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Miter saw keeps tripping breaker

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I just finished a new house, and have been working on getting my garage workshop organized.  Today I got my new miter saw set up and ready to go, but every time I pull the trigger it immediately trips the breaker.  Only thing i can think is the 15 amp saw is overloading the 15 amp circuit BUT... I ran my 15 amp table saw a few days ago, without a problem, and I never had a problem running it at the old house, which had the same standard 15 amp setup.  What am I missing here?

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Welcome to the forum.

 

Is the problematic breaker GFCI or regular thermal/magnetic?

 

If GFCI, the inductive motor load presents a phase (time) difference between voltage and current that could exceed the trip threshold.

 

In either case, breakers can go bad, but not typical on a new house. Is retaining the house wiring, but swapping two usage of a physically different but equivalent breaker in the panel an option? If so, please report on the results.

 

Use all appropriate safety procedures and qualified personnel when working on electrical systems.

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It is gfci, and swapping it out might be an option.  My builder's site manager is supposed to be out in the next couple of days to check out a couple things, gonna have him look at it. 

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There are some of my motorized tools that are always problematic on GFCI breakers, on different breakers, even after the breaker is swapped to a new breaker. 

 

The issue is the inductive load. 

 

One option that may work differently is to use a GFCI breaker in the circuit panel, instead of a GFCI receptacle at the first location in the string of receptacles. This option maintains the ground fault protection required for building codes, but could be more tolerant of the inductive load.

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Actually, I'm pretty sure that's how they have it set up, because none of the outlets have the gfi switches, they're just plain ole regular outlets.

 

I had my dad come over and take a look at it today, and he said everything seemed ok with the circuitry and whatnot, and when he tried the saw, it tripped a couple times, but then worked fine.  

Probably just me and technology not getting along again...

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17 hours ago, PocketsRN said:

Actually, I'm pretty sure that's how they have it set up, because none of the outlets have the gfi switches, they're just plain ole regular outlets.

It sounds like the regular 15A thermal / magnetic panel breakers are tripping when using the saw, with zero GFCI protection in the garage.

 

My understanding is that GFCI receptacle protection is required in areas where water shock is possible, like: kitchen; bathroom; garage; basement and outside. It is surprising to me that new construction would be built without that protection.

 

On every house I've wired, I always run 20A circuits w/ 12AWG wire for the receptacles. I also put lighting on different circuits than receptacles so visual light dimming doesn't happen when high-draw receptacle loads are turned on.

 

An ammeter with peak current hold could be used to measure the current drawn by the saw, to determine if the saw is problematic.

 

The OP's garage workshop would be significantly improved if additional 20A receptacle circuits were added. Those new 20A receptacles could be used for this saw, plus other high-draw tools.

 

When I did my shop, linked below, the receptacles on each wall, plus the bench strip, was on it's own 20A circuit breaker.

 


http://forum.toolsinaction.com/topic/14373-wingless-basement-workshop/
 

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It sounds like the regular 15A thermal / magnetic panel breakers are tripping when using the saw, with zero GFCI protection in the garage.
 
My understanding is that GFCI receptacle protection is required in areas where water shock is possible, like: kitchen; bathroom; garage; basement and outside. It is surprising to me that new construction would be built without that protection.
 
On every house I've wired, I always run 20A circuits w/ 12AWG wire for the receptacles. I also put lighting on different circuits than receptacles so visual light dimming doesn't happen when high-draw receptacle loads are turned on.
 
An ammeter with peak current hold could be used to measure the current drawn by the saw, to determine if the saw is problematic.
 
The OP's garage workshop would be significantly improved if additional 20A receptacle circuits were added. Those new 20A receptacles could be used for this saw, plus other high-draw tools.
 
When I did my shop, linked below, the receptacles on each wall, plus the bench strip, was on it's own 20A circuit breaker.
 

http://forum.toolsinaction.com/topic/14373-wingless-basement-workshop/
 
Per NEC, a 20 amp circuit (20A breaker and #12) may have 15A or 20A receptacles. I see no advantage to installing receptacles to accept plugs that barely exist. The miter saw has a 15A plug. I have come across 20A plugs exactly once, in a commercial setting on semi stationary equipment.

