Motor Fuses

[godsquadgeek]

Most of the guys I know have one of them sliders that show the amps of motors and what entails overload, feeders, and branch GF protection fuses.

but I am starting to see some of them get outdated, and was wanting to see what the 2014 code states regarding this.

I used an Uglies, and it said a 25HP 480 continuous duty motor (with O.L.) was 50 AMPS.

even thought the FLA was only 34.

then I used an online calculator

http://www.electrician2.com/calculators/motor_ver_1.html

(based on the 2011 code the above states the fuse size at 60AMP or at 175%

both the 2014 uglies, and 2011 calculator were using dual element time delay.

so I put 50's in, as that seems to have a greater consensus.

but I was currious if the section changed between 2011 and 2014, or even 2008 to 2014.

I also used a 2008 handbook and it seemed to want to say 175% was the right ratio.

it's just that 175% seems awful high to me,

almost double.

many guys I know are uncomfortable fusing even higher than 125%

any thoughts?

 

[jumper]

Numbers look right, both tables are the same as 2011.

Standard breaker is 250% of FLA.

 

[jumper]

The above is for short circuit and ground fault protection. Overload protection is less and covered in 430.32.

 

[petersonra]

Edited after I realized what the Op was actually saying.

you can use upto 175% if it is TD fuses or 250% if TM CB.

so nothing wrong with 50A fuses. could go as high as 60 amp fuses if you wanted to though.

 

[bphgravity]

Start with the FLC values in A430 Tables (430.247, 430.248, 430.250).

Size your conductors from 430.22 &/or 430.24 using the above FLC.

Size your OLs from 430.32(A)(1) &/or 430.32(C) using the motor nameplate rating.

Size your OCD (SC/GF) from 430.53 &/or 430.62(A) using the above FLC.

This over simplifies the matter, but that's the basic idea...

 

[godsquadgeek]

Start with the FLC values in A430 Tables (430.247, 430.248, 430.250).

Size your conductors from 430.22 &/or 430.24 using the above FLC.

Size your OLs from 430.32(A)(1) &/or 430.32(C) using the motor nameplate rating.

Size your OCD (SC/GF) from 430.53 &/or 430.62(A) using the above FLC.

This over simplifies the matter, but that's the basic idea...

thanks,

I also notice a discrepancy in article 430 about motors with separate overload devices, being derated as far as smaller short/ ground fault protection?

It didn't make a bunch of sense.

I tried using the handbook but sometimes, the extra notes are not always helpful.

 

[godsquadgeek]

The above is for short circuit and ground fault protection. Overload protection is less and covered in 430.32.

I believe, If I interpreted it correctly that the fuses were for branch circuit protection, while the O.L was for protection after the disconnect, the motor, and wire branch going to the motor.

but the fuses were to protect the whole load side of disconnect from overloading and damaging the primary feeds.

if I am not mistaken, hence the 175% rating.

but what I don't know is if this is for a motor with no overload (thermal on it), or if it's for a motor with a separate overload (heaters at starter).

But the overload, I think was derated to 125%-140% I believe.

but the branch circuit or ground fault protection was 175%,

see they put 40 Amp fuses in it, and it was blowing every time.

they needed 50A at least, and up to 60A if needed.


due to the branch circuit requirments.

my boss basically told me today, (we don't put any fuses over 125%)

which I think is a common misconception of the code.

but then again, he never did a code class in his life.

so I just showed him the uglies they issued us, and showed him where is showed 50 amp.

but he didn't like it.

(didn't like to be told he had been doing something wrong for the last 10 years)

no on likes that.

so I simply showed him the book that he gave us, and said, that we should go by that.

instead of making it sound like a know it all.

 

[jumper]

thanks,

I also notice a discrepancy in article 430 about motors with separate overload devices, being derated as far as smaller short/ ground fault protection?

It didn't make a bunch of sense.

I tried using the handbook but sometimes, the extra notes are not always helpful.

What was the discrepancy? Post the section and what you feel is unclear.

The handbook has its merits, but I have found this forum a way better source of information.

 

[templdl]

I first look at the purpose of the fuse and what benefit that it brings to the party. Many are under the misconception that the fuse provides motor protection when it actually is the job of the an OLR relay with properly sized heater .
The fuse actually protects the motor circuit should the motor go into a failure mode such as a winding going ground. Your intent is to remove that fasling motor from the circuit reducing the chance on a catastrophic failure which would affect he motor circuit as well as the upstream system supplying it.
To want that fuse to supply such protection as close as practical without needlessly blowing relying on the OLR to provide the OL protection.
If a fuse goes blow it is either because it has been sized incorrectly or that the motor or failing and it is your best interest not to let the smoke out.

