Motor Breaker Sizing

[mikkel1]

I have a 480 volt, 3 phase, 100 HP, motor for a pump at a Water Treatment Plant. It currently has a C/H breaker Cat # HMCP150U4c protecting it, which originally was for a 60HP motor. It trips ocasionally on start up, which I believe is from inrush current (the KVA code is "F") and the FLA is 127. I believe this breaker is too small, but am getting lost on sizing it in NEC 430. Can anyone help?

 

[Cow]

We have several 100HP sewage pumps on a lagoon for one of our customers, same FLA as yours tripping 150 amp breakers occasionally also. We replaced most of the pump panels that were deteriorating(sp?) with Allen Bradley size 4's. They come with a 225-250 amp HMCP breaker in them. No issues so far.

If you want to calculate breaker size yourself, take a look at 430.250 and Table 430.52 along with the exceptions.

Have you turned the instanteous trip dial up all the way on your breakers?

 

[mayanees]

I have a 480 volt, 3 phase, 100 HP, motor for a pump at a Water Treatment Plant. It currently has a C/H breaker Cat # HMCP150U4c protecting it, which originally was for a 60HP motor. It trips ocasionally on start up, which I believe is from inrush current (the KVA code is "F") and the FLA is 127. I believe this breaker is too small, but am getting lost on sizing it in NEC 430. Can anyone help?

Table 430.250 gives you the fla as 124 amps. Table 430.52 tells you that you can size the Short-circuit device at 250% of that number or up to 300% per exception c(1)c. So you should start with a breaker as large a 2.5*124=310.. say 300 amps. It's no wonder the breaker is tripping at 150 amps!
I would asusme that your cable was upsized when the 60 was changed to 100 hp, to at least 124 * 1.25 = 155 amps.
You may be getting confused by the fact that 155 amp cable is currently landing on a 150 amp breaker. Realize that breaker is not there for protection of the conductor at its ampacity. Conductor protection comes from the motor overloads located in the starter.
John M

 

[mikkel1]

Cow
thanks, as far as turning the trip settings up, it is currently on "E" which is for the in rush current of 1750A's, the settings go up as high as "H" which is for an inrush of 2500A's.
Mayanees
thanks, I realized that the breaker should be upsized but was having trouble sizing it, as far as this problem goes, our company has come in behind another company and the Plant's maintenance people, there seems to be quite a few problems with Soft Starts and VFD's that are undersized

 

[Cold Fusion]

Cow
thanks, as far as turning the trip settings up, it is currently on "E" which is for the in rush current of 1750A's, the settings go up as high as "H" which is for an inrush of 2500A's.
Mayanees
thanks, I realized that the breaker should be upsized but was having trouble sizing it, as far as this problem goes, our company has come in behind another company and the Plant's maintenance people, there seems to be quite a few problems with Soft Starts and VFD's that are undersized

mikkel -
As you know, this is an instantaeeous only CB. It does not have any thermal element.

The limits are as specifed in:
430.110.A: Disconnect is required to be at least 115% FLC. The 150A HCMP meets that. Unless there is somethng strange with your motor, such as low speed/high torque the CB you have is sufficient.

As noted by others, the max setting is from 430.52.C.3 and exception 1 OR 430.52.C.7.

Either one limits you to:
1700% FLC => 2108A Design B energy efficient
1300% FLC => 1612A other than Design B energy efficient.

You have already exceeded 1300% with the existing setting. So your next option is to determine if the motor is "energy efficient". If it doesn't say so on the name plate, then get a copy of MG1. I had to do that for one batch of motors where the "energy efficient" status was not clear from the nameplates, and 1300% was causing occasional nuisance trips. It is tedious, but not difficult.

If the CB is set as high as the NEC allows and you are still getting nuisance trips, there is another option I have used successfully.
1. Test the motor thoroughly to insure it is not defective.

2. Put a power analyzer on it to determine there is infact an inrush current bump above the CB setting. (It's good to know that the CB is not defective)

Once you show those two things, document and then set the CB up to where it does not trip. I have had to do this before and the AHJ was agreeable.

There is one other option that I have not tried yet. See the FPN after 430.52.C.3. I have not used a CB with a damping means cause I have not yet seen one. First time I get a chance, I will though.

cf

 

[Jraef]

That breaker is an MCP, "Motor Circuit Protector" or "Mag-Only" breaker, also known as an Instantaneous Trip (IT) breaker, which is how it is referenced in the NEC. The rules for sizing it are both in table 430.52, and also in paragraph 430.52.C.3 (on the next page in most editions) where is references exceptions. Essentially you can set the IT breaker to 800% of the motor FLA, but if you show that this doesn't work, you can increase that to 1100% for a "standard" motor, or even 1700% if it is an energy efficient motor.

You have a 150A continuous rated breaker, that is not the correct size MCP for a 100HP motor, you need a 250A frame. The 150A frame is good only up to 60HP, as it was apparently originally used. Fix that issue first.

Your FLA is 127A, so you can set the IT breaker trip point at 1000A without question, 1397A if you demonstrate that doesn't work, and 2159A if it is a newer energy efficient motor.