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Lots of misinformation here. A breaker is no longer just a breaker. There are three different devices: breakers, GFCIs, AFCIs, and they detect and do different things. There is no one size fits all. I’ve “fixed” a lot of situations where somebody felt they were doing the right thing for instance by putting in combination AFCIs (and the moron inspector pushed for this) for ALL breakers in a machine shop! Not only is it not required but it caused all kinds of problems.

First a 15 A breaker is not 15 A. Actually they are rated to trip between 80 and 100% of that number so at 12-15 A after a while (generally minutes). This is due to a small heating element that heats up and bends a bimetallic strip until it trips due to an overload which based on the description isn’t happening. So a tool rated for a 15 A circuit will be 12 A max. A true 15 A tool has one prong 90 degrees from the other one so it can only plug into a 20 A receptacle and can go up to 16 A. I’ve seen a few IT UPS systems with it. Using a 20 A breaker and 15 A receptacles or 20 A receptacles (these are dual rated) and heavier wire is Code and often done in garages and kitchens for a little extra margin but that’s not your issue. It’s easy to do and a cheap upgrade but doesn’t solve an issue that’s not overload.

Second standard breakers are thermal-magnetic. There is a solenoid too that is set so that if the current exceeds 10 times the rated current so 150 A, it trips as soon as possible, generally in about 1/60th of a second. In industrial systems this is adjustable but this situation is residential where it isn’t.

During starting the coils in the motor have to build up a magnetic field. At that point it’s just a coil of wire so it’s almost a dead short. There is an initial inrush that is very high, up to 17 times rated current by Code but I’ve measured some European motors at 22 times. These are very problematic with today’s industrial plant breakers. The inrush current is independent of the load. Industrially with faster breakers in common use these days as well as very high energy efficient motors with very high inrush, nuisance tripping is a common issue. But on ordinary breakers without microprocessor tripping (under 100 A) this is not an issue. Once this passes the motor is stalled (0 speed) so it pulls heavy starting currents. Stall current is still very high, 6-10 times rated current, but it’s low enough the breaker won’t trip. That’s a hint why the breaker magnetic trip is 10 times the rating, to avoid motor stall currents. On most motors within about 2-6 seconds it gets up to speed and the current drops down to nameplate or less. This will not trip the breaker normally because it takes time for the wire (and the breaker heater) to warm up so these initial surges go through without causing a trip except in a true overload condition say with a stalled motor or too many things plugged in. So inrush and stall should not be a factor and measuring “peak” or “inrush” current is just going to confuse things unless it finds a shorted motor coil.

The GFCI comments are getting there but way off. A GFCI has two current sensors. One is on the hot and one on the neutral. Ground is not monitored and voltage has nothing to do with it. If the hot and neutral currents differ by even a few milliamperes (around 0.010 A but I forgot the exact number) again in about 1/60th if a cycle it trips. Inductive loads work just fine. The trip circuit is very simple but GFCIs tend to drift out of calibration pretty easily especially as cheap and mass produced as they are. The problem is it takes very little leakage to set the GFCI off so even very subtle problems are hard to spot on top of cheaply built GFCIs. As motors age the varnish gets cracked and crazed from heating and cooling. Then moisture and dirt gets in the cracks causing leaks which trips the GFCI but is really hard to diagnose with an ordinary meter. You need an insulation resistance meter known in the trade by the most famous brand name, Megger.

Finally the latest one and this is a nightmare is the AFCI. An arcing fault has a minimum arc voltage which is pretty high when the arc ignites. Then as the current (not voltage) goes through zero (AC does this 120 times per second) the arc goes out then restrikes again as the voltage gets high enough. The current waveform looks like a squarish wave because of this on/off pattern. Simple electrical circuits won’t work. Various microprocessor programs are used to detect it. This explains the price...you are buying a computer stuffed in a breaker. There are lots of things even just semi-slow starters or the very quick current increase on starting that the AFCI often confuses so there are lots of nuisance trips with these. The worst are variable speed drives that basically have a “rabbit ear” shaped current that...surprise...looks an awful lot like an arc waveform. Supposedly
GE has some industrial AFCIs that don’t trip with variable speed drives but my success rate on making one work is 0.0% and I’m a drives specialist for a motor shop! If you have one on your garage, get rid of it! It is not Code required for a reason in that location. The requirement is: “All 120 volt, single-phase, 15- or 20-amp branch circuits supplying outlets [includes both lighting outlets and receptacle outlets] and devices [including switches] installed in dwelling unit kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry rooms, or similar rooms or areas shall be protected.” Garages, shops, and outdoor locations are purposely not on the list. That doesn’t mean GFCI might be required and as I mentioned earlier, that’s not an issue.

