more current when starting than running.

As previously stated here by me, and later agreed upon by others, if you have an individual (dedicated) circuit for this saw, you are allowed to upsize your breaker to 25, 30, or even 40 Amps on your 12 gauge wired circuit. The Code specifically allows this in many instances to prevent nuisance tripping problems you are experiencing.

Quote:
430.52(B) All Motors. The motor branch circuit short circuit and ground-fault protective device shall be capable of carrying the starting current of the motor.
(emphasis added)

Since the Code demands that your circuit breaker must carry the starting current of your motor, then I postulate that failure to upsize your breaker could be in itself a violation. The Code makes no distinction between residential, commercial, or industrial applications in this section.

I'd be willing to bet a 3rd saw will exhibit the same problems as the other 2.
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Old 11-17-2008, 05:17 PM   #35
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Hitachi Miter saw trip 20A


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Originally Posted by kbsparky View Post


Since the Code demands that your circuit breaker must carry the starting current of your motor, then I postulate that failure to upsize your breaker could be in itself a violation. The Code makes no distinction between residential, commercial, or industrial applications in this section.

I'd be willing to bet a 3rd saw will exhibit the same problems as the other 2.
I don't know. I have the Ridgid 12" miter. It's about as hoss of a saw as they get, and it doesn't trip the breaker that's literally 10 ft. away. It didn't trip the 15 A circuit that I plugged it into on a trim job a while back. I think he still has a problem with the saw. It may last forever like that, but it is still defective.
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Old 11-17-2008, 08:28 PM   #36
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Hitachi Miter saw trip 20A


Quote:
Originally Posted by kbsparky View Post
I would say that you have plenty of supply voltage available. You don't need to upgrade your panel, nor increase the size of the supply wire.
It is literally about 8 feet from the panel with about 10-12 feet of wire running to the outlet, the outlet on the other circut that we have had it on is about the same distance from the panel on the other wall. So your theory makes sense.

Now that I think about it, we have one of those 40'-50' cord reels on the celing, and many of the other tools get moved around the shop and get run off that cord, the table saw often gets run off that cord... This may explain why we are not finding that other tools have the same problem, not only is that outlet on the ceiling farther away from the panel, it has a 50' extension cord to go through...

I just e-mailed my dad and asked him to perform some saw tests with it pluged into the drop down reel extension cord.

Thanks again,
Jamie
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Old 11-17-2008, 08:32 PM   #37
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Hitachi Miter saw trip 20A


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Originally Posted by Yoyizit View Post
BTW, what is the voltage at this outlet? If Hitachi is cutting it close and your house is right next to the pole transformer, this could be the problem.

120v/26A = 4.6 ~ 5 ohms trips half (?) the time.
120v/15A = 8 ohms won't trip at all.
Current draw at no load = 2A?

So you've got up to 3 ohms to add to the wall outlet to use as a diagnostic tool to check breaker tripping on startup.

A normal wall outlet gives you about 1/4 ohm.
If the outlet farthest from the panel in your house gives you no trips most of the time this is also a clue as to how much resistance you need.

Another way is to add ~1.5 ohms with skinny extension cords and see how you do. If it doesn't trip the breaker on, let's say, 10 consecutive startups you could go lower.
Don't try to cut wood with this cord in place.
To be fair, give the breaker a 5 min rest between attempts.
I will have to go over there with a digital meter and check the voltage. In my previous post, you can see that I think I may have discovered that what you saying is the correct answer - I am awaiting my dad to do some tests with the extension cord that the other tools use.

How do you test the resistance at an outlet? Can you do this with a fairly inexpensive tester or do you need a pretty good meter?

Thanks
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Old 11-18-2008, 06:00 AM   #38
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Hitachi Miter saw trip 20A


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Originally Posted by jamiedolan View Post
How do you test the resistance at an outlet?
It's a very low ohms. Most meters are not accurate that low. Like I posted earlier, 100 feet of 12 gauge is only 0.15 ohms.

Also, the instantaneous inrush current isn't only affected by ohms unfortunately. The inductive properties of the wire will slow the inrush current, for a very short time (microseconds, likely).
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Old 11-18-2008, 09:41 AM   #39
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Hitachi Miter saw trip 20A


A hair dryer or a toaster pulls ~10A. If this load drops the outlet voltage from 123v to 120v then the resistance is (123-120)v/10A = 3/10 = 0.3 ohm.
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Old 11-18-2008, 10:59 AM   #40
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Hitachi Miter saw trip 20A


Long post so I might have missed something, Have you tried this saw in another circuit? Maybe kitchen or other 20 amp.
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Old 11-18-2008, 02:59 PM   #41
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Hitachi Miter saw trip 20A


http://www.google.com/search?hl=en&q=%22back+emf%22

As a fix, or as another diagnostic tool, before you apply power, if you can spin the saw blade in the direction of normal rotation just a few RPMs, this will take some of the kick out of the startup surge.

Do not qualify for a Darwin Award. If you do not fully understand what is below, don't do it.

With this marginal saw, possibly putting the tip of a phillips screw driver blade into the hole that all these blades have, and giving it a shove, may be enough.

