Minimum size feeder for motor on soft starter

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philly

Senior Member
What is the minimum size feeder required for feeding a softstarter?

I have a softstarter which I will be connecting to motor. The softstarter will be fed from an MCC by cable for some distance. What are the requirements for the size of the cable and breaker in the MCC.

I would think the softstarter would be similar to a VFD which would require a feeder circuit and breaker rated for 125% of the rated input current to the softstarter. Or if not maybe just the typical 125% rating of the motors FLA? The input current and the motors FLA are the same so either way the feeder cable needs to be 125% of motor FLA but I was wondering for future reference what this was based off of?

Am I correct on the 125%?

So for a motor with FLA of 180A I am going to use a 4/0 cable with 250A breaker. Does this sound o.k.?
 

Jraef

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I'm not aware of any special considerations for sizing of conductors on soft starter circuits as there are for VFDs. I've always just used basic motor circuit rules per article 430.
 

philly

Senior Member
I'm not aware of any special considerations for sizing of conductors on soft starter circuits as there are for VFDs. I've always just used basic motor circuit rules per article 430.

So you would just treat this as a normal motor circuit requiring 125% of the motor FLA.

At the MCC I will have a feeder breaker that must be a thermal magnetic breaker since it is only a feeder breaker. This will protect the cables feeding the softstarter correct?

Now the cables on the ouput of the softstarter of bypass contactor must also be sized at 125% of motor and will be protected by the softstarters internal overload sensing or external overloads if used with a bypass contactor.

Where are the short circuit requiremnts of 430.52 met in this case. Is the drive considered the short circuit interrupting device as well as the overloads. What about when the softstarter is bypassed by the bypass contactor? Would the upstream feeder breaker then have to provide this short circuit protection according to the requirements of 430.250?

Also is something were to happen to the softstarter and we were starting the motor across the line with the bypass contactor it is possible that the upstream feeder breaker at the MCC may not be large enough to handle the inrush. However if we increase the size of this breaker as allowed by 530.52 wont we be comprimising the overload protection of the feeder cable? Or will the external overloads even though they are located downstream on this long feeder cable still provie overload protection for the cable? In other words does it matter where on the circuit the overlaods are located (begining or end?)
 

Jraef

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So you would just treat this as a normal motor circuit requiring 125% of the motor FLA.
Yes

At the MCC I will have a feeder breaker that must be a thermal magnetic breaker since it is only a feeder breaker. This will protect the cables feeding the softstarter correct?
Should be, but I am not there to see what you have. But a word of advice, assume nothing. I have seen a lot of people put MCPs into feeder buckets even though they are not supposed to. If that's the case, you might have a problem.

Now the cables on the ouput of the softstarter of bypass contactor must also be sized at 125% of motor and will be protected by the softstarters internal overload sensing or external overloads if used with a bypass contactor.
Correct

Where are the short circuit requiremnts of 430.52 met in this case.
Assuming there is no fused disc. or CB in the soft starter, then the FCB would be the SCPD for the entire circuit.

Is the drive considered the short circuit interrupting device as well as the overloads.
Be careful not to mix up the issues of "drives" vs soft starters. They are different animals.

Softstart, not "drive"; No. Some soft starters offer Short Circuit protection as a feature, but I don't know of any that are UL listed as an SCPD. They are like any other "starter" in that they need to be part of a circuit that has an SCPD.

VFDs are different in that they are like a new power source for the motor circuit. For this reason, most UL listed VFDs are also listed as the SCPD for the motor side of the circuit. But soft starters are not.

What about when the softstarter is bypassed by the bypass contactor? Would the upstream feeder breaker then have to provide this short circuit protection according to the requirements of 430.250?
Yes, but I'm assuming you mean 430.52 because 430.250 is the FLC chart.

Also is something were to happen to the softstarter and we were starting the motor across the line with the bypass contactor it is possible that the upstream feeder breaker at the MCC may not be large enough to handle the inrush.
Depends on the sizing of the FCB, but it should be.

However if we increase the size of this breaker as allowed by 430.52 wont we be comprimising the overload protection of the feeder cable? Or will the external overloads even though they are located downstream on this long feeder cable still provie overload protection for the cable? In other words does it matter where on the circuit the overlaods are located (begining or end?)
No, Yes, No in order of those questions. But bear in mind this has only to do with the protection scheme, you still have to determine all the other aspects such as sizing for VD, LOTO etc. etc.
 

philly

Senior Member
Depends on the sizing of the FCB, but it should be.