So suggestions are:
1. Check if it’s GFCI, AFCI, or regular breaker.
2. If it’s AFCI try plugging it in somewhere else with a GFCI. If it works, the issue is improper installation. Replace AFCI with GFCI. Get a credit for it. AFCIs are double or triple the cost of GFCI. If it trips, there is something wrong with the tool. Replace or repair.
3. If it’s GFCI again try somewhere else. Otherwise find an electrician friend with a Megger and test it. If it passes, defective GFCI. Replace. Again trying somewhere else may work,too.

The trick is you’re getting an “instantaneous” trip but it’s unclear if it’s an electrical short, ground fault, or arcing fault, or a nuisance trip. Need to narrow down the cause.



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Hmm thats nice to know. I always thought gfci outlets were for rooms exposed to high amounts of water. I didn't know you had to have them in all those other rooms.

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No. No. No.

 

A 15A breaker doesn’t trip at 12A.

 

You are confusing a continuous rated breaker with a non-continuous breaker.

A non-continuous breaker can only be loaded to 80% as per the CEC/NEC.

A continuous rated breaker (expensive) can be loaded right to 100%.

Residential code is based off the assumption that non-continuous rated equipment is installed and almost all home loads are cyclical.

(A continous load is one where the maximum current is expected to continue for 3 hours or more, non-continuous load is less than 3 hours.)

Hence the 12a load on a 15a breaker limit.

 

Also, the first thing to look for with GFCI related nuisance trips is electronic loads.

Electronic Dimmers, occupancy sensors, etc.

Combine that with an inductive load you are asking for trouble.

GFCI monitors the current and trips when there is 6mA or greater ground fault.

Also, sometimes you just need to replace the breaker. Over time and multiple trips, they can get temperamental!

I have a GFCI in bathroom that loves to trip when you turn off the LED lights while the fan is running!

 

Read this:

https://www.fluke.com/en-ca/learn/blog/grounding/chasing-ghost-trips-in-gfci-protected-circuits

 

 

 

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First you confirmed a “15 A” breaker trips at 12 A if you give it long enough. Hence “continuous”
vs. noncontinuous loads. Continuous is defined as 3+ hours. See this from the horses mouth:

https://www.ul.com/wp-content/uploads/2014/09/CircuitBreaker_MG.pdf

See section 38. Miniature molded case breakers under 250 V cannot be 100% rated at 15 A.

For normal cases the “15 A” rating applies. It might take 3 hours (Code maximum) to trip at 80% but it will get there eventually. I can personally confirm some brands are more aggressive about the 80% rule than others. See for instance the trip curves for Siemens QPs used in residential panels:

https://w3.usa.siemens.com/download-center/default.aspx?pos=details_mobile&fct=downloadasset&assetid=2218136&page=1&search_str=&languagefilter=&displayfiltercolumn=&displayfiltervalue=&language=en&datapool=&sortcolumn=&sortorder=ascending&

The curve only goes to 10,000 seconds but it is obviously trending towards tripping at 80% eventually. This is one of the least aggressive breakers out there as far as the 80% rule,

The list I posted is for AFCI, not GFCI. Since new construction AFCI must be combination (AFCI+GFCI) and trust me, you want that version of you must install an AFCI, it sort of back doors GFCI into areas most people never considered. The general list for GFCI is bathrooms, garage and outbuildings, outside receptacles, unfinished basements and crawl spaces, kitchens, laundry, wet bars, and around pools. As the commenter said, almost any place associated with water or moisture. But there are almost more exceptions than rules for each of these so don’t take this list as gospel without reading the Code closely. For instance in kitchens it is for receptacles feeding a dishwasher, countertop areas, and “within six feet of a sink”. Not required elsewhere such as a refrigerator (if more than six feet from the sink) or over the counter microwaves. So you need both.


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First you confirmed a “15 A” breaker trips at 12 A if you give it long enough. Hence “continuous”vs. noncontinuous loads. Continuous is defined as 3+ hours.   

