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Protecting Wires that Serve Motors

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    Protecting Wires that Serve Motors

    We are replacing an existing motor, its controller, and the mechanical components driven by the motor. “We” (i.e., my group, but long before I started work here) gave the Contractor a schematic diagram of what is to go inside the controller. The Contractor will build this thing. So right off the bat I don’t see a UL listing being given, but I don’t think that is an issue. Here is my issue: There is a 50 amp breaker in the upstream distribution panel.
    There is a 50 amp “motor circuit protector” as the first component inside the controller.
    We told the Contractor to run #10 wires from the controller to the motor. It is a 5HP, 208V, 3 phase motor – 16.7 FLA). I think that is OK.
    However, upstream of the #10 wires the only OCPD is set at 50 amps.

    Article 430 has never been a friend of mine. Is this OK? Is there something I should look for in our specifications and/or the Contractor’s submittal to verify that some other protective device is included?

    FWIW, the motor will likely be run only for periodic testing, perhaps four times a year. Each time, however, it will run for about 45 minutes before it is stopped.

    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

    #2
    Originally posted by charlie b View Post
    We are replacing an existing motor, its controller, and the mechanical components driven by the motor. “We” (i.e., my group, but long before I started work here) gave the Contractor a schematic diagram of what is to go inside the controller. The Contractor will build this thing. So right off the bat I don’t see a UL listing being given, but I don’t think that is an issue. Here is my issue: There is a 50 amp breaker in the upstream distribution panel.
    There is a 50 amp “motor circuit protector” as the first component inside the controller.
    We told the Contractor to run #10 wires from the controller to the motor. It is a 5HP, 208V, 3 phase motor – 16.7 FLA). I think that is OK.
    However, upstream of the #10 wires the only OCPD is set at 50 amps.

    Article 430 has never been a friend of mine. Is this OK? Is there something I should look for in our specifications and/or the Contractor’s submittal to verify that some other protective device is included?

    FWIW, the motor will likely be run only for periodic testing, perhaps four times a year. Each time, however, it will run for about 45 minutes before it is stopped.
    The overloads in your controller will provide the cable protection at 110-140% of the motor fla, thus protecting the cable from overload.

    Comment


      #3
      If this project is in Washington State, I believe the contractor can call for a state inspector to inspect the panel in lieu of a UL listing. Years ago when I had a panel shop in Seattle, I allowed UL to set up a training program at my shop for state inspectors so they could see what was involved. Boeing lobbied for that change in the state code and it passed, sometime around 1990 I think.

      Your contractor however cannot fabricate a starter in the field using an MCP (mag-only) breaker, it can only be used as part of a factory assembled and listed combination starter. If he is using a factory built combo starter and modifying it with other controls, that’s OK, but if the entire assembly is going to be built in the field, the breaker will have to be a Themal-Mag.

      But as to the conductors, mayanees is correct, the OL relay in the starter (assuming there is one) provides that protection.

      The conductors from the distribution panel to the controller however would need to be rated for 50A because it is that feeder in the panel protecting those.
      Last edited by Jraef; 10-22-19, 11:46 AM.
      __________________________________________________ ____________________________
      Many people are shocked when they discover I am not a good electrician...

      I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

      Comment


        #4
        The way I read 430.22(a) I would size the conductors from the 50A OCPD at 125% of the FLA's from Table 430-150. So 125% of 16.7A is 20.87A. #10AWG. (of course this is assuming the circuit feeds ONLY the motor and not any other loads). Am I missing something here?
        Facilities Electrical Engineer
        Quando Omni Flunkus Moratati

        Comment


          #5
          Originally posted by RD35 View Post
          The way I read 430.22(a) I would size the conductors from the 50A OCPD at 125% of the FLA's from Table 430-150. So 125% of 16.7A is 20.87A. #10AWG. (of course this is assuming the circuit feeds ONLY the motor and not any other loads). Am I missing something here?
          I'm with you on this RD35, 430.22.