 

[kwired]

I believe, If I interpreted it correctly that the fuses were for branch circuit protection, while the O.L was for protection after the disconnect, the motor, and wire branch going to the motor.

but the fuses were to protect the whole load side of disconnect from overloading and damaging the primary feeds.

if I am not mistaken, hence the 175% rating.

but what I don't know is if this is for a motor with no overload (thermal on it), or if it's for a motor with a separate overload (heaters at starter).

But the overload, I think was derated to 125%-140% I believe.

but the branch circuit or ground fault protection was 175%,

see they put 40 Amp fuses in it, and it was blowing every time.

they needed 50A at least, and up to 60A if needed.


due to the branch circuit requirments.

my boss basically told me today, (we don't put any fuses over 125%)

which I think is a common misconception of the code.

but then again, he never did a code class in his life.

so I just showed him the uglies they issued us, and showed him where is showed 50 amp.

but he didn't like it.

(didn't like to be told he had been doing something wrong for the last 10 years)

no on likes that.

so I simply showed him the book that he gave us, and said, that we should go by that.

instead of making it sound like a know it all.

Motors are different from most other things we run into in that they have a high current inrush when first energized, but this rapidly falls to close to normal operating levels within less then a second. Trip curves of overcurrent devices often will not allow starting of the motor if sized at only 125%. So for motor applications we typically have two kinds of protection.

The branch circuit device (usually a fuse or a circuit breaker) is intended for short circuit and ground fault protection, and is anywhere from 150% to 1100% depending on circumstances (see Table 430.52)

The motor overload protection protects the motor from overloading, but at same time also protects the rest of the circuit from overloading as well. (If the motor can not draw too much current because of the setting of the motor overload protection, how will the rest of the circuit draw too much outside of short circuits and ground faults which is what the branch circuit protection device provides?)

Unlike short circuit and ground fault protection that need to be at the source end of the circuit overload protection can be anywhere in the circuit as long as it can accurately monitor circuit current. For three phase motors it is common to have overload protection associated with the motor controller but it can be most anywhere in the circuit and still be effective.

Single phase motors - what current comes in one line must go out the other line - so the overload protection only needs to be "single pole design" and can be inserted anywhere in the circuit - even internally within the motor itself.

 

[godsquadgeek]

Motors are different from most other things we run into in that they have a high current inrush when first energized, but this rapidly falls to close to normal operating levels within less then a second. Trip curves of overcurrent devices often will not allow starting of the motor if sized at only 125%. So for motor applications we typically have two kinds of protection.

The branch circuit device (usually a fuse or a circuit breaker) is intended for short circuit and ground fault protection, and is anywhere from 150% to 1100% depending on circumstances (see Table 430.52)

The motor overload protection protects the motor from overloading, but at same time also protects the rest of the circuit from overloading as well. (If the motor can not draw too much current because of the setting of the motor overload protection, how will the rest of the circuit draw too much outside of short circuits and ground faults which is what the branch circuit protection device provides?)

Unlike short circuit and ground fault protection that need to be at the source end of the circuit overload protection can be anywhere in the circuit as long as it can accurately monitor circuit current. For three phase motors it is common to have overload protection associated with the motor controller but it can be most anywhere in the circuit and still be effective.

Single phase motors - what current comes in one line must go out the other line - so the overload protection only needs to be "single pole design" and can be inserted anywhere in the circuit - even internally within the motor itself.

I agree,

we had a pump trip our main plant building one year,

because the fuses were not ground fault enabled.

and/or too large or something.

possibly too out dated, not sure.

but it tripped the main high voltage coming into the building, stopping production in all areas.

just from one 50 horse pump ground faulting.

So I see the need for ground faults, and shorted motors is probably 25 % of my issue. (as well with shorted wire in flex going to motor)

the other 24% is bearings, and or other mechanical issue.

and the other 1% single phasing motors. or bad conections, disconnects etc.

 

[godsquadgeek]

I first look at the purpose of the fuse and what benefit that it brings to the party. Many are under the misconception that the fuse provides motor protection when it actually is the job of the an OLR relay with properly sized heater .
The fuse actually protects the motor circuit should the motor go into a failure mode such as a winding going ground. Your intent is to remove that fasling motor from the circuit reducing the chance on a catastrophic failure which would affect he motor circuit as well as the upstream system supplying it.
To want that fuse to supply such protection as close as practical without needlessly blowing relying on the OLR to provide the OL protection.
If a fuse goes blow it is either because it has been sized incorrectly or that the motor or failing and it is your best interest not to let the smoke out.

I guess this is the reason why I question the code in article 430 mentioning a separate overload.

why would not want an overload?

