Instantaneos motor breakers

[mull982]

What is the rule of thumb when sizing motor circuit breakers uses in a breaker/starter/overload arrangement for a motor. I am aware that the instantaneous breakers in this case are only for short circuit protection and that the NEC allows the max short circuit setting to be 8X FLA. I also understand that with adjustable trip instantaneous breakers, the trip rating is not necessarily the rating of the breaker but is a range from 3X to 10X the rating of the breaker. In other words an adjustable 100A instant breaker has a trip setting range of 300 to 1000A.

So with that said, is there a rule of thumb or a general range for using these breakers with certain motors. As an example, if i have a 40hp motor @480V with a FLA of 47A what size breaker can I use. Can I theoretically use any size that will let me go up to 8X the FLA rating or in this case 376A. I know the code has an exception to allow the breaker setting to be up to 17X FLA for hard to start motors, should this number be taken into consideration.

Back to the 40hp, 47A example, if all I have avaliable is a 100A breaker I feel comfortable using it b/c 8X 47A is 376A which is well within the span of this breaker setting. Is this correct?

 

[wbalsam1]

Perhaps Table 430.52 will be of help. Also, 240.G.

Table 430.250 says 40 hp at 460 volts = 52 amperes.
430.22 says 52 amperes times 125% = 65 amperes
430.32 says for 1.5 SF use 125% of nameplate current
Nameplate is _________? X 125% = ________amps max O.L.P.
430,52 says for DETD (dual element time-delay) fuse max %@ 175% of Table amps
430.52 Code F (I'm guessing) 52 X 175% =91
drop back to 90 as per 430.52 (A) or 430(B) to the highest setting of the ATCB.
430.52(C)(1) permits increase to 225% max for DETD device where the above is not sufficient for starting current of the motor.

 

[weressl]

What is the rule of thumb when sizing motor circuit breakers uses in a breaker/starter/overload arrangement for a motor. I am aware that the instantaneous breakers in this case are only for short circuit protection and that the NEC allows the max short circuit setting to be 8X FLA. I also understand that with adjustable trip instantaneous breakers, the trip rating is not necessarily the rating of the breaker but is a range from 3X to 10X the rating of the breaker. In other words an adjustable 100A instant breaker has a trip setting range of 300 to 1000A.

So with that said, is there a rule of thumb or a general range for using these breakers with certain motors. As an example, if i have a 40hp motor @480V with a FLA of 47A what size breaker can I use. Can I theoretically use any size that will let me go up to 8X the FLA rating or in this case 376A. I know the code has an exception to allow the breaker setting to be up to 17X FLA for hard to start motors, should this number be taken into consideration.

Back to the 40hp, 47A example, if all I have avaliable is a 100A breaker I feel comfortable using it b/c 8X 47A is 376A which is well within the span of this breaker setting. Is this correct?

If the 376A is close to the MIDDLE setpoint of the adjustment then you probably selected a MCP that will fulfill the needs of almost all potential motors of that HP. The manufacturer's recomendation of NEMA standard size starters are usually give you the same result.

 

[Jraef]

Magnetic-Only breakers (MCPs) are NOT something that can be legally used to construct your own combination motor starter. They can only be used in LISTED manufactured assemblies that have been tested as complete units. So to start off with, if you want to build your own, you are relegated to using Thermal-Mag breakers.

Cheaper T-M breakers do not have adjustable magnetic trips, so make sure you find one that does. As you noticed, most of them have an adjustment range that goes up to 10x the rating. As weressl mentioned, it's a good idea to size the breaker so that your expected inrush falls somewhere in the middle of the range, but within the allowable limits in the NEC article 430.

The actual amp rating of the breaker in a motor starter circuit is somewhat irrelevant (other than as mentioned above) in that the thermal overload in the motor starter is providing the conductor protection, the breaker only needs to provide the short circuit and ground fault protection. Just follow the rules.

 

[brian john]

Magnetic-Only breakers (MCPs) are NOT something that can be legally used to construct your own combination motor starter. They can only be used in LISTED manufactured assemblies that have been tested as complete units. So to start off with, if you want to build your own, you are relegated to using Thermal-Mag breakers.

I have never done this but I also did not know this. Where is this information published?

The only place I have seen mag only CB's (that I remember) is in MCC's.

 

[jim dungar]

Magnetic-Only breakers (MCPs) are NOT something that can be legally used to construct your own combination motor starter. They can only be used in LISTED manufactured assemblies that have been tested as complete units. So to start off with, if you want to build your own, you are relegated to using Thermal-Mag breakers

Magnetic only circuit breakers are UL Component Recognized, you may use them to build your own combination starter if the specific breaker and starter are listed together (i.e. no using a SqD breaker with a CH starter).

