Breaker size for inrush with high efficiency motors

[philly]

We are trying to find a reasonable solution to a problem we are having with MCP instantaneous breakers installed in a combination starter tripping during inrush of a high effeciency motor. The motor is a 480V 150hp motor with a FLA of 175A. The breaker that is installed currently is a 250A instantaneous breaker set to the maximum setting giving us a 2500A trip point. Several times when we try to start this motor we trip this breaker on inrush and have measured inrush currents in excess of the 2500A setting (2800-3000A). We are trying to come up with possible solutions for getting over this inrush issue and still staying code compliant.

I know the NEC allows a maximum setting of 17x FLA with an instantaneous breaker for a high efficiency motor. A 17x setting for this motor would equate to a setting of 2975A. Would it be possible to install a 400A instantaneous breaker (next size up) and dial the breaker down to 3000A? Even with this i'm not sure it would be enough to get over the inrush.

The other option we looked at was installing a 250A thermal-magnetic breaker but I noticed that even with this breaker the adjustable instantaneous trip setting only allowed a maximum of 2500A which puts us in the same position as the origonal breaker. The code allows for a inverse time breaker to be used up to 250% of motor FLA. Does this inverse time breaker pertain to a thermal-magnetic breaker or a thermal only type breaker. If it applies to a thermal-magnetic type breaker with an adjustable instantaneous setting does the 17x that applies for an instantaneous breaker still apply to the instantaneous setting on the thermal magnetic breaker. In other words if we install a 400A thermal-magnetic breaker are we restricted in how high we adjust the instanteous setting? Would this violate any NEC or UL ratings?

Are there breakers that are stricly thermal elements on dont have an instantaneous element but rather have a trip curve similar to a fuse that would allow us have a longer delay on this high inrush.

What is typcially the solution to this inrush problem for these high efficiency motors. Others are stating that we need to install soft starters or VFD's that are very costly and I find it hard to believe that a soft starter that may cost upwards of $10k is the only solution to this problem. Are there any additional provisions in the NEC that allow us to get around this inrush issue with upsizing the short circuit protection while not violating the UL isting of the starter?

 

[topgone]

We are trying to find a reasonable solution to a problem we are having with MCP instantaneous breakers installed in a combination starter tripping during inrush of a high effeciency motor. The motor is a 480V 150hp motor with a FLA of 175A. The breaker that is installed currently is a 250A instantaneous breaker set to the maximum setting giving us a 2500A trip point. Several times when we try to start this motor we trip this breaker on inrush and have measured inrush currents in excess of the 2500A setting (2800-3000A). We are trying to come up with possible solutions for getting over this inrush issue and still staying code compliant.

Since you have the motor, is it possible telling us what the LRA on the nameplate says? Otherwise, try reduced-voltage starting.

 

[philly]

Since you have the motor, is it possible telling us what the LRA on the nameplate says? Otherwise, try reduced-voltage starting.

I dont have the motor in front of me but I belive the LRA is aproximately 1200A. Obviously inrush current will be greater than this value in order to produce magnetic field in windings etc...

Is spending money on additional reduced-voltage starting the only solution? This seams un-economical. Is there no work around with another breaker size/type or using fuses?

 

[ptonsparky]

Ask at least three suppliers for a price on a soft start. Should be considerably less than 10K.

 

[winnie]

Since the problem is magnetizing inrush, I wonder if some sort of contact timing would solve the problem. If you close the switch on an inductor at the peak of the AC cycle, you avoid any inrush.

As another approach, I wonder if some sort of 'precharge' would get the job done, eg closing a contactor that feeds the motor via a set of resistors prior to closing the main contactor; essentially a very short duration soft start that only deals with the magnetizing inrush.

