# Motor Overloads



## Wirenuting (Sep 12, 2010)

tarmy78 said:


> Hey Guys got a question when you are sizing an overload for a motor and you can't find the exact size do you go the next step up or the next step down? Settle a argument!


Never go over, going under is fine. But it's based on the loading. 
Keep in mind that o/l's have a range,, so it's hard to think you can't get the proper size. 

What do you have and what amperage do you need?


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## Wirenuting (Sep 12, 2010)

Here is a page for CH/Westinghouse. 


http://www.pioneerbreaker.com/v/vspfiles/V4_Backup/Tools/west_FH.htm


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## JRaef (Mar 23, 2009)

Wirenuting said:


> Never go over, going under is fine. But it's based on the loading.
> Keep in mind that o/l's have a range,, so it's hard to think you can't get the proper size.
> 
> What do you have and what amperage do you need?


Yeah, I agree. The ranges generally overlap, it shouldn't be possible to not have a heater element that will not cover your nameplate FLA. 

But as to never going over, that will depend, and this may be where your friend is getting his info. The old rule of thumb is that you cut the motor life by 50% for every heater size over the correct one. But selecting the correct one has a LOT to do with what TYPE of OL relay you have. Most, but not all, bi-metal overload relays are what is called "ambient compensated" or "ambient insensitive". That means that it doesn't matter (much) if the OL relay (starter) is in an area where the ambient temperature is different from the motor. But for those that are NOT ambient comp./insensitive, and for eutectic melting alloy (solder pot) OL relays, then there are rules about how the heaters should be selected based upon the different ambient conditions. Here is what A-B says about it regarding the Type W eutectic alloy type heater elements:



> The following is for motors rated for Continuous Duty:
> For motors with marked service factor of not less than 1.15, or motors with a marked temperature rise not over +40 °C (+104 °F), apply application rules 1 through 3. Apply application rules 2 and 3 when the temperature difference does not exceed +10 °C (+18 °F). When the temperature difference is greater, see below.
> 
> 1. The Same Temperature at the Controller and the Motor — Select the “Heater Type Number” with the listed “Full Load Amperes” nearest the full load value shown on the motor nameplate.
> ...


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## Wirenuting (Sep 12, 2010)

JRaef said:


> Yeah, I agree. The ranges generally overlap, it shouldn't be possible to not have a heater element that will not cover your nameplate FLA.
> 
> But as to never going over, that will depend, and this may be where your friend is getting his info. The old rule of thumb is that you cut the motor life by 50% for every heater size over the correct one. But selecting the correct one has a LOT to do with what TYPE of OL relay you have. Most, but not all, bi-metal overload relays are what is called "ambient compensated" or "ambient insensitive". That means that it doesn't matter (much) if the OL relay (starter) is in an area where the ambient temperature is different from the motor. But for those that are NOT ambient comp./insensitive, and for eutectic melting alloy (solder pot) OL relays, then there are rules about how the heaters should be selected based upon the different ambient conditions. Here is what A-B says about it regarding the Type W eutectic alloy type heater elements:
> 
> ...


[/QUOTE]

I agree, about the old hard rule of never going over. 
Due to his limited question, I gave a limited answer. But I did post that link so they could see the difference between compensated and noncompensated o/l's.

I don't think he's going to get the left or right argument answer he was looking for.. Maybe they will have to split the cost of that case of beer.


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## tarmy78 (Apr 3, 2011)

The overloads are the piggy type for motor disconnects which falls into the motor control spectrum sorta and what happened is someone ordered an overload let's say at 5.63 and the math tells us that what's needed is 5.52 and what we should have is 5.43 I don't think These overloads have ranges as they are fixed. I'm certain that most small motors 1hp or less have internal overloads which makes these extra overloads at switch redundant no?


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## micromind (Aug 11, 2007)

tarmy78 said:


> The overloads are the piggy type for motor disconnects which falls into the motor control spectrum sorta and what happened is someone ordered an overload let's say at 5.63 and the math tells us that what's needed is 5.52 and what we should have is 5.43 I don't think These overloads have ranges as they are fixed. I'm certain that most small motors 1hp or less have internal overloads which makes these extra overloads at switch redundant no?


I've connected tons of small motors that had no O/L built in. 

If you want to get ultra technical with O/Ls, look in the manufacturers tech section and see where the O/L will actually trip. It's usually a percentage of whatever current is given in the tables. 

Select the O/L that is just under the SFA of the motor. 

The majority of the time, this will result in the same unit as using the table values though.


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## xlink (Mar 12, 2012)

What type of overload block do you have and what is the heater number. Maybe someone can give more precise answers if they have more precise info. Lets say for example someone ordered an overload at 5.63. Is that the overload number or the amperage rating for a specific overload.


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## tarmy78 (Apr 3, 2011)

That would be the amperage the number for overload would be an Allen Bradley p29 or p30


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## Introyble (Jul 10, 2010)

We need more information to answer this question with 100% accuracy. Is the motor continuous or non continuous?

Assuming the motor in question is continuous then sizing overloads is pretty simple.

FLA x 1.25

This is the universal accepted calculation *Unless* your continuous duty motor falls into one of the two exceptions.