          The 50-amp breaker feeding the controller is sized as an inverse time breaker with a maximum of 250% of the fla. (430.52) and the motor and controller feeders are ampacity protected by the overloads.

          The I hope jraef returns to comment because he's one sharp fellow.

          I don't think the new forum format provides the same access and exposure as the previous did so he may not see it. Before we saw any and all active topics with a single click.

          Comment


            #6
            Originally posted by charlie b
            ...There is a 50 amp breaker in the upstream distribution panel.
            There is a 50 amp “motor circuit protector” as the first component inside the controller. ...
            My thinking is that the rules of article 430 pertaining to the motor conductors and OCPD sizes apply for the BRANCH circuit, defined as the circuit from the last OCPD to the load. From the distribution panel to the controller, it is a FEEDER circuit. Conductor size must match the feeder OCPD, in this case a 50A breaker; so those conductors would need to be rated for 50A. It COULD have been a 30A feeder circuit using #10 conductors, or even a 40 or 45A breaker and #10 conductors (see below) but someone chose to put in a 50A breaker. The 50A MCP (Instantaneous Trip only) breaker was fine as the BRANCH OCPD if it were part of a factory assembled combo-starter with an OL relay, because the rule on MCPs is up to 800%, so no problem there. But that does not transfer up to the Feeder ahead of it.

            Assuming that the 50A breaker is feeding only this one motor, the Feeder circuit and OCPD could be sized the same as the Branch as per article 430. So while the #10 wire was always sufficient for the 16.7A load, the 50A breaker is too big for that load (250% of 16.7 = 41.75, as the MAXIMUM size inverse-time breaker). So by using a 50A breaker, they had made it a Feeder, ergo the conductors would have to match it. But what I guess that you guys are doing is "rounding up" on the 41.75A max to the next closest standard size as if that were 50A? There is 45A before you get there, which technically could have been allowable by that rule. But that's not what they put in, it's a 50A breaker, so it's now a 50A Feeder circuit and needs 50A conductors, IMHO.
            __________________________________________________ ____________________________
            Many people are shocked when they discover I am not a good electrician...

            I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

            Comment


              #7
              Originally posted by Jraef View Post

              My thinking is that the rules of article 430 pertaining to the motor conductors and OCPD sizes apply for the BRANCH circuit, defined as the circuit from the last OCPD to the load. From the distribution panel to the controller, it is a FEEDER circuit. Conductor size must match the feeder OCPD, in this case a 50A breaker; so those conductors would need to be rated for 50A. It COULD have been a 30A feeder circuit using #10 conductors, or even a 40 or 45A breaker and #10 conductors (see below) but someone chose to put in a 50A breaker. The 50A MCP (Instantaneous Trip only) breaker was fine as the BRANCH OCPD if it were part of a factory assembled combo-starter with an OL relay, because the rule on MCPs is up to 800%, so no problem there. But that does not transfer up to the Feeder ahead of it.

              Assuming that the 50A breaker is feeding only this one motor, the Feeder circuit and OCPD could be sized the same as the Branch as per article 430. So while the #10 wire was always sufficient for the 16.7A load, the 50A breaker is too big for that load (250% of 16.7 = 41.75, as the MAXIMUM size inverse-time breaker). So by using a 50A breaker, they had made it a Feeder, ergo the conductors would have to match it. But what I guess that you guys are doing is "rounding up" on the 41.75A max to the next closest standard size as if that were 50A? There is 45A before you get there, which technically could have been allowable by that rule. But that's not what they put in, it's a 50A breaker, so it's now a 50A Feeder circuit and needs 50A conductors, IMHO.
              There is 45!

              250% of 16.7 = 41.75 so that 50-amp breaker should technically be a 45.

              I think RD35 will agree.

              Thanks jraef

              Comment


                #8
                430 part V is for short SC/GF protection of feeders supplying motor loads. You can have situations where you have higher than "usual" feeder protection device, especially if only motors are being supplied.
                I live for today, I'm just a day behind.

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