-Jon

 

[topgone]

NEC workaround

NEC workaround

My understanding on 430.52 (3) Instantaneous Trip Circuit Breaker:
The instantaneous breaker is allowed by the National Electrical Code (NEC) if certain criteria are met:
1. The breaker must be fully adjustable over the range of anticipated inrush currents.
2. The breaker must be part of a listed combination motor controller having coordinated motor-overload, short circuit and ground-fault protection. "

From what you said, the inrush current you have measured was way higher than the max breaker setting (2500), so you're not compliant. Your workaround then will be 430.52 (C)(1) Exception No. 1, that says:

Exception No. 1: Where the values for branch-circuit short-circuit and ground-fault protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal protective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere rating shall be permitted.

You can go with a 400A breaker, IMO.

 

[Jraef]

My understanding on 430.52 (3) Instantaneous Trip Circuit Breaker:
The instantaneous breaker is allowed by the National Electrical Code (NEC) if certain criteria are met:
1. The breaker must be fully adjustable over the range of anticipated inrush currents.
2. The breaker must be part of a listed combination motor controller having coordinated motor-overload, short circuit and ground-fault protection. "

From what you said, the inrush current you have measured was way higher than the max breaker setting (2500), so you're not compliant. Your workaround then will be 430.52 (C)(1) Exception No. 1, that says:

You can go with a 400A breaker, IMO.

This is the common solution to this problem. You cannot use a larger MCP type breaker unless the starter manufacturer is listing it as an option in that specific combination starter. Some are now doing that, but if you already have the starter, you are not permitted (legally) to change it in the field; you would have to order a starter that way from the factory. But you CAN remove the MCP and install a next size larger TM breaker per that exception. Handle mechanisms etc. may not work because you are going to be changing frame sizes, but with a little tweaking it will probably work.

The trick is, you have to be able to support (if challenged by an AHJ) that it would not work otherwise, which you have already observed. Keep your notes.

 

[philly]

This is the common solution to this problem. You cannot use a larger MCP type breaker unless the starter manufacturer is listing it as an option in that specific combination starter. Some are now doing that, but if you already have the starter, you are not permitted (legally) to change it in the field; you would have to order a starter that way from the factory. But you CAN remove the MCP and install a next size larger TM breaker per that exception. Handle mechanisms etc. may not work because you are going to be changing frame sizes, but with a little tweaking it will probably work.

The trick is, you have to be able to support (if challenged by an AHJ) that it would not work otherwise, which you have already observed. Keep your notes.

For clarification purposes, does an Inverse Time Breaker as referenced in 430.52 refer to a thermel-magnetic breaker. When using a thermal magnetic breaker are there any limitations on how high you can adjust instantaneous settings.

So with exception No. 1 as listed above how do you interpret that this applies to a thermal-magnetic breaker and not an instantaneous? Is this becasue the instantaneous exceptions are handled seperately in 430.52(C)(3)?

So if we follow the exception and go to the next standard size of a 400A thermal-magnetic as you mentioned are we free to adjust the instantanous setting on this breaker as high as necessary to get over inrush? Will this still keep the UL listing of the combination starter intact?

Just out of curiousity are you ever permitted to change instantaneous breakers in the field as long as the manufacturer lists a particular breaker for a starter or must you always buy a new complete starter when you need a new breaker?

 

[Jraef]

For clarification purposes, does an Inverse Time Breaker as referenced in 430.52 refer to a thermel-magnetic breaker. When using a thermal magnetic breaker are there any limitations on how high you can adjust instantaneous settings.

Yes. Inverse-time = thermal-mag for all practical purposes. The limitations are as stated in Exception 2:c: "The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes." Although it doesn't expressly state this, I also would not exceed the 17X FLC (or as close as you can get with the breaker adjustments) for the magnetic trips as specified in 430.52.3 Exception 1, even though that technically would apply only to instantaneous trip breakers. The intent is the same: allowance for high magnetizing currents that cause nuisance tripping. Using a T-M breaker in a combination motor starter where you have a separate OL relay means you are essentially just using the magnetic instantaneous trips for the breaker.