 

[Jraef]

Read 430.52.C.3

Although technically, a UL shop could build them and list them, even if they used the exact same UR (UL recognized, not listed) components that were tested together by the manufacturer in their own assemblies, the UL shop would still be required to submit the entire assembly to UL for testing and listing as a complete package, something that costs upwards of $10,000 and takes as much as 6 months (5 months 30.5 days of waiting, 2 minutes of testing, 3:58 worth of paperwork). So for all practical purposes, 430.52.C.3 makes it so that Instantaneous circuit breaker usage is relegated to mass produced (i.e. factory built) combination starter assemblies.

 

[templdl]

Jraef makes valid points with his posts.
With an understanding of the role that an MCP plays in the motor starter circuit NEC 430-52 should make some sense.
An MCP consists of a magnetic (instantaneous) trip element only, most of the smaller sizes commonly rated up to 150a continuos, consisting of a magnetic element that is basically a solenoid. The size of the wire that the solenoid is constructed with will vary from a smaller gauge wire in the very low ampacity MCPs to larger gauges in the higher ampacities. All of the current that flows through the MCP must travel; through the solenoid. If you attempt to apply a 3A MCP with a 50A load for example that 50A will flow through the coils of the solenoid, overload it and it will fry. It will destroy the MCP. It is often very evident if you pick up the MCP an simply smell it.
Remember that a motor starter consist of a contactor and overload relay (OLR). A combination motor starter adds either a set of fuses of an MCP on the load side of the motor starter. I prefer MCPs because they can be adjusted for closer protection.
The contactor job is to control the motor,
The OLR with a properly sized set of heaters or an integral adjustment based upon the full load current of the motor including a service factor continually monitor the current the motor is drawing to assure that the motor is protected from overload. Should the OLR detect an overload a set on normally closed contacts in the OLR that is wired in series with the contactor coil will open the contactor.
Remember that neither the contactor, which is essentially a dumb device, and the OLR, are basically useless should there be a motor winding failure of a fault between the contactor and the motor. Without short circuit protection a failed winding can cause a motor fire. A fault in the wiring from the OLR to the motor can damage the OLR and/or the contactor.
The MCP give the combination started the benefit of having there magnetic protection. A properly adjusted MCP will monitor the motor circuit for both winding failure and circuit faults.
Because an MCP is not a stand alone device, having a reverse UR label, you will notice that it will have a continuous rating be will not have an interrupting rating.
You will find that when used in combination with a motor started the combination will be tested together and the assembly will then be given an interrupting rating.
Regarding setting an MCP up until the mid 90s the NEC limited its adjustment to a maximum of 13 times the FLA and that was it. This did not work well with the high efficiency motors which were being introduced during that time which drew more that 13x inrush an nuisance tripped the MCPs.
Thus, the maximum was increased to 17x. Please note that and MCP doesn't protect the motor but protects the motor circuit (motor cicuit protector) while limiting damage to a motor that is in the begining stages of failure.
My basic recommendation is to disregard the recommended settings allowed and use the lowest setting that will not cause nuisance tripping. Since an MCP will most likely only trip when a winding starts to fail by arcing to ground the quicker that I can pick that failure up and clear it the less motor damage will occur.

Also, one last word of caution, unless you have access to a method to check the calibration of a MCP please don?t attempt to test them. If done incorrectly it will with almost all certainty fry the magnetic element in the MCP and destroy it. MCPs do not have any overload protection which the common thermal magnetic breaker has.

 

[coulter]

Consider a 100A frame, molded case, instantaneous only, say an FA style. Common ratings are 3A to 50A(?). I highly suspect the contacts are all the same size - but I haven't ever taken any apart to see.

Has anyone ever opened any up to see - or have any other verifiable knowledge?

carl

 

[templdl]

I've taken plenty of HMCPs apart on can't recall any contact difference. The only reason to change the contact sized is for cost reasons. Considering that the cost of a set of contacts on an MCP is so minor when you consider the cost of trying to keep different contact assemblies in stock for assembly it is much most of a hassle than any minute cost savings that may be realized.
O the small residential breakers If I recall there may be 2 or 3 families between 15 and 100a.
The sensitive part on an MCP in the 3a -15-a range the reside in the 150a frame is the solenoid type magnetic tip element.

I am also aware where in order to get an HID rating a 100a assembly may be used for a 15 or 20AT HID breaker to increase contact life.

 

[Jraef]

The actual contacts are usually all the same on all CBs of a given frame size; thermal-mag or MCP. That fact can be seen in looking at pricing; a 10A CB is usually the price as a 50A CB. With some manufacturers, in the 100A or 125A frame sizes there may be a difference in the contact material at maybe 70 or 80A and up, it depends from mfr to mfr. Once you get above that 100AF or 125AF range though, everything becomes the same. it isn't worth the trouble for a mfr to make, stock and support multiple sizes of contacts just to save a smidgen of material.

The real difference is in the trip devices. The coil of wire in a magnetic trip element IS different based upon the expected continuous current. There is a lot less copper in a trip coil for a 3A MCP than there is for a 100A MCP even though the frame size and maybe the contact material is the same. There is also a difference in the heat rise, so maybe the frame ventilation design etc. They may look and mount the same, but may be very different on the inside.