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    Help With Subpanel Circuit

    I am trouble shooting a problem for a customer of mine. He has a main breaker panel in his home, the previous owner ran a 240 circuit out to the garage, via underground. Problem is that just recently rather than tripping any of the breakers in the garage sub panel, it is tripping the 30 amp double pole breaker in the house. I had shut off all the 15 & 20 amp breakers in the garage sub panel and after about an hour the 30 amp breaker trips. This is starting to baffle me a bit.

    #2
    My guess is that when the feeder was run underground, it was not done properly. If the wrong type of cable was run directly buried, its insulation system may have deteriorated. That could be the cause of the trips. I don't know if there is a test you can run to check out this possibility. I don't know if a megger test would tell you anything. Do you know how long ago the feeder was run underground? The older it is, the more likely it will have deteriorated to the point of causing the trip.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

    Comment


      #3
      If you disconnect the feeder conductors completely at the remote panel, wire nut them and then turn on the 2P-30 you will be able to confirm that the problem is between the end of the feeder and the 30 amp CB.
      Rob

      Moderator

      All responses based on the 2017 NEC unless otherwise noted

      Comment


        #4
        Originally posted by infinity View Post
        If you disconnect the feeder conductors completely at the remote panel, wire nut them and then turn on the 2P-30 you will be able to confirm that the problem is between the end of the feeder and the 30 amp CB.
        Keep in mind he means to cap them individually, not as a single bundle. It took me a moment to catch this.

        You can also wire a high-wattage incandescent bulb in series with each pole of the breaker to act as a combination current limiter (so you the breaker can stay on) and current indicator. The bulbs should obviously be dark with the garage disconnect off unless the problem is in the garage.

        You might find an issue you can correct above ground by wiggling where you have access, and look in LBs, etc.
        Master Electrician
        Electrical Contractor
        Richmond, VA

        Comment


          #5
          Bad underground. Isolate both ends and meter it. Wire to wire and wire to ground.
          Tom
          TBLO

          Comment


            #6
            191114-2030 EST

            mike4326:

            Based on what you described you have a problem between the main panel breaker, and subpanel, including the subpanel bus bars.

            This may be a large load right from when you start your test, or an abrupt overload at the time of tripping.

            Allow some time for the tripping breaker to cool. Possibly 5 minutes, or at least 1 minute. This is also cooling time for the cable.

            With all breakers in the garage off, and the main panel breaker off. Measure each main panel bus voltage to neutral.

            With main panel breaker on, then using a current clamp-on meter measure the current on each hot line to the garage. If either line immediately has a large current, then trouble shooting is simple. A current around the trip point of a breaker won't immediately trip a breaker.

            If both hots have a large current, and about equal, then you have a short between them. If only one is high, yhen check the neutral for the high current.

            You have not indicated the cable length. At 60 Hz, 5000 pfd, and 120 V current would be about 120/500,000 = 0.2 milliampere. Thus, if you read a number of milliamperes of current you probably have a leakage problem. This alone and unchanged won't trip your breaker. However, after your time delay time this might change to a low resistance short.

            This website is screwed up tonight. I loose what I am composing.

            Let's assume you initially see a large current. Identify the wires where this current flows. May or may not be useful. At 240 V a 30 A breaker will trip with a load of approximately 8 ohms, 4 ohms at 120 V.

            Do some measurements, and tell us what you see.

            .

            Comment


              #7
              I want to thank all of you guys for your great responses. I did have a feeling that it might be the underground feeder insulation deteriorating. I'm not sure how long ago it was installed. I'm going to isolate the feeder and do some tests tomorrow. Once again thanks

              Comment


                #8
                Originally posted by mike4326 View Post
                I am trouble shooting a problem for a customer of mine. He has a main breaker panel in his home, the previous owner ran a 240 circuit out to the garage, via underground. Problem is that just recently rather than tripping any of the breakers in the garage sub panel, it is tripping the 30 amp double pole breaker in the house. I had shut off all the 15 & 20 amp breakers in the garage sub panel and after about an hour the 30 amp breaker trips. This is starting to baffle me a bit.
                Mike are you telling us that you have a 30 amp service for your garage

                Comment


                  #9
                  Originally posted by German shepards View Post

                  Mike are you telling us that you have a 30 amp service for your garage
                  I think he is saying that he has a 30 amp feeder not service
                  They say I shot a man named Gray and took his wife to Italy
                  She inherited a million bucks and when she died it came to me
                  I can't help it if I'm lucky

                  Comment


                    #10
                    Originally posted by Dennis Alwon View Post

                    I think he is saying that he has a 30 amp feeder not service
                    That was my assumption as well.
                    Rob

                    Moderator

                    All responses based on the 2017 NEC unless otherwise noted

                    Comment


                      #11
                      Is garage only thing supplied by the breaker in main house panel? Make sure something else didn't get tapped off it or elsewhere on the circuit.

                      Is any current drawn when everything in garage is turned off?

                      Meg conductors at least twice the test voltage as operating voltage.
                      I live for today, I'm just a day behind.

                      Comment


                        #12
                        Originally posted by ptonsparky View Post
                        Bad underground. Isolate both ends and meter it. Wire to wire and wire to ground.
                        Sure, if you have an insulation leak that's bad enough to trip a 30 Amp breaker it's not going to be that hard to find.
                        The 95% of people that you can't trust give the other 5% a bad name.

                        Comment


                          #13
                          Originally posted by kwired View Post

                          Is any current drawn when everything in garage is turned off?
                          He could/should pull that 30 Amp breaker in the main panel and check for arcing and arcing at surrounding breakers to see if they are really heating up.

                          The 95% of people that you can't trust give the other 5% a bad name.

                          Comment


                            #14
                            Originally posted by growler View Post

                            He could/should pull that 30 Amp breaker in the main panel and check for arcing and arcing at surrounding breakers to see if they are really heating up.
                            won't hurt, but seems less likely it will trip his 30 amp breaker if it isn't carrying any load, and you would think others would be tripping if the heat is coming from other breakers.
                            I live for today, I'm just a day behind.

                            Comment


                              #15
                              Disconnect the cable at both ends and megger it.

                              Comment

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