So with exception No. 1 as listed above how do you interpret that this applies to a thermal-magnetic breaker and not an instantaneous? Is this becasue the instantaneous exceptions are handled seperately in 430.52(C)(3)?

Yes, see above.

So if we follow the exception and go to the next standard size of a 400A thermal-magnetic as you mentioned are we free to adjust the instantanous setting on this breaker as high as necessary to get over inrush?

Not unlimited, but you can always make an argument to the AHJ if, at 17X, it still nuisance trips. I had to once, he allowed it. magnetizing current was 20x FLC.

Will this still keep the UL listing of the combination starter intact? Just out of curiousity are you ever permitted to change instantaneous breakers in the field as long as the manufacturer lists a particular breaker for a starter or must you always buy a new complete starter when you need a new breaker?

Technically, any change to a listed assembly is a violation of the UL listing of that assembly. But in cases like this, where it's proven to be a problem, most AHJs will allow the replacement of a component if the replacement is UL LISTED. So T-M breakers are UL LISTED. The problem with Instantaneous-Trip (MCP) breakers is that they are NOT UL LISTED, they are only UL RECOGNIZED components (UR), meaning they can only be used as parts of tested listed assemblies. So you can replace them in the field with the exact same unit, but not with something else or another brand, unless the OEM of the assembly has that specific combination of devices listed, and even then, it's a chance.

 

[Jraef]

By the way, this issue is a problem going on inside of the starter manufacturer's halls right now as well. Many of them are having to go through considerable expense to re-list combo-starters and MCC buckets with breakers that have higher mag trip settings because of this issue of energy efficient motors causing nuisance trips. It causes serious problems when the breaker frame sizes jump, especially with MCC buckets.

 

[weressl]

We are trying to find a reasonable solution to a problem we are having with MCP instantaneous breakers installed in a combination starter tripping during inrush of a high effeciency motor. The motor is a 480V 150hp motor with a FLA of 175A. The breaker that is installed currently is a 250A instantaneous breaker set to the maximum setting giving us a 2500A trip point. Several times when we try to start this motor we trip this breaker on inrush and have measured inrush currents in excess of the 2500A setting (2800-3000A). We are trying to come up with possible solutions for getting over this inrush issue and still staying code compliant.

I know the NEC allows a maximum setting of 17x FLA with an instantaneous breaker for a high efficiency motor. A 17x setting for this motor would equate to a setting of 2975A. Would it be possible to install a 400A instantaneous breaker (next size up) and dial the breaker down to 3000A? Even with this i'm not sure it would be enough to get over the inrush.

The other option we looked at was installing a 250A thermal-magnetic breaker but I noticed that even with this breaker the adjustable instantaneous trip setting only allowed a maximum of 2500A which puts us in the same position as the origonal breaker. The code allows for a inverse time breaker to be used up to 250% of motor FLA. Does this inverse time breaker pertain to a thermal-magnetic breaker or a thermal only type breaker. If it applies to a thermal-magnetic type breaker with an adjustable instantaneous setting does the 17x that applies for an instantaneous breaker still apply to the instantaneous setting on the thermal magnetic breaker. In other words if we install a 400A thermal-magnetic breaker are we restricted in how high we adjust the instanteous setting? Would this violate any NEC or UL ratings?

Are there breakers that are stricly thermal elements on dont have an instantaneous element but rather have a trip curve similar to a fuse that would allow us have a longer delay on this high inrush.

What is typcially the solution to this inrush problem for these high efficiency motors. Others are stating that we need to install soft starters or VFD's that are very costly and I find it hard to believe that a soft starter that may cost upwards of $10k is the only solution to this problem. Are there any additional provisions in the NEC that allow us to get around this inrush issue with upsizing the short circuit protection while not violating the UL isting of the starter?

Combination starters are listed with various size MCP's for OCP. As long as your MCP is listed for the size starter, you should be OK. Relabeling could be performed by the authorized repair shop.