# Chop saws tripping breaker



## macmikeman (Jan 23, 2007)

Type of breaker? Direct drive chop saws? Your social security number? Date of Birth? Mothers maiden name? These are all critical questions we need to know in order to properly help you fix the issue with the tripping circuit breakers.


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## emtnut (Mar 1, 2015)

High magnetic breaker will fix it up.


I'd say the resistance of the longer ext cord 'slightly' reduced the inrush.


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## frenchelectrican (Mar 15, 2007)

Those chop saw they do draw insane inrush current when they start up from 0 RPM to full running speed useally about 4K rpm under a second or so.

I did took a current reading one of them before and I do recall they can hit over 150 amps and some case pretty close to 200 amp when you plug it close to the load centre but with very long extendison cord it act like a choke to reduce the voltage some to reduce the current draw under about 100 or so amps.

it dont matter if that chop saw is wired for 120 volts or 240 volts it the same on universal motor the only difference is voltage/ amp drawage that it. 

If ya understand series motor they are basically about the same on Universal motor. they draw insane current at low or zero rpm. 

the other good place you will get the idea is engine starter that do the same thing.


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## Bird dog (Oct 27, 2015)

Another possibility is the motor is starting to go bad and the breaker is just doing its job. Btw, remember generally speaking LRC (Locked Rotor Current) could be six times running load current. Maybe experiment with different gauges/lengths of drop cords? Get a smaller chop saw on the same size ckt?


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## brian john (Mar 11, 2007)

It is an inrush issue. Identical CBs will have slightly different instantaneous pickups.


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## MDShunk (Jan 7, 2007)

The extension cord serves as a current limiting inductor, calming the problem. I agree that a -HMI breaker will solve the issue.


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## joebanana (Dec 21, 2010)

How old are these saws? Is this a new issue? Or, have they always done this? You say these are in a metal/wood shop? When's the last time someone blew out the innards with compressed air?


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## Wirenuting (Sep 12, 2010)

Replace the cord, tie an underwriters knot to slow the inrush. lain:

If that don't work, go with EMTNUT's plan. :smile:


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## tjb (Feb 12, 2014)

You need to start the blade spinning by hand. Get it to a decent rpm before hitting the switch. Should be good.


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## paulengr (Oct 8, 2017)

Inrush depends on phase angle so it’s “random”.


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## Speedy Petey (Jan 10, 2007)

Thanks guys!!
The resistance of the longer cord was a clue, but not sure why. Now I know. 

I know the inrush can be huge, especially with these types of saws. It just seemed way to quick to be an overcurrent thing.

One brand new saw and two older, both DeWalt. Relatively clean since it's an artist/sculptor shop, so they are pretty clean compared to a metal/welding shop.
QO snap-in breakers, typical.


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## MDShunk (Jan 7, 2007)

QO120HM would be the part number then.


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## Bird dog (Oct 27, 2015)

MDShunk said:


> QO120HM would be the part number then.


So, is this basically a SP HACR type CB or is the trip curve different?


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## emtnut (Mar 1, 2015)

Bird dog said:


> So, is this basically a SP HACR type CB or is the trip curve different?


QO120 regular breaker is HACR rated.
the HM curve is different.

QO120 ....
https://download.schneider-electric...80672&p_File_Name=730-3.pdf&p_Reference=730-3

QO120HM
https://download.schneider-electric...80887&p_File_Name=730-4.pdf&p_Reference=730-4


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## micromind (Aug 11, 2007)

MDShunk said:


> QO120HM would be the part number then.


Beat me to it, Marc.....I've solved this issue a number of times by using the high-magnetic (HM) version of the breaker.


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## Bird dog (Oct 27, 2015)

emtnut said:


> QO120 regular breaker is HACR rated.
> the HM curve is different.
> 
> QO120 ....
> ...


Cool, 200A difference at 1 cycle. 500A vs 700A if I read it correctly.


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## emtnut (Mar 1, 2015)

Bird dog said:


> Cool, 200A difference at 1 cycle. 500A vs 700A if I read it correctly.


For one cycle, I'm reading 300A vs about 1600A (max clearing fault)


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## telsa (May 22, 2015)

Commutator & brushes problem.

This is causing the back-EMF to come up too slow...

And then you're hitting the trip curve of your breaker.

1) Replace saws... replace brushes... add a choke ( extension cord of, say, #14, loop it in a coil )

2) ...

( The high inertial load of this machine really beats up the commutator to brushes interface. )

&&&

In a shop setting -- for production work -- 14" chop saws are not ideal. They're too loud.

There are trick shears (and band saws) made to operate on tin studs, strut, etc. 

They even cut faster and leave no burrs.

Portable chop saws are for field work.


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## MDShunk (Jan 7, 2007)

I don't think chop saws have used brushes for a long time.


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## MTW (Aug 28, 2013)

I've seen chop saws repeatedly trip Homeline and QO arc fault breakers.


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## RePhase277 (Feb 5, 2008)

MTW said:


> I've seen chop saws repeatedly trip Homeline and QO arc fault breakers.


I sure hope this metal shop doesn't have arc fault breakers.


