The manufacturer said

[Pharon]

Let's all keep in mind that the only reason we can have a scenario where the wire is sized smaller that the breaker is because the motor has built in thermal overloads. If a motor does not have those, then you need to install them. And the upstream breaker (which is only used for short circuit and ground fault, now) is sized higher in order to prevent nuisance tripping on startup, since startup current can be 6 to 8 times higher than steady state.

As an academic exercise, I'm thinking that you could potentially even use a molded case switch as your upstream protection, since it also has built-in magnetic/instantaneous protection. The thermal part of the breaker is not used.

Also note that in Article 440, the value of the OCPD is limited to 225% of the motor FLA. The gap is wider for generic Article 430 motors.

 

[jap]

Let's all keep in mind that the only reason we can have a scenario where the wire is sized smaller that the breaker is because the motor has built in thermal overloads. If a motor does not have those, then you need to install them. And the upstream breaker (which is only used for short circuit and ground fault, now) is sized higher in order to prevent nuisance tripping on startup, since startup current can be 6 to 8 times higher than steady state.

As an academic exercise, I'm thinking that you could potentially even use a molded case switch as your upstream protection, since it also has built-in magnetic/instantaneous protection. The thermal part of the breaker is not used.

Also note that in Article 440, the value of the OCPD is limited to 225% of the motor FLA. The gap is wider for generic Article 430 motors.

Which is a good point seeing as how in our test question that percentage (using a FLA of 21 amps) would be the difference of choosing a 50 amp breaker or a 60 amp breaker

21 x 2.50 = 52.50
21 x 2.25 = 47.25

 

[david luchini]

Which is a good point seeing as how in our test question that percentage (using a FLA of 21 amps) would be the difference of choosing a 50 amp breaker or a 60 amp breaker

21 x 2.50 = 52.50
21 x 2.25 = 47.25

A 45A breaker (Art. 440) or a 60A breaker (Art. 430.)

 

[jap]

Thanks,


JAP>

 

[tkb]

 

[kwired]

Let's all keep in mind that the only reason we can have a scenario where the wire is sized smaller that the breaker is because the motor has built in thermal overloads. If a motor does not have those, then you need to install them. And the upstream breaker (which is only used for short circuit and ground fault, now) is sized higher in order to prevent nuisance tripping on startup, since startup current can be 6 to 8 times higher than steady state.

I got lost in the debate and not even sure what we are debating to some extent, but kind of have to agree that you are on the right track to what seems to be the topic. Most motor applications have two forms of overcurrent protection, and refrigeration compressors are usually this way as well. The branch circuit device provides short circuit and ground fault protection and is typically higher then conductor ampacity often 2-2.5 times higher with circuit breakers or as low as only 150% with fuses, AND the motor overload protection - sort of intended to protect the motor but inherently protects the conductor from overload as well.

As an academic exercise, I'm thinking that you could potentially even use a molded case switch as your upstream protection, since it also has built-in magnetic/instantaneous protection. The thermal part of the breaker is not used.

Also note that in Article 440, the value of the OCPD is limited to 225% of the motor FLA. The gap is wider for generic Article 430 motors.

A molded case switch is just a switch and provides no overcurrent protection, if you have magnetic trip only you have an instantaneous trip circuit breaker, you can have much higher trip setting device for those units then for an inverse time unit - but still need to supplement with motor overload protection.

Most breaker manufacturers have suggested breaker sizes for specific motors, I use a lot of Square D products and have a slide ruler motor calculator. Their recommended breaker size for most motors is less then 250% that NEC gives as a maximum. In many cases I have seen even smaller breakers then the slide calculator suggests and they still hold when starting, but I still try to use what they recommend in general and have little or no call backs on tripping during starting if I do follow the chart. If I do have a call back I often find some problem with driven load or something other then just the breaker as the root cause of trouble.

 

[jim dungar]

I use a lot of Square D products and have a slide ruler motor calculator.

The slide rule provides information on Thermal-magnetic breakers not the magnetic only style.

 

[Mgraw]

@kwired I have come across A/C units protected at barely over MCA and work for years. But the units were well maintained and didn't become a problem until the compressors were getting old and hard to start. A/C manufactures have to account for worst case though and this is why there is a high MOCP.

 

[kwired]

@kwired I have come across A/C units protected at barely over MCA and work for years. But the units were well maintained and didn't become a problem until the compressors were getting old and hard to start. A/C manufactures have to account for worst case though and this is why there is a high MOCP.

Really the motor design is a big factor along with how much impedance you will get from the supply leads as well as the source. Large motor on marginally sized transformer - is sort of an unintentional reduced voltage starter in a way and will limit current to some extent. NEC requirements for art 430 applications take into account the worst possible designed motor for a particular horsepower rating. They want conductor sized for most inefficient motor and do give you some leeway on overcurrent devices - but the 250% allows for many of worst case LRA you would ever find.

I agree, I have seen motors significantly lower breaker settings then recommended or even well below the 250%. 10 HP three phase 480 volt motors - NEC basically says can be on a 35 amp inverse time breaker - no further questions unless the motor won't start on that breaker. But I have seen many that were in operation for a very long time on 15 amp breakers and never have any tripping issues.

I have also in the past run into some simpler situations with 120 volt power tools. In particular some table saws, chop saws and some air compressors seem to be most common culprits. They have a high starting current - and if you put them on the right system and on standard QO single pole circuit breakers they will trip when trying to start. Plug same tool into a 50 foot extension cord and you have added enough resistance to limit the starting current enough it will not trip the breaker. Plug it in at a different site that has longer circuit, longer feeder or service conductors, or maybe just a smaller POCO transformer and put on same QO breaker and it will not trip.

