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    3 Phase Motor Breaker Sizing

    The branch circuit breaker trips intermittently for a 3 phase motor circuit. It is a 5 Hp, 208 V motor FLA 12.4 SF 1.15. I believe it is a severe duty rated motor. The branch breaker is 30 A, and the contractor has megged the wiring, changed the motor and the breaker, and the problem persists. I don't have access to NEC tables, but understand they must be used for conductor sizing and SC/GF protection. I suspect the breaker is sized too tight for in-rush, and a 35 A breaker is allowed. I'm trying to learn Art 430, so could someone verify if I'm heading in the right direction.

    #2
    Originally posted by USMC1302 View Post
    The branch circuit breaker trips intermittently for a 3 phase motor circuit. It is a 5 Hp, 208 V motor FLA 12.4 SF 1.15. I believe it is a severe duty rated motor. The branch breaker is 30 A, and the contractor has megged the wiring, changed the motor and the breaker, and the problem persists. I don't have access to NEC tables, but understand they must be used for conductor sizing and SC/GF protection. I suspect the breaker is sized too tight for in-rush, and a 35 A breaker is allowed. I'm trying to learn Art 430, so could someone verify if I'm heading in the right direction.
    See table 430.250 and get the NEC full load current which is 16.7. Then look at 430.52 and you will see that an inverse time breaker can be 250% which is 41.75. Next size up is allowed here so you could use a 45 amp breaker. The conductors must be sized for 16.7 X 125% for ampacity of 21.
    Note that the name plate current is not relevant for the branch circuit short circuit/ground fault protection. Name plate current is only used to size the OL protection.
    Last edited by texie; 11-06-19, 07:14 PM.

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      #3
      430.52 not 250.52
      If Billy Idol is on your playlist go reevaluate your life.

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        #4
        Originally posted by ActionDave View Post
        430.52 not 250.52
        Yep. I had 250 on the brain but of course it is 430. Old age.
        I fixed it.
        Last edited by texie; 11-06-19, 07:15 PM.

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          #5
          Thanks fellas, much appreciated. I found some info from manufacturer's data, but will go through tomorrow. Thanks again.

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            #6
            It's hard if you only have a circuit breaker with fixed instantaneous trip settings! What type of circuit breaker are you using, if i may ask?

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              #7
              If you are dealing with a " High Intertia " load, you are in trip class 20 to 30 range for your starter. I would be curious to see the inrush on a recording meter such as Fluke 87 and also see where the current goes when it ramps up. VFDs have helped me in this case.
              Microwave Poison will be seen to be a Trillion times worse than Asbestos.

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                #8
                When is it tripping, during startup or when it has been running for some time?

                During startup - many cases a 30 amp breaker likely holds here even though code does allow higher setting. Mostly has to do with total circuit impedance (including source impedance as well as conductors) limiting the starting current. If you have a really stout source and a pretty short circuit length, you may have some issues with a 30 amp breaker.

                If it is tripping after some run time has passed, 30 amps is well over motor full load rating and it should not trip here, that means something else is happening like possibly a bad connection either on output terminals or on the bus to breaker connection is causing excess heating and eventually causing thermal trip to occur, or other internal breaker failure causing excess heating.
                I live for today, I'm just a day behind.

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                  #9
                  It is a Square-D breaker, although I have not been out to see what type/style. I tried to ask as many questions as possible to try and get better info on what the cause might be, and it appears this occurs during first use. The breaker has been changed once already. I came up with the same conclusion Texie provided. I'm going out tomorrow to gather more info, and will report back.

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                    #10
                    Originally posted by USMC1302 View Post
                    It is a Square-D breaker, although I have not been out to see what type/style. I tried to ask as many questions as possible to try and get better info on what the cause might be, and it appears this occurs during first use. The breaker has been changed once already. I came up with the same conclusion Texie provided. I'm going out tomorrow to gather more info, and will report back.
                    Normally, you have around 1 second trip time window, when your motor inrush is around 6 times the rated amps (120 - 180A). An FA 30 circuit breaker will be better at about 5 seconds trip delay.

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                      #11
                      Update....I did not get to review on-site, but was able to find drawings. Another lesson to find out what was SUPPOSED to be there in the first place. Surprise, 35A breaker was called out. Thanks everyone.

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                        #12
                        Originally posted by USMC1302 View Post
                        Update....I did not get to review on-site, but was able to find drawings. Another lesson to find out what was SUPPOSED to be there in the first place. Surprise, 35A breaker was called out. Thanks everyone.
                        35 amp breaker is what is suggested on Square D motor calculator (slide chart). NEC allows 45 amp breaker before it has to fail to start then you can possibly increase it some.

                        That said many cases a 20 or 25 might hold, all depends on conditions of your install and how much starting inrush current you are actually experiencing. Stout source and short circuit length, will tend to need higher breaker setting than weaker source and/or longer circuit length as those items alone will have current limiting effects on how much starting current you will see.
                        I live for today, I'm just a day behind.

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                          #13
                          If you are dealing with a " High Intertia " load, you are in trip class 20 to 30 range for your starter. I would be curious to see the inrush on a recording meter such as Fluke 87 and also see where the current goes when it ramps up. VFDs have helped me in this case.





                          mybkexperience
                          Last edited by Erin7798; 11-14-19, 11:12 PM.

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                            #14
                            Originally posted by Erin7798 View Post
                            If you are dealing with a " High Intertia " load, you are in trip class 20 to 30 range for your starter. I would be curious to see the inrush on a recording meter such as Fluke 87 and also see where the current goes when it ramps up. VFDs have helped me in this case.
                            You cannot increase trip class without a motor specifically built for it. Trip class is 20 under NEMA unless they name plate says otherwise. IEC the default is 10. The curve is slightly different but close. Low values are for things like hermetically sealed motors. I’ve only seen class 30 on crusher duty motors.

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