 

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 I confirmed nothing.And thanks for repeating what I already stated.

 

A 15a amp breaker is rated for 15a.

 

It says so from the manufacturer.

Nowhere does it state that they trip at 12a.

Nowhere.

Linking a UL white paper on breaker terminology does what exactly? There is no proof.

Show me proof that states they are designed to trip at 12a.

Because they don’t. Or they wouldn’t be rated for 15a, they would be 12a.

I can run 14a all damn day and the breaker will hold.

 

The 80% rule is Electrical Code. It’s not the manufacturers spec. It is meant to ensure that the continuous load of of a non-continuous rated breaker is no more than 80% of its rating.

That means if I can prove that my load is non-continuous, I can load that breaker right up to 100% of it’s rated current.

 

Continuous rated breakers are designed for the increased thermal demands over longer periods of time.

 

 

Do you know what trip coordination is?

If breakers behaved they way you have misinterpreted, I would be losing a great deal of money!

Continuous and non-continuous rated breakers use the exact same trip-curves.

 

Here is a nice, easy to understand link:

https://www.schneider-electric.ca/en/faqs/FA104355/

 

Are you an electrician?

It sure doesn’t seem like it or you would know better.

 

 

 

 

 

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If you prefer Schneider, fine.

https://www.schneider-electric.us/en/faqs/FA236739/

“An 80% overcurrent device, either circuit breaker of fused switch, will carry 80% of its rating continuously. Loading above that level will eventually result in the overcurrent device’s activation if maintained. The higher the loading level the faster the time for activation.”

It isn’t an engineering thing. The trip curves are indeed the same and work the same outside of an enclosure but in an assembly the standard breakers are rated at 80%. It isn’t just that the breaker is overloaded (ampacity) but it will actually trip. I have had this happen first hand.

What’s going on? It’s a panel heating thing. See below.

https://iaeimagazine.org/magazine/2016/07/19/100-vs-80-choosing-the-right-ocpd-solution/

The issue is inside a distribution panel temperatures go up which changes the calibration of the thermal element. UL, NEC, and CEC account for this by adding 125% to continuous loads which is the same as allowing a maximum 80% load on the breaker, or 12 A vs. 15 A.

This isn’t just an engineering or design thing. The first time I ran into this with multiple nuisance trips on a 400 A standard Siemens breaker in a panelboard. Not picking on Siemens here, that’s just what it was. It tripped multiple times. We recorded the current and found we were running at 350 A, on a mining machine that runs aroond the clock with almost no break at shift change. We changed the breaker twice figuring we just got a dud. I even pulled a “failed” breaker and tested it...trip curve was spot on. THD was very low...not a harmonic problem. It kept tripping. Siemens tech support pointed out the issue and we switched to 100% rated...problem vanished. I’ve seen this happen once in a while since then. It’s not just an obscure or bogus Code or engineering thing.

This usually happens when someone especially an engineer designs for the rating (100%) and loads the breaker to 90-100%. Engineers don’t learn Code in school and mist industrial electricians don’t either.

I do breaker testing. I get calls when this obscure trip issue happens. I usually test the “defective” breaker, especially after swapping breakers a couple times. In this relatively obscure case they pass but trips installed in the panel. So then we do power system testing looking for the actual load which is sometimes in the mystery 80-100% zone. That’s when you get all the highly credentialed engineers, electricians, and management people all upset. I get called names. Often they don’t want to pay for the testing and send the business to a competitor. Or they accuse the manufacturer of selling cheap junk. They just can’t accept the idea that a breaker can test at 15 A then actually trips at a lower value. Then they go around and around on this before they accept what NEC, UL, and the manufacturers are telling them. Going to a 100% rated breaker (when available), or sizing for 125% fixes it.



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Lots of misinformation here. First a 15 A breaker is not 15 A. Actually they are rated to trip between 80 and 100% of that number so at 12-15 A after a while (generally minutes). 

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  This statement is false.

 

They are not “rated” to trip at 12a, nor will they trip in “minutes” of running above 12a (unless operating in abnormally high temperatures or other abnormal contributing factors).

 

Please show the 12a “rating”.

 

I certainly understand that non-continuous rated breaker will trip when under an increased continuous load.

That was never a contentious point.

 

You claimed a 15a breaker wasn’t actually rated for 15a.

 

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