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  • Electrical current for sewage pump

    I got a call from a commercial place where a small canister sewage pump for basin hand sink in first aid room is overflowing. The pump is 120V, 1/2HP, 13A running current and 60A for start current. this pump is connected 120V 15A circuit. I found total 8 receptacles the circuit.

    I found circuit breaker was trip which is 120V 15A. I assume that some thing was connected while the pump kicks in.

    Tested for a while with running water to pump and working ok.

    I told manager that the pump is working ok but it may trip again if other appliance is pluged into the circuit.

    My question is what is the proper way to do wiring.

    Does the pump should have it's own circuit or just change to 12 AWG with 20A breaker?

    These are just wall receptacles so these are not connected heavy appliances such as refrigerator or micro wave.

    What do you think about this?

    Thank you.
    Last edited by khlee815; 02-03-2010, 07:23 PM.

  • #2
    Re: Electrical current for sewage pump

    I would put the pump on it's own 20A dedicated circuit. The way it is now, with 8 other receptacles on the same circuit, you have no control over what gets plugged in.

    Comment


    • #3
      Re: Electrical current for sewage pump

      How many receptacles do you install with a 15A circuit in commercial and residencial?

      Comment


      • #4
        Re: Electrical current for sewage pump

        Article 220.3(B) of the NEC covers this. Max is 10 on a 15A circuit and 13 on a 20A circuit. Residential there is no limit. I generally put about 6 on each though.

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        • #5
          Re: Electrical current for sewage pump

          When a motor starts, it pulls approximately 8 times the full load current. In order to start a motor, you must size the breaker or fuses to handle this "inrush" current. So here is an example of how to size the breaker and wire. This is for a single motor only.

          13 amps x 2.50 (art. 430.152) = 32.5 amps. This is the current you should use to size the breaker. Since there is no such thing as a 32.5 amp breaker, you could use a 40 amp breaker on this circuit. This would be the maximum.

          Now your wire. You have a 13 amp motor. So look at Article 310.16. 13 amps requires a #14 wire. But we have a motor here so we must use the following formula.

          13 amps x 1.25 (art.430.22) = 16.25 amps. So we must use use a #12 wire.

          Motors are treated differently than regular branch circuits. This is just one instance where a breaker can be much larger than allowed in other circuits for this size wire.
          As said above the motor should be on its own circuit.

          Edit: To end any confusion I have edited this post. Motors and appliances with cord and plug assemblies installed at the factory are to be connected and protected by the manufacturers printed instructions. The example above is for a single motor. Not an appliance. Most all appliances have motors or compressors. The manufacturer has engineered this appliance to their specification and they are not governed the NEC. Follow all manufacturers instructions.
          Last edited by John Valdes; 02-13-2010, 12:42 PM.
          Licensed Electrician

          Comment


          • #6
            Re: Electrical current for sewage pump

            Originally posted by John Valdes View Post
            When a motor starts, it pulls approximately 8 times the full load current. In order to start a motor, you must size the breaker or fuses to handle this "inrush" current. So here is an example of how to size the breaker and wire. This is for a single motor only.

            13 amps x 2.50 (art. 430.152) = 32.5 amps. This is the current you should use to size the breaker. Since there is no such thing as a 32.5 amp breaker, you could use a 40 amp breaker on this circuit. This would be the maximum.

            Now your wire. You have a 13 amp motor. So look at Article 310.16. 13 amps requires a #14 wire. But we have a motor here so we must use the following formula.

            13 amps x 1.25 (art.430.22) = 16.25 amps. So we must use use a #12 wire.

            So, if you are having tripping issues with this motor you can go all the way up to a 40 amp breaker on #12 wire to correct this issue.
            Motors are treated differently than regular branch circuits. This is just one instance where a breaker can be much larger than allowed in other circuits for this size wire.

            As said above the motor should be on its own circuit. I would try a 30 amp breaker first. If it still trips and you are positive the motor is good install a 40 amp breaker.
            Thank you for your detailed answer.

            I am still not sure one thing.

            As your calculation, this circuit needs 30A or 40A, but the pump comes with regular 120V, 15A, 3 prong plugs (attached yellow picture)
            Why not comes with 30A or 40A plug?

            What do you think about that?
            Attached Files

            Comment


            • #7
              Re: Electrical current for sewage pump

              Originally posted by John Valdes View Post
              When a motor starts, it pulls approximately 8 times the full load current.
              Ok, so if my shopvac is drawing 8A, it's inrush is 64A so the breaker should be 100 A? That makes no sense. Please explain.

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              • #8
                Re: Electrical current for sewage pump

                Always follow manufacturers instructions. If the motor or appliance is fitted with a cord and plug, the manufacturer has determined this is sufficient for starting and running the appliance at the given current. They have engineered these products to their specifications. Manufacturers are under different rules than we are. They are not governed by the NEC. They are governed by UL.

                The NEC standard in my example is for one free standing motor. Not for manufactured equipment or appliances.

                A shop vacuum pulling 8 amps requires what the manufacturer says it requires. You do not calculate an appliance as you would a