AMB rating is greater 40 C
The Service Factor (SF) is less than 15% 

SF is typically denoted as 1.15, 1.25 on the name plate.

If your continuous duty motor falls into 1 of those two exceptions then you size overloads by FLA x 1.15

Over sizing by greater then 25% often offers no protection at all and will result in damage under normal failure conditions.

Under sizing will cause over load trip failures under healthy conditions. Don't do either. Find the correct OL rating. This is simple as heaters are sized in hundreds of combinations. Or, if you are using a non solid state starter you can simply buy programmable OL and dial in your exact specification.


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## hardworkingstiff (Jan 22, 2007)

Introyble said:


> Assuming the motor in question is continuous then sizing overloads is pretty simple.
> 
> FLA x 1.25


Are you sure? I was always instructed to use the FLA to size the OL. I use FLA x 1.25 to size the conductors and the OCP, but not the "heaters".


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## Introyble (Jul 10, 2010)

hardworkingstiff said:


> Are you sure? I was always instructed to use the FLA to size the OL. I use FLA x 1.25 to size the conductors and the OCP, but not the "heaters".


I'm 100% positive


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## BBQ (Nov 16, 2010)

Introyble said:


> I'm 100% positive


I think you are incorrect, I have never read anything about adding 125% to a motor overload size for continuos duty. In my opion you are over sizing them.


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## Introyble (Jul 10, 2010)

Here is a nice tutorial I found:

http://www.wisc-online.com/objects/ViewObject.aspx?ID=IAU10308

I have seen motors with SF less than and greater than 15%

Motors that have a percentage of less than 15% SF are far cheaper than normal duty. They are bought because the customer is usually pinching pennies or the motor was designed for OEM specifications.

Heavy duty motors that have identical name plate specifications but have SF greater than 15% are very expensive.


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## BBQ (Nov 16, 2010)

Introyble said:


> Here is a nice tutorial I found:
> 
> http://www.wisc-online.com/objects/ViewObject.aspx?ID=IAU10308
> 
> ...



My IPad can't open that link, but if you go to this link http://www.westernextralite.com/resources.asp?key=66 it says the 25% extra is already included in the heater size. 




> Heater coils are rated to protect standard 40°C rise motors, and open and drip proof motors having a service factor of 1.15 at approximately 125% of rated motor current, and where controller and motor are at same ambient.


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## Introyble (Jul 10, 2010)

BBQ said:


> My IPad can't open that link, but if you go to this link http://www.westernextralite.com/resources.asp?key=66 it says the 25% extra is already included in the heater size.


Sorry about the link, I just googled and picked the first search result. Ended up being a pretty cool interactive and animated site. 

Honestly, when possible I have never simply set to the FLA. Almost always use programmable on high end applications. Dial in the shunt trips, delays, ect ect ect ect.

Not practical on 5 or 10 HP obviously so I simply amp clamp under the maximum load and dial in a simple OL. 

Motors running in the SF significantly decrease in life (typical 15% anyway) so I do my best to avoid that scenario. 

Solid State starters? No VFD obviously? Turn it on and go I take it.

Generic calculation is 25% Here is another link

Hard to argue with the NEC:

http://www.electricneutron.com/motor-control/nec-calculation-for-overload-sizing/


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## JRaef (Mar 23, 2009)

The 25%, or 125% depending on how you look at it, is what is called the "pick-up point" of the thermal tripping curve. Below that point, the OL is not likely to ever trip. But whether you _*ADD*_ that to the motor FLA, or _*USE*_ the motor nameplate FLA, is TOTALLY dependent upon what the motor OL relay mfr _*SAYS *_you must do _*in their instruction manual*_. 

There is NO CONSENSUS among manufacturers! Some say that you add the 25% to the nameplate FLA and _then _select the heater, others have ALREADY factored the 125% pick-up point into the selection criteria and you must select based SOLELY on the nameplate FLA. If you add the 25% to those, you are now at 156% of the motor nameplate FLA!

Bottom line, ASSume nothing, RTFM.


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## RICHGONZO1 (Mar 5, 2012)

I'm a maintenance electrician and have worked on basically all the common motor controllers. I've found that Allen Bradley wants you to size them to FLA. Westinghouse however wants you to size them according to the FLA X the service factor. Also depends on the ambient compensated but ours always were. On some ol blocks you can select 100%, 115% or 125% depending on your service factor of the motor. Sometimes when its too close to call I go lower, trend the current on the control system or recording amp meter and see if it nuisance trips in a week or 2 while within FLA, I bump up. 

Hell, process supervisions signs papers to increase the heaters and the millwrights sometimes put different motors in without telling us so I guess throw em out the window cause they are pretty much bypassed. I love it when a man with no training tells me to up size the heater with absolutely no knowledge. Gotta love process, instead of fixing problems they just bandaid. Just gotta have a paper trail...


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## Search Party (May 23, 2013)

Go the next size up

Going the next size down could very well annoy you with not being able to get the motor up and running.

You should get the correct size, but if you cannot; go the next size UP, not down.

No point puttng in a breaker that will render the motor a paper weight.


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