If you have an older motor, NOT energy efficient, and the breaker is tripping at 1750A, then you have a problem in the motor that must be addressed; you cannot have it set that high legally. If it is a newer EE motor, then you have a little more wiggle room on the allowable settings. That does not solve the breaker size issue, but until you can get the proper sized breaker it can allow you to run temporarily.

Another issue is that of it being an MCP in the first place. You CANNOT use an MCP in a "home brew" starter arrangement, it is only allowed to be used in "listed combination motor controller" (430.52.C.3), which effectively means built by the starter manufacturer. If you have built your own controller, you must use a Thermal-Magnetic (Inverse time) circuit breaker, which changes the sizing rules.

PS: I'm pretty much redundant to Cold Fusion's post, although I guess I need to check my math...
But the last part is important.

 

[augie47]

I was suprised to see in the Cutler-Hammer catalog that the HMCP150 was listed for both 60HP and 100 HP on the Size 4 starter.

 

[Cold Fusion]

...You have a 150A continuous rated breaker, that is not the correct size MCP for a 100HP motor, you need a 250A frame. ...

jr -
Give me a litle education here. How come a 150A MCP is not big enough for a 480V, 3ph, 100Hp motor? (assuming nothing weird about the motor)

430.110.A would appear to say that the 150A rating is okay. As Gus noted, the CH catalog appears to say it is okay (with the U4 trip curve instead of a T4).

cf

 

[mikkel1]

Cold Fusion and Jraef
Thanks for the help, yes someone previous to our company has made this setup a "homebrew" by taking out the original starter and installing a C/H Soft Start in it's place, which is also rated for only up to 60 HP. Needless to say we have a multitude of problems we are addressing at this site.

 

[Jraef]

jr -
Give me a litle education here. How come a 150A MCP is not big enough for a 480V, 3ph, 100Hp motor? (assuming nothing weird about the motor)

430.110.A would appear to say that the 150A rating is okay. As Gus noted, the CH catalog appears to say it is okay (with the U4 trip curve instead of a T4).

cf

To be honest, I didn't look in a C-H catalog, I looked at Siemens and ABB. Siemens stops at 60HP 480V for their 150A frame, ABB at 40HP. At first I too thought it would be OK off the top of my head, but after I saw the ABB catalog and then the Siemens, I was trying to figure out why they showed lower values and came up with 80% of a 150A breaker is only 120A. So maybe C-H rated their MCPs at 100%? Don't know, it doesn't say why.

But the weird thing is, the 60HP limit appears on Siemens' IT breaker application chart, yet if you look at their combination starters, they too use 150A IT breakers internally. So it must be right on the ragged edge and they are considering the breakers at 100% rating when they build and test the starters themselves.

 

[Jraef]

Cold Fusion and Jraef
Thanks for the help, yes someone previous to our company has made this setup a "homebrew" by taking out the original starter and installing a C/H Soft Start in it's place, which is also rated for only up to 60 HP. Needless to say we have a multitude of problems we are addressing at this site.

You may not be able to use that MCP to protect a soft starter unless C-H has specifically listed the two together. Their catalog doesn't say, but everyone else I know of uses Thermal Mag breakers (or fuses) in front of their soft starters because of UL listing issues. Check with C-H.

 

[Cold Fusion]

Cold Fusion and Jraef
Thanks for the help, yes someone previous to our company has made this setup a "homebrew" by taking out the original starter and installing a C/H Soft Start in it's place, which is also rated for only up to 60 HP. Needless to say we have a multitude of problems we are addressing at this site.

Your original post asked for help on a mag-only Cb tripping. However, now that we have the rest of the story, you really don't need any help for a tripping CB.

So what do we have:
1.As jraef said, the installation is likely not a listed combination starter

2. The collection of parts is wrong for the application - 60 hp soft-start on a 100hp motor.

So, no thanks necessary - I didn't help you. You are outside of the NEC and any reasonable design criteria - none of my posts apply. I wouldn't have any other suggestions.

Well maybe one

cf

 

[mayanees]

mikell1,
sorry for my confusion... I didn't pay attention to the fact that it was a motor circuit protector with just instantaneous, and went down a thermal magnetic path.
John M

 

[RETRAINDAILY]

if you change from a 60 hp to a 100 hp the whole circket needs to up grade the pipe, wire, breaker, and motor control

 

[dtonjes]

Cold Fusion and Jraef
Thanks for the help, yes someone previous to our company has made this setup a "homebrew" by taking out the original starter and installing a C/H Soft Start in it's place, which is also rated for only up to 60 HP. Needless to say we have a multitude of problems we are addressing at this site.