 

[iwire]

I think a much bigger factor than maintenance is how stout the supply is.

If the supply is stout (the voltage does not plummet at start up) the unit should start up fine on the MCA.

On the other hand if your in an area where voltage is on the low end or the supply is not so stout that is when the unit may struggle to start and might trip an MCA sized breaker.

From my perspective there is no good reason not to use a MOCP sized breaker and perhaps eliminate a call back.

JMPO.

 

[kwired]

I think a much bigger factor than maintenance is how stout the supply is.

If the supply is stout (the voltage does not plummet at start up) the unit should start up fine on the MCA.

On the other hand if your in an area where voltage is on the low end or the supply is not so stout that is when the unit may struggle to start and might trip an MCA sized breaker.

From my perspective there is no good reason not to use a MOCP sized breaker and perhaps eliminate a call back.

JMPO.

I think the stout supply allows for more inrush current and does trip the low level breaker easier then if there is some impedance in the supply. By having extra impedance in the supply, anywhere from a marginally sized transformer, generator, etc. to voltage drop in the supply conductors creates a reduced voltage starter to some degree. It makes the acceleration time longer, but does shave off some of the peak of the inrush as well. Too low of a breaker setting may not hold long enough for acceleration on some higher inertia types of loads.

I have experienced these issues many times. Most common is with some power tools. Plug them into a circuit with low impedance to the source and they trip breakers when trying to start. Plug them into an extension cord but via the same premises receptacle and you created a choke for the inrush current and have little or no troubles. (This seems to be common with standard QO single pole breakers, the solution is to use the "high magnetic" breakers that you will only find at a Square D distributor - they have a higher magnetic trip point then the standard QO single pole does.)

Also have had larger motors that ran fine for years on a marginally low setting for circuit breaker, then you come and make changes to the facility that increase conductor sizes, make conductor runs shorter, or otherwise allow for less impedance between the motor and source - and now that breaker trips occasionally when starting, because of the less impedance and higher inrush it allows.

 

[jap]

Your right. I've had my same chop saw plugged in the same place on the same ciruit for 20 years.
Although on a Sq.D Homeline, 2 times out of 10 it will trip the breaker just a soon as I pull the trigger but I've left it that way just to keep me guessin.

JAP>

 

[jap]

Also, just so everyone wont think I'm a complete outlaw, I do have my air compressor running on 12/3 sj cord on a 2p 30 amp breaker.hmy:

JAP>

 

[iwire]

I think the stout supply allows for more inrush current and does trip the low level breaker easier then if there is some impedance in the supply.

For sure the inrush is higher but the duration is less

So the question is are the breakers tripping on instantaneous or thermal portion.


Boy you can ramble.

 

[Mgraw]

You would be looking at a LR condition for 6-30 cycles at 5-8 times FLA. I would say thermal.

 

[kwired]

For sure the inrush is higher but the duration is less

So the question is are the breakers tripping on instantaneous or thermal portion.


Boy you can ramble.

Once the field has saturated to a certain level the impedance of the motor increases and current is lowered pretty significantly, before then is the part that draws the most current and sometimes gets the instantaneous trip.

This is exactly what they are trying to minimize with reduced voltage starting methods - usually on larger motors.

Impedance of that motor winding when not energized and during the first few cycles after energizing is very low and is like closing the source into a short circuit for just a few cycles.

Get past that first part and you are drawing heavy power - but it is accelerating the motor and not just trying to establish the field in the motor windings.

Sorry for rambling, I think I often know what it is I want to talk about, but am not always good at getting it out in a simple and easy to understand manner.

 

[jap]

Once the field has saturated to a certain level the impedance of the motor increases and current is lowered pretty significantly, before then is the part that draws the most current and sometimes gets the instantaneous trip.

This is exactly what they are trying to minimize with reduced voltage starting methods - usually on larger motors.

Impedance of that motor winding when not energized and during the first few cycles after energizing is very low and is like closing the source into a short circuit for just a few cycles.

Get past that first part and you are drawing heavy power - but it is accelerating the motor and not just trying to establish the field in the motor windings.

Sorry for rambling, I think I often know what it is I want to talk about, but am not always good at getting it out in a simple and easy to understand manner.

oooohhhhhhh,,,,, so thats why my chop saw trips sometimes....... what he said....

Jap>

 

[Pharon]

A molded case switch is just a switch and provides no overcurrent protection, if you have magnetic trip only you have an instantaneous trip circuit breaker, you can have much higher trip setting device for those units then for an inverse time unit - but still need to supplement with motor overload protection.

Yes, I did not mean to imply that thermal overload protection would not be required -- only that, in these particular instances where the OCPD is used only for short circuit protection, that a molded case switch (which is a breaker with magnetic only, not thermal) would also provide that, though you'd still need thermal overloads at the motor, obviously.

 

[kwired]

Yes, I did not mean to imply that thermal overload protection would not be required -- only that, in these particular instances where the OCPD is used only for short circuit protection, that a molded case switch (which is a breaker with magnetic only, not thermal) would also provide that, though you'd still need thermal overloads at the motor, obviously.

Not the point I was trying to make, but a little research makes me retract what I did say - there are at least some molded case switches that do have magnetic trip devices in them - I had thought they were a switch only and no trip mechanism.

I did find some that say they have no overcurrent protection as well as some that say they have magnetic trip only.

 

[Pharon]

My understanding is that all molded case switches have a magnetic (short circuit) tripping function in order to satisfy their UL Listing requirements.

Do you have a link to molded case switches without magnetic functions? Because if so, I would be very interested in seeing how they get around that.