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Thread: using motor starter overloads as motor branch circuit short circuit protection

  1. #1
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    using motor starter overloads as motor branch circuit short circuit protection

    I am asked to install a control cabinet that is field built with a distribution block supplied with a 100 amp circuit and tapped for 5 individual motor branch circuits
    The individual motor starters have no individual breakers or fuses in their tap conductors
    The individual that built the controller says his starter overloads will function as motor branch circuit short circuit protection
    This is a new one on me
    Please advise
    I think I need individual branch circuit protection in the form of either fuses or inverse time breakers in front of each starter in the tapped motor circuit?

  2. #2
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    Quote Originally Posted by jim gage View Post
    I think I need individual branch circuit protection in the form of either fuses or inverse time breakers in front of each starter in the tapped motor circuit?
    I'd have fuses ahead of the starters and the starters would include inverse time overload protection. But I don't know what your rules are.
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  3. #3
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    using motor starter overloads for branch circuit short circuit protection

    The motor circuit tap conductors exit the controller to 2 - 1 hp motor loads 1- 5 hp motor load 1- 3 hp motor load and 1- 7.5 hp motor load with existing conductors sized from 14 AWG to #10 AWG I am leaving the office in a few minutes hope to hear some sort of reply soon
    Thanks

  4. #4
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    It is possible but in all likelihood there would need to be overcurrent protective device for each motor. The overloads will act as over current protection and the 100 amp breaker will act as a ground fault and short circuit device but the 100 amps may be too high for the motors. In most cases the max short circuit and ground fault device for a motor is 250% of the fla.
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  5. #5
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    using motor starter overloads for branch circuit short circuit protection

    The loads are existing motors with overload protection and fed with existing buss rail disconnects but we are doing a dust collector system and local jurisdiction is requiring the orderly shut down of all dust creating equipment connected to the vac system to stop when the collector is not in use or has a shut down due to the flamex control cabinet spark detection.
    I want to assure that each existing circuit that is controlled by this multi motor feeder that is tapped has short circuit protection and I do not want to count on an overload assembly to accomplish this. In my experience I am not aware of any overload for a motor starter that is capable or listed for this use. I find no need for a motor starter in this application and a simple motor rated and current rated relay would serve the purpose but individual circuit protection for short circuit sized to the individual conductors leaving the cabinet should still be installed in front of each relay ie. contactor .

  6. #6
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    Quote Originally Posted by jim gage View Post
    I am asked to install a control cabinet that is field built with a distribution block supplied with a 100 amp circuit and tapped for 5 individual motor branch circuits
    The individual motor starters have no individual breakers or fuses in their tap conductors
    The individual that built the controller says his starter overloads will function as motor branch circuit short circuit protection
    This is a new one on me
    Please advise
    I think I need individual branch circuit protection in the form of either fuses or inverse time breakers in front of each starter in the tapped motor circuit?
    No, not true, if what you are referring to is a simple thermal or solid state overload relay. They do not provide the required Short Circuit and Ground Fault protection, they are only providing Over Current protection.

    Where it would be true is if the “overload” is actually an IEC style motor protection device, what’s called a “Motor Protection Circuit Breaker” (MPCB) here, that is a 3 function device similar to a manual motor starter because it can disconnect the load and has thermal OL protection, but also includes a magnetic trip element which provides the SC and GF protection required by the NEC. They look like this:
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    It is perfectly legitimate to mount these down stream of the contactor if that’s what they show. But these are not just an overload relay, they are different.
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  7. #7
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    using motor starter overloads for branch circuit short circuit protection

    Thanks for that but they are simply square d motor starters with overload assembly on them I guess I stick with my thoughts and the advise of my engineer and install din rail mounted feed through motor breakers in front of the individual starters
    That is how I build things and that is how I will continue to do so.
    Thanks and I would welcome any added comments available to this question.

  8. #8
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    Quote Originally Posted by jim gage View Post
    Thanks for that but they are simply square d motor starters with overload assembly on them I guess I stick with my thoughts and the advise of my engineer and install din rail mounted feed through motor breakers in front of the individual starters
    That is how I build things and that is how I will continue to do so.
    Thanks and I would welcome any added comments available to this question.
    That’s the right way to approach it.

    Another issue is the available fault current and the SCCR of the components used. In the vast majority of cases, thermal OL relays are only rated for 5kA all by themselves, but will be series rated with fuses or a circuit breaker at much higher levels, up to 100kA. 5kA is virtually impossible to live with in any kind of industrial facility. But if using breakers, you might want to cross check WHICH breakers those starters will be listed with, it will not be anything made by another mfr. you will have to use Sq.D and it may only be a specific breaker.
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