Thanks for the great answers.

You mentioned the above in your repsponse to weather or not the FCB as large enough to handle the inrush. This is a 250A thermal magnetic breker with the motor load being 150hp. I agree with you that this should be large enough to handle the inrush but in the event that it is not, we are allowed to increase this breaker size to 250% of motor FLA according to 430.52. Is this correct.

Now there is a breaker in the softsatarter which I guess would serve as the sort circuit protection. This breaker is the same size as the FCB upstream. Even though now there are two breakers, can either one be considered the short circuit protection and would both apply to table 430.52? I think that it would make sense that if we increased one in size we would need to increase the other since the smaller one would be the weakest link?
 

kwired

Electron manager
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NE Nebraska
Does the soft start have any instructions stating required overcurrent protection requirements?

Motor circuits, as well as pretty much any continuous load circuit requires branch circuit conductors to be sized 125% of the load.

Motor circuits are allowed more than 125% for overcurrent device to allow for starting as long as there is additional protection for overload.
 

Jraef

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Thanks for the great answers.

You mentioned the above in your repsponse to weather or not the FCB as large enough to handle the inrush. This is a 250A thermal magnetic breker with the motor load being 150hp. I agree with you that this should be large enough to handle the inrush but in the event that it is not, we are allowed to increase this breaker size to 250% of motor FLA according to 430.52. Is this correct.

Now there is a breaker in the softsatarter which I guess would serve as the sort circuit protection. This breaker is the same size as the FCB upstream. Even though now there are two breakers, can either one be considered the short circuit protection and would both apply to table 430.52? I think that it would make sense that if we increased one in size we would need to increase the other since the smaller one would be the weakest link?

250A CB on a motor with a possible 220A FLC seems a little small to me. What you are playing with is the time-current curves of the respective breakers. You can get hold of the trip curves for the breakers and compare them to an expected current draw with the soft starter if you know what it will be. But really, your OL relays should be set to trip before a breaker does, so if you get the trip curve of the OL and the thermal trip curve of the CBs, as long as the OL curve is under the CBs, you are fine. That's still not going to say that the motor will start without tripping though, that's a load related issue. What is your load? If it's a pump, 350% for 10 seconds is going to be fine but if it's a centrifuge where you may need a Class 30 OL curve to accelerate it, it may not work...

As to the two breakers, the one in the soft starter becomes the one that falls into 430.52. The one in the MCC then becomes a Feeder (branch) breaker and falls under those rules (don't have my NEC here), even if they are the same size and the cable is the same size. Branch breakers protect the cable feeding the starter from thermal overloads and short circuits, motor circuit breakers only need to protect the motor from short circuits and the OLs protect the motor windings and leads thermally. Technically you may not have NEEDED the breaker in the soft starter, but it's not a bad thing to have it there, I like it better that way myself. I would set the Branch breaker mag trips (if possible) higher than the Soft Starter breaker mag trips so that any short circuit fault is cleared at the lowest possible level.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Nice discussion. I'm wondering about the CB inside the softstart: does the bypass mechanism isolate the CB and connect through a seperate cabinet or does the bypass happen downstream of this 2nd CB? If the latter is the case I agree with Jraef that the MCC CB is a branch circuit breaker and needs to be sized as such (not upto 250%). If the bypass mechanism isolates the softstarter AND the CB, I think you would need to size the MCC CB per article 430 based on your start up current. Since the original topic was sizing the feeder, keep in mind that if you size the MCC CB as a motor circuit breaker (upto 250%) you will also need to adjust the size of your conductors so that they are protected, unless the circuit breaker downstream at the softstart is of a size that protects the feeder conductors.
 