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## MadSparky (Mar 2, 2018)

You could get a variable frequency drive

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## MTW (Aug 28, 2013)

RePhase277 said:


> I sure hope this metal shop doesn't have arc fault breakers.



I missed that since I didn't even read the OP.


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## frenchelectrican (Mar 15, 2007)

MDShunk said:


> I don't think chop saws have used brushes for a long time.


Hey Marc .,, they still using brushes on universal motors so on modern chopsaw they still useing brushes.

that one reason why they stay with brushes due most modern chop saw do have electric brake ( plugging the universal motor in reverse or shunt it out to slow down quick. )


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## OldMasterTech (Mar 12, 2014)

Speedy Petey said:


> Here is the scenario, and one which I think we've all come across.
> A friend has a metal and woodworking shop. He has 2 or 3 of those typical 14" abrasive chop saws, these happen to be DeWalt.
> They almost always trip the breaker the instant the trigger is pulled but spins the motor a slight bit. This happens with older saws and brand new as well. They do it on different circuits.....but not all of them that's one mystery. The other thing is if the motor is spinning at all it does not happen. This clues me into an armature problem, but not sure.
> The other big mystery is that they only do it if plugged directly into the wall, which of course I recommended. If he plugs them into a long cord, like 50'-100', even #14ga, they do not trip at all, regardless of where they are plugged in.
> ...


Exactly the same thing happened in my own garage, ended up replacing 4 breakers in a 5 year old panel.


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## RePhase277 (Feb 5, 2008)

If you want high acceleration, several kRPM, and bone-snapping torque, and you want it cheap and reliable, you still can't beat the brushed universal motor.


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## MadSparky (Mar 2, 2018)

RePhase277 said:


> If you want high acceleration, several kRPM, and bone-snapping torque, and you want it cheap and reliable, you still can't beat the brushed universal motor.


#agree

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## emtnut (Mar 1, 2015)

electrokinetix said:


> You could get a variable frequency drive
> 
> Sent from my A574BL using Tapatalk


Good point ! I carry around a portable VFD for all my corded power tools:biggrin:


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## MadSparky (Mar 2, 2018)

emtnut said:


> Good point ! I carry around a portable VFD for all my corded power tools


🤣

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## emtnut (Mar 1, 2015)

electrokinetix said:


> 🤣
> 
> Sent from my A574BL using Tapatalk





Welcome to ET btw :rockon:


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## oliquir (Jan 13, 2011)

I have a makita one and it does the same at my shop with regular qo breakers (1 out of 5 times they trip)


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## Spark Master (Jul 3, 2012)

See, those cheap orange 16ga extension cords at the Home Depot really do serve a purpose !!


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## mitch65 (Mar 26, 2015)

emtnut said:


> High magnetic breaker will fix it up.
> 
> 
> I'd say the resistance of the longer ext cord 'slightly' reduced the inrush.


yep. Sounds like a SQD panel.


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## micromind (Aug 11, 2007)

mitch65 said:


> yep. Sounds like a SQD panel.


If so, a 'high magnetic' breaker will likely solve the problem. 

It'd be a QO120HM.


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## emtnut (Mar 1, 2015)

mitch65 said:


> yep. Sounds like a SQD panel.


You make that sound like a bad thing :wink:


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## mitch65 (Mar 26, 2015)

emtnut said:


> You make that sound like a bad thing :wink:


We did a shop where we had to change all the breakers for the wall plugs for exactly this reason. Fortunately the manager at our wholesalers was a good guy so swapped the twenty odd breakers straight across for HMs.
Do other brands even need to make HM breakers? only ever seen it be an issue with SQD


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## emtnut (Mar 1, 2015)

mitch65 said:


> We did a shop where we had to change all the breakers for the wall plugs for exactly this reason. Fortunately the manager at our wholesalers was a good guy so swapped the twenty odd breakers straight across for HMs.
> Do other brands even need to make HM breakers? only ever seen it be an issue with SQD


SqD has a tighter trip curve. In a short circuit, that makes it a superior breaker (little bit easier on the wiring) ... IMO

I know GE and Eaton (BR) have them too ... you're right thou, I've never seen one needed with those panels !


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## mitch65 (Mar 26, 2015)

emtnut said:


> You make that sound like a bad thing :wink:


I'm a little down on SqD these days, had a nuisance trip on a cct with area lights. 4 HIDs running at just under 14A on a 20A circuit, would trip once a week like clockwork, nobody knew when. Tried a new breaker, no change, tried an HM in case it was doing it on startup, no change. Ended up moving the breaker to the bottom of the row, on the same phase, in case it was getting warm and it hasn't tripped in 3 months. Didn't feel warm when I took it out to move it.


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## Spark Master (Jul 3, 2012)

Very interesting discussion. I have a SD QO panel in my house. Using a chop saw, big contractor grade chop saw. It tripped 3 breakers. and totally destroyed a 2 pole 50 amp breaker for my oven !!

All over the panel. Not even adjacent breakers, and the single pole breaker for the chop saw just needed a reset. Some how, the startup surge went through the entire panel. Problem happened 1 time, and never again..... go figure.


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