A safer but more inconvenient way is to spin up the blade with a drill motor, 3/8" drive socket and drill-chuck/socket adaptor. The socket mates with the saw bolt head.

You are looking for the minimum RPMs to prevent the breaker from tripping, most or all of the time.


BTW, I'm just about "out of cards to play."
At this point I'd decide on the most likely cause and pursue that cause.
For the money riding on this, it's almost worth it to rent additional test equipment to trace the cause.

Or, if you can get past the Marketing People at Hitachi, a Hitachi Application Engineer will probably help you (at risk to his job). This problem and its solution is probably common knowledge on the factory floor at Hitachi.
Try to do this by phone. If the main Hitachi number ends in a number like 8000 or 6000 or something, try dialing 8001 or 8002 to get to speak to an ordinary person. Then ask for "the Application Engineer that you were just talking to."

That's how I got to talk to a person at the IRS who actually helped me. Do not underestimate the arrogance of large organizations.

Or, tell Hitachi you have a question on the Technical Service Bulletin that is out on this problem. Or the (hidden) warranty on this problem.

Getting your foot in the door may also work with e-mail. Address it to something like John.Jones@Hitachi.xxx

I had a slime-ball Auto Dealership deny that there was a problem with their cars' headlights burning out, but he did say that there were aftermarket solutions to this (problem which he never admitted was widespread in some of his cars).
Hitachi may admit to a similar solution if they don't want to lose face but still want to help you.

The more knowledgeable you sound, the less likely they will give you BS. Probably all Public Relation Types don't want to risk being caught in a deception. The appearance of credibility is all they have.
And you can always say that you will run their answer past someone knowledgeable who chooses to remain unidentified (this Forum!!!).

Last edited by Yoyizit; 11-18-2008 at 03:29 PM.
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Old 11-19-2008, 01:39 PM   #42
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Hitachi Miter saw trip 20A


If your saw is like mine, you'll need a breaker that can stand 60A for .0083 seconds; maybe even 90A.
The trip curves will show what the breaker can stand.

You are supposed to already have that much capacity.

Sounds like a bad breaker.

The reason your saw trips the breaker only occasionally is maybe because you happened to turn it on when the AC waveform was at its peak of ~170v. This might happen 1 or 2 times for every 4 times you turn it on.

Please post back with what you finally did on this.

Last edited by Yoyizit; 11-19-2008 at 05:37 PM.
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Old 11-24-2008, 08:14 PM   #43
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Hitachi Miter saw trip 20A


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Originally Posted by Yoyizit View Post
If your saw is like mine, you'll need a breaker that can stand 60A for .0083 seconds; maybe even 90A.
The trip curves will show what the breaker can stand.

You are supposed to already have that much capacity.

Sounds like a bad breaker.

The reason your saw trips the breaker only occasionally is maybe because you happened to turn it on when the AC waveform was at its peak of ~170v. This might happen 1 or 2 times for every 4 times you turn it on.

Please post back with what you finally did on this.
Yoyizit & everyone else invloved;

I wanted to post back on this topic with some results and Thank everyone again for all the imput.

I went over to my dad's yesterday and did some testing. He had been out of town for days and that delayed the testing.

I pluged the saw into the extension cord, and I put a digital meter into the same outlet that the cord is pluged into. The voltage droped from 120(sometimes reads 121) to 114 when the saw was started / run. The saw did NOT trip the breaker during our testing while it was connected to the extension cord.

The saw pluged into one of the circuits on one of the outlets that is close to the panel, where it trips the breaker, reads 120 to 121 regular voltage. Then it only every drops to 116 at the lowest when the saw is run - This is the connection where the breaker will trip.

So from the testing we have done, it appears that using the extension cord (about 40') causes enough voltage drop to prevent nusiance triping.

While this does not necessarly 100% solve the mystery, it does offer an easy solution to the problem.

My dad and I appricate all the information, and he is greatful that a easy solution was arived at, nice and easy - use an extension cord.

Some day I will get a better meter and be able to take some more accurate readings. However for now the mystery will remain, and I will be happy that there is a easy solution.

Thanks again,
Jamie
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Old 11-25-2008, 01:06 PM   #44
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Hitachi Miter saw trip 20A


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Originally Posted by jamiedolan View Post
I pluged the saw into the extension cord, and I put a digital meter into the same outlet that the cord is pluged into. The voltage droped from 120(sometimes reads 121) to 114 when the saw was started / run. The saw did NOT trip the breaker during our testing while it was connected to the extension cord.

The saw pluged into one of the circuits on one of the outlets that is close to the panel, where it trips the breaker, reads 120 to 121 regular voltage. Then it only every drops to 116 at the lowest when the saw is run - This is the connection where the breaker will trip.

So from the testing we have done, it appears that using the extension cord (about 40') causes enough voltage drop to prevent nusiance triping.
What gauge extension cord? It will be stamped on the insulation.

If you cut wood with this cord in place, it should be at least as heavy as your manual recommends for a cord of this length, so you don't shorten the life of the motor.

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