 

[godsquadgeek]

I also notice some panel builders in our shop will put 3 phase breakers instead of fuses.

but the clamp of that breaker is often too, high, and/or not sensitive enough.

I like that it prevents single phasing in that the breaker opens all three legs.

but in the case of a ground fault in a winding or feed wire, it doesn't matter if you single phase or not, your overloads should trip when high current happens from single phasing.

right?

I also notice that many overload selector pots, are already pre adjusted with the 125% or the 140% depending on type.

so you simply put in your FLA ,and your done.

if they calculate two instances of 125% for example, now their overload is at 150%

which may tend to blow feeder protection, and or mains in the distribution.

 

[godsquadgeek]

Motors are different from most other things we run into in that they have a high current inrush when first energized, but this rapidly falls to close to normal operating levels within less then a second. Trip curves of overcurrent devices often will not allow starting of the motor if sized at only 125%. So for motor applications we typically have two kinds of protection.

The branch circuit device (usually a fuse or a circuit breaker) is intended for short circuit and ground fault protection, and is anywhere from 150% to 1100% depending on circumstances (see Table 430.52)

The motor overload protection protects the motor from overloading, but at same time also protects the rest of the circuit from overloading as well. (If the motor can not draw too much current because of the setting of the motor overload protection, how will the rest of the circuit draw too much outside of short circuits and ground faults which is what the branch circuit protection device provides?)

Unlike short circuit and ground fault protection that need to be at the source end of the circuit overload protection can be anywhere in the circuit as long as it can accurately monitor circuit current. For three phase motors it is common to have overload protection associated with the motor controller but it can be most anywhere in the circuit and still be effective.

Single phase motors - what current comes in one line must go out the other line - so the overload protection only needs to be "single pole design" and can be inserted anywhere in the circuit - even internally within the motor itself.

you mention trip curves, and I was wondering about that too.

how do you feel about putting breakers instead of fuses on a motor starter?

I realize sneider electric (I.E. via Square D), has a combo overload, and breaker unit.

I call those overloads.

but I am talking a standard three pole single throw breaker.

 

[don_resqcapt19]

I agree,

we had a pump trip our main plant building one year,

because the fuses were not ground fault enabled.

and/or too large or something.

possibly too out dated, not sure.

but it tripped the main high voltage coming into the building, stopping production in all areas.

just from one 50 horse pump ground faulting.

So I see the need for ground faults, and shorted motors is probably 25 % of my issue. (as well with shorted wire in flex going to motor)

the other 24% is bearings, and or other mechanical issue.

and the other 1% single phasing motors. or bad conections, disconnects etc.

There is nothing special in the motor branch circuit and ground fault protective devices. It is just the rating of the fuse of breaker.

If the upstream device tripped, that is just a coordination issue. That is the ground fault trip device in the upstream breaker had a setting lower than the current that it takes to open the downstream device.

 

[godsquadgeek]

There is nothing special in the motor branch circuit and ground fault protective devices. It is just the rating of the fuse of breaker.

If the upstream device tripped, that is just a coordination issue. That is the ground fault trip device in the upstream breaker had a setting lower than the current that it takes to open the downstream device.

well every year we have buckets trip, and they are usually 3, 5, or 7 amp 480 volt buckets.


the overloads are fine.


it's just a ground fault usually, like a short or something that does it.

we have gotten to where when we see it, we know it is probably water in a motor, or disconnect, or shorted wire in a pipe, or shorted windings.

not an overload.

So I guess what I was saying with my other post was that whatever the pumps had protecting them, were not rated for what they should have been rated to trip.

maybe a simple adjustment or whatever.

I tend to think the error is on the side of the local equipment, as that is not professionally inspected regularly.

or maintained typically.

we maintain all the switches, and MCC's but not the smaller motor panels annually.

 

[GoldDigger]

You will always have coordination problems when you have a non-GFP breaker whose fault trip is higher than the GF trip level at an upstream breaker.

 

[iceworm]

well every year we have buckets trip, and they are usually 3, 5, or 7 amp 480 volt buckets.

the overloads are fine.

it's just a ground fault usually, like a short or something that does it.

we have gotten to where when we see it, we know it is probably water in a motor, or disconnect, or shorted wire in a pipe, or shorted windings.

not an overload.

So I guess what I was saying with my other post was that whatever the pumps had protecting them, were not rated for what they should have been rated to trip.

maybe a simple adjustment or whatever.

I tend to think the error is on the side of the local equipment, as that is not professionally inspected regularly.

or maintained typically.

we maintain all the switches, and MCC's but not the smaller motor panels annually.