Jraef

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Nice discussion. I'm wondering about the CB inside the softstart: does the bypass mechanism isolate the CB and connect through a seperate cabinet or does the bypass happen downstream of this 2nd CB? If the latter is the case I agree with Jraef that the MCC CB is a branch circuit breaker and needs to be sized as such (not upto 250%). If the bypass mechanism isolates the softstarter AND the CB, I think you would need to size the MCC CB per article 430 based on your start up current. Since the original topic was sizing the feeder, keep in mind that if you size the MCC CB as a motor circuit breaker (upto 250%) you will also need to adjust the size of your conductors so that they are protected, unless the circuit breaker downstream at the softstart is of a size that protects the feeder conductors.
Generally (there are always exceptions...) the Bypass Contactor circuit is tapped off downstream from any disconnect or breaker in a soft starter panel.
 

philly

Senior Member
As to the two breakers, the one in the soft starter becomes the one that falls into 430.52. The one in the MCC then becomes a Feeder (branch) breaker and falls under those rules (don't have my NEC here), even if they are the same size and the cable is the same size. Branch breakers protect the cable feeding the starter from thermal overloads and short circuits, motor circuit breakers only need to protect the motor from short circuits and the OLs protect the motor windings and leads thermally. Technically you may not have NEEDED the breaker in the soft starter, but it's not a bad thing to have it there, I like it better that way myself. I would set the Branch breaker mag trips (if possible) higher than the Soft Starter breaker mag trips so that any short circuit fault is cleared at the lowest possible level.

I went to the field and verifed what was installed currently. The breaker in the softstart enclosure upstream of both the softstart and contactor was a thermal magnetic breaker with a 225A rating and an adjustbald instantaneous trip setting. The feeder breaker in the MCC however was a cuttler hammer motor circuit breaker (MCP) with an instantaneous trip only. Now since you said that this breaker in the MCC is a feeder breaker and no longer applied to 430.52 but rather article 240 this breaker should not be allowed. Besides the fact that I thought MCP's were only UL listed for use in a listed combination starter which would not allow them to be used outside of a starter in the first place. In other words even if there was not a breaker in the softstart enclosure and this breaker in the MCC applied to 430.52 then it could still not be an MCP breaker since it is outside of a listed starter, and must be a thermal magnetic? Is this correct.

So with my current arrangement then the breaker in the enclosure will be the short circuit protection applying to 430.52 and the breaker in the MCC will be the feeder breaker. Since the feeder circuit must be 125% of motor FLA then this cable and therefore breaker must be at least 125% of FLA. FLA in NEC for a 150hp motor is 180A so this gives me a value of 125A. So I would therefore have to use a 4/0 cable with a 225A breaker.

If I remove the breaker from the enclosure then the breaker in the MCC becomes the short circuit protection for the branch circuit and can apply to 430.52 correct? I guess the difference between the feeder and branch circuit is that the branch circuit is anything after the final OCPD in the circuit.




250A CB on a motor with a possible 220A FLC seems a little small to me. What you are playing with is the time-current curves of the respective breakers. You can get hold of the trip curves for the breakers and compare them to an expected current draw with the soft starter if you know what it will be. But really, your OL relays should be set to trip before a breaker does, so if you get the trip curve of the OL and the thermal trip curve of the CBs, as long as the OL curve is under the CBs, you are fine. That's still not going to say that the motor will start without tripping though, that's a load related issue. What is your load? If it's a pump, 350% for 10 seconds is going to be fine but if it's a centrifuge where you may need a Class 30 OL curve to accelerate it, it may not work...
.

Since my feeder breaker now is limited with OCPD to provide thermal protection of the feeder cables I am concerned about motor inrush current tripping this breaker upstream. If I increase the size of the breaker in the enclosure applicable to 430.52 and I raise this up to a value that gets beyond the inrush current wont I potentially sill have an issue with the feeder breaker upstream being to small? The only way to increase this breaker would be if the feeder cable size was increased (all of this ignoring voltage drop for now). This is a pretty long feeder run so is it possible that the resistance in the cable will lessen the effects of the upstream feeder breaker? Or would it be wise to make the feeder breaker a thermal magnetic breaker so that the instantaneous setting can be adjusted along with the one in the enclosure?

I have had several nusience tripping issues lately with high efficiency motors and want to make sure I cover all possibilities in this design. Do you see any problems or limitations with this upstream feeder breaker?

In response to your overload question, this application is a conveyor system. If I compare the trip curves of the breakers and the overloads (both inside the softstart, and external overload relay) as long as the overload curve is below the breaker curves I should be o.k. for starting current (not inrush)?

Thanks for all the help.
 

philly

Senior Member
Do you agree that the instantaneous mag only breaker in the MCC is not correct and should be a thermal magnetic breaker?
 

don_resqcapt19

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Do you agree that the instantaneous mag only breaker in the MCC is not correct and should be a thermal magnetic breaker?
You are correct...that type of breaker can only be used as part of a listed combination starter. It cannot be used to supply the power to a drive.
 
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