The softstart changes EVERYTHING. The soft start is basically an adjustable speed drive, just adjustable on the way up to full speed. The breaker issue is not full load current protection for the motor, but branch circuit protction. Look closer at the soft start and see if you can adjust it to be "softer". It may help on the tripping. If the tripping is a recent development. Meg out everything to see if something may be failing (shorting) which should put this on your bosses higher priority list since you have the data that your pump system is getting ready to take a dump (at 2am). Take all the readings you can as soon as you can in as many places on the system as you can, volts, amps, resistance. "Then compare these readings with your baseline?? readings when they modified the system and did their startup TVMX???, anyway I digress, just keep your numbers as a baseline and keep logging. Maybe you can tell your boss which night the mess will burn down-THEN-you will be sizing to code and whatever other information you can lay your hands on. Check out some stuff on efficiency and wire sizes. (other thread) Good application for over sizing the wires. Also, basically by code 430.52.3 exception you can put any size breaker you want with engineering (AHJ, your boss??) In fact if you need to you can take CH (crap here) out and tie it right to the bus if it keeps whatever it is pumping from creating an envionmental issue which may save your and your bosses job.

 

[philly]

So what is the rule of thumb for selectng the size of an instantaneous circuit breaker for a combination starter? I have always heard to use 250% of FLA. But as long as the breaker had a range of adjustable settings that allowed you to set up to 800% or maybe higher, then can you use a lower size?

All this of course is a hypothetical question realizing that these breakers are only part of a listed combination starter.

 

[Cold Fusion]

So what is the rule of thumb for selectng the size of an instantaneous circuit breaker for a combination starter? ....

Mostly from post 5: The limits are as specifed in:

430.110.A: Disconnect is required to be at least 115% FLC. This would be the number on the front. For example For the size ), AB uses 3A, 7A, 15A. Which practicaly speaking doesn't mean a thing. For this example, the CBs are all 100A frame and all have the same contacts. But each rating has a different instantaneous trip unit - and that matters.

430.52.C.1, 430.52.
1100% FLC => Design B energy efficient
800% FLC => Other than Design B energy efficient.

When required by practical experience, or engineering evaluation:
430.52.C.3 and exception 1: The max instantaneous setting:
1700% FLC => Design B energy efficient
1300% FLC => Other than Design B energy efficient.

My practice is to set them right up to the max allowed by 430.52.C.3 and exception 1.

cf

 

[philly]

Mostly from post 5: The limits are as specifed in:

430.110.A: Disconnect is required to be at least 115% FLC. This would be the number on the front. For example For the size ), AB uses 3A, 7A, 15A. Which practicaly speaking doesn't mean a thing.

cf

I'm assuming here, you are referring to the ratings of the contactors and not the breakers correct?


So as an example lets say I had a motor with a FLA of 7A. If I was going to select a circuit breaker would I start with 2.5* this value and select maybe a 15A breaker. However with this 15A breaker it is adjustable between 45A and 150A, so even the lowest setting gives you 6x full load. Could you technically select a 7A breaker which would be adjustable between 21A and 70A thus allowing you to get to 800%. Or should you use a breaker that allows you to get to 1300% at least?

 

[Cold Fusion]

Instantaneous trip settings in a listed combo starter

Instantaneous trip settings in a listed combo starter

I think this fits in this thread, but if not I won't feel bad if it gets moved.

Question for discussion:
Why does the NEC limit the instantaneous trip setting to any value less than the conductor withstand rating? Except for small systems, where the setting would have to be less than the system will deliver to an arcing ground fault.

Here's my thinking - cf's Law of motor starter circuit protection:

Overload:
The only thing that protects the motor during normal operation is the machine design. Design the machine - including motor selection, so that the motor is not overloaded. However, the overload can protect the motor from burning up during some, limited machine malfunctions such as draging bearings. Mechanical overloads will not stop the operator from continued resetting, which will generally burn up the windings.

Mostly the overload protects the conductors from over-heating during abnormal operation.

Instantaneous CB
This doesn't protect the motor at all. It doesn't operate until a motor has faulted. In which case what is to protect. The windings are finished - rewind or replace.

The instantaneous does protect the the conductors in the event the motor windings fault and the current goes up extremely high - well above the LRA, say 6x FLA. With the setting allowed by the NEC, the instantaneous will trip well inside of the cable damage curve.

Now if the cable is damaged, say fork lift attack, the role of the cb is to keep the structure from catching fire. You might call that personnel protection.

The questions:
If the instantaneous trip is set inside of the cable damage curve, what is to be gained by setting the trip any lower?

What is the driving factor for the NEC to base the instantaneus trip setting to a percentage of the FLC?

Why does the NEC base the instantaneous trip setting on the Design letter? Why didn't the NEC use the Code letter (T430.7.B)?

I'm pretty sure I understand the code sections and how to apply them. And I'm familiar with pertinent sections of MG-1.

And I've had a few conversations with "nationally known NEC experts".

I still can't figure the NEC code panel reasoning.

cf

 

[skeshesh]

The questions:
If the instantaneous trip is set inside of the cable damage curve, what is to be gained by setting the trip any lower?

Being able to adjust the instantaneous trip beyond the damage curve of conductors allows for selective coordinations of that device with others in the system. The discussion in this thread has been limited to the one machine as described by OP and sizing of the OCPD to protect said unit only. Depending on the system, the inrush from a transformer or the startup from a different machine may require a breaker to be adjusted accordingly. I think the mfg. (CH or whoever else the case may be) accounts for these posibilities in designing their product.