Disclaimer: My experience is all medium/heavy industry. That will be my point of view. I don't do IEC motors, very little with combo IEC overload/contactor/disconnect/cb units. Everything I will refer to will be NEMA standard stuff.

There are some on here that do not agree with my design philosophy. So if you find yourself disagreeing - you are in good company - wrong, but in good company:roll:.

Assumption:
When you say, "every year we have buckets trip, and they are usually 3, 5, or 7 amp 480 volt buckets", you are refering to a bucket with separate CB, contactor, overload relay. The "3, 5, 7" is the number on the CB handle, and the trip was the CB. If not pitch the rest of the post.

The circuit breakers will be mag-only (instantaneous). The "3, 5, 7" doesn't have much to do with breaker setting. Most all of these I have seen will have three dials near the bottom, usually marked A -G (or so) These are what set the trip level. Using the chart that came with CB, one selects the setting that cooresponds with a suitable fault level, usually 8X to 17X the motor FLA. For example, for a CB marker "7', and the dials are set for a trip of 8X, the load can draw 50A for ever and the CB will not trip - yep, truth

Mag-only CB can only be part of a listed combination controller. This type ese should be part of a bucket that was purchased assembled (CB, Contactor, overload) and they should have a NRTL sticker. The sticker applies to all the parts, not just the CB.

So, if the CB is tripping on a GF of short Circuit and ovlds are not tripping, the devices are working exactly per plan.

This is short and there are a bunch of design and code issues that I didn't go into. I'm working today (someone is paying my bill) so all for now. Do a search. I've got some posts as well as others. There's plenty of detail.

later

ice

 

[kwired]

Disclaimer: My experience is all medium/heavy industry. That will be my point of view. I don't do IEC motors, very little with combo IEC overload/contactor/disconnect/cb units. Everything I will refer to will be NEMA standard stuff.

There are some on here that do not agree with my design philosophy. So if you find yourself disagreeing - you are in good company - wrong, but in good company:roll:.

Assumption:
When you say, "every year we have buckets trip, and they are usually 3, 5, or 7 amp 480 volt buckets", you are refering to a bucket with separate CB, contactor, overload relay. The "3, 5, 7" is the number on the CB handle, and the trip was the CB. If not pitch the rest of the post.

The circuit breakers will be mag-only (instantaneous). The "3, 5, 7" doesn't have much to do with breaker setting. Most all of these I have seen will have three dials near the bottom, usually marked A -G (or so) These are what set the trip level. Using the chart that came with CB, one selects the setting that cooresponds with a suitable fault level, usually 8X to 17X the motor FLA. For example, for a CB marker "7', and the dials are set for a trip of 8X, the load can draw 50A for ever and the CB will not trip - yep, truth

Mag-only CB can only be part of a listed combination controller. This type ese should be part of a bucket that was purchased assembled (CB, Contactor, overload) and they should have a NRTL sticker. The sticker applies to all the parts, not just the CB.

So, if the CB is tripping on a GF of short Circuit and ovlds are not tripping, the devices are working exactly per plan.

This is short and there are a bunch of design and code issues that I didn't go into. I'm working today (someone is paying my bill) so all for now. Do a search. I've got some posts as well as others. There's plenty of detail.

later

ice

I took his 3,5, and 7 to mean he has smaller motors in these instances and the fuse settings associated with them are in the 3-7 amp ranges. But I agree if that is the case that he likely has ground fault issues and not motor overload issues when it takes out those fuses

 

[kwired]

you mention trip curves, and I was wondering about that too.

how do you feel about putting breakers instead of fuses on a motor starter?

I realize sneider electric (I.E. via Square D), has a combo overload, and breaker unit.

I call those overloads.

but I am talking a standard three pole single throw breaker.

Trip curves is something that doesn't always get as much attention as it should.

Different types of fuses will have different trip curves. You put in a non time delay fuse on a motor circuit and you will need to select one with a higher rating then if you selected a time delay fuse. A 10 amp time delay vs non time delay will both still trip at some point if loaded more then 10 amps, but the time delay version will allow more through for a longer time before it trips. This is one reason why time delay fuses are preferred for motor circuits. But they still don't ordinarily monitor current to close enough variance to allow using them for motor overload protection. Motor overload protection devices have longer time delay so to speak to allow motor starting, but they still respond to long term lower level overload conditions. Properly selected thermal type units actually are designed to take on similar thermal characteristics to the motor they are protecting. The hotter the motor is running the hotter the thermal unit is running. Short term current surges do add heat to both the motor and the thermal unit, but it takes time to heat motor enough to damage it - as well as it takes time to heat the thermal unit enough to cause a trip. Electronic overload units do the same thing but is more of a calculation then an actual thermal reaction to the current.