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Old 11-13-2008, 03:26 PM   #1
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Hitachi Miter saw trip 20A


HI;

My dad has a 12" Hitachi miter saw in his work shop. This is the second saw he has had and they both have the same problem. Often when the saw starts up, it trips the breaker. He has had the same problem on different branch circuits. Today, I put a clamp meter on the circuit he was using. No draw until he started the saw, then a spike to 26 amps. It does not trip every time.

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.

Could there be some kind of electrical problem that I am not seeing, or do you think this is just a issue with the Hitachi saws, and if it is, is there anything else that can be done about it? It is a standard plug, so 20A is the largest you can go right?

Thanks

Jamie

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Old 11-13-2008, 03:35 PM   #2
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Hitachi Miter saw trip 20A


A 20A breaker that trips on a 26A motor start leads me to believe the breaker is the problem. Change the breaker and see if the problem goes away.

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Old 11-13-2008, 03:37 PM   #3
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Hitachi Miter saw trip 20A


I'm interested in the solution to this problem. I have the same problem in my woodshop when I fire up my dust collector which is on a dedicated 20 amp circuit. It trips the breaker only when it starts getting colder. I had a similar problem with my Dewalt mitre saw as well in my previous shop. I would think it is a weak breaker problem. I would definitely not put a larger breaker in the circuit as it probably has only #12 wire at the most.
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Old 11-13-2008, 03:41 PM   #4
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Hitachi Miter saw trip 20A


How long is the circuit run? Perhaps voltage drop is causing the saw to draw more amps for longer than it normally would. Usually, the breaker should handle the start up current. Try the saw on a receptacle near the panel. Maybe a new 20 A breaker on a 10-2 run is in order.
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Old 11-13-2008, 04:06 PM   #5
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Hitachi Miter saw trip 20A


Quote:
Originally Posted by HouseHelper View Post
A 20A breaker that trips on a 26A motor start leads me to believe the breaker is the problem. Change the breaker and see if the problem goes away.
It happens with 2 different breakers, on 2 different circuits. - both from the same panel. But 2 different breakers.

Jamie
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Old 11-13-2008, 04:10 PM   #6
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Hitachi Miter saw trip 20A


Quote:
Originally Posted by InPhase277 View Post
How long is the circuit run? Perhaps voltage drop is causing the saw to draw more amps for longer than it normally would. Usually, the breaker should handle the start up current. Try the saw on a receptacle near the panel. Maybe a new 20 A breaker on a 10-2 run is in order.
22'x22' shop. Wired with NM. The outlet being used today that tripped it is about 10-12' from the panel. Outlets on the far side of the shop can't be more than a total of 50' - 70' of cable depending on exactly how they are routed.

At what distance does voltage drop start to become an issue? I was under the impression that it is a fairly good distance, say over 100'.

Thanks
Jamie
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Old 11-13-2008, 04:19 PM   #7
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Hitachi Miter saw trip 20A


Are there any outlets that accept standard plugs that can be wired 10gage / 30 A or is it required to have a 30A style plug for a 30A circuit.

Upgrading to a 30 might solve the problem, but I am curious if there isn't something else going on, as I can't belive that Hitachi makes saws that are not able to operate properly on a 20A breaker.

It is a square D commercial line Main lug only 8 space panel. Fed from a main panel off of a 30A breaker in the main panel. Distance of wire from the main is about 50'. The main (30 amp in main service panel that supplies this) has never triped.

The the hot is fed by 2 #6, and the neutral is fed by a #10 - hence the reason it is protected by a 30A. Since the main isn't tripping, I can't see an issue here. The box is bonded to a grounding rod with #6. This is a detached garage.

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Old 11-13-2008, 04:20 PM   #8
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Hitachi Miter saw trip 20A


Is the motor in good condition? Does the saw blade move freely or are the bearings in need of lube? Is there an electronic brake on that model?
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Old 11-13-2008, 04:25 PM   #9
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Hitachi Miter saw trip 20A


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Originally Posted by handyman78 View Post
Is the motor in good condition? Does the saw blade move freely or are the bearings in need of lube? Is there an electronic brake on that model?
I belive this one does have a break on it, but am not positive, I would have to check with my dad, I don't use it often enough to remember.

Yes the saw is in good condition, about 8 months old - used a few times a week or so. Saw moves freely, no problems with the saw as far as we can tell. Saw previous to this one was a very similar model, same brand, and cause the same problem.

10" table saw, Band saw, planer, shaper, joinner, very large bench grinders, sanders, stick welder, wirefeed welder, etc, never trip any of the breakers.

Only these stupid hitichi chop (miter) saws.

So I don't know if it just draws more power more quickly (spikes on startup) or something than other equiptment does or what? I would blame the saw only and say get a new saw, but this is the new saw, it's the second saw with the same problem.

Jamie

Last edited by jamiedolan; 11-13-2008 at 04:26 PM. Reason: more info
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Old 11-13-2008, 04:45 PM   #10
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Hitachi Miter saw trip 20A


12 gauge is 0.001588 ohm/foot. 100 feet is 0.15 ohms for the wire. It's not going to be an issue.

Maybe it's the outlets? Is the plug loose? If the outlet is high resistance it could be causing a voltage drop.
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Old 11-13-2008, 04:47 PM   #11
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Hitachi Miter saw trip 20A


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Originally Posted by Gigs View Post
12 gauge is 0.001588 ohm/foot. 100 feet is 0.15 ohms for the wire. It's not going to be an issue.

Maybe it's the outlets? Is the plug loose? If the outlet is high resistance it could be causing a voltage drop.
Plug isn't loose. Happens in 2 different outlets. They seem hold the plug in just fine. Knowing my dad, they are not premium outlets, but I am postive all of his connections are rock solid, all looped firmly around the screw and fully tightened down.

thanks
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Old 11-13-2008, 04:58 PM   #12
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Hitachi Miter saw trip 20A


Could be the start capacitor on the motor. When those start to go it causes it to spike more current.

Your clamp meter might be too slow to see it. The breaker can trip in milliseconds if the surge is steep enough.
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Old 11-14-2008, 12:32 AM   #13
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Hitachi Miter saw trip 20A


This is a very common problem.

A circuit breaker has two separate trip units built into it, thermal and magnetic.

The thermal trip unit is designed to protect the wiring from excessive heating during a sustained overload. Typically, a 20 amp breaker will hold about 120 amps for about a second or two, 40 amps for about 10 seconds, and 30 amps for about a minute.

The magnetic trip unit is designed to protect against short circuits and ground faults. A typical 20 amp breaker will trip in about one cycle (1/60 of a second) at over about 150 amps.

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.

It's entirely possible that the magnetic part of the breaker sees the starting current as a short, and trips instantly. If you plug a couple hundred feet of extension cord into the outlet, and the saw into the cords, I'd bet the breaker will hold. If so, you've identified the 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.

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Old 11-14-2008, 12:39 AM   #14
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Hitachi Miter saw trip 20A


I'd blame the saw(s) and not the wiring, based on what you've said.
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Old 11-14-2008, 02:46 AM   #15
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Hitachi Miter saw trip 20A


Quote:
Originally Posted by micromind View Post
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.
This is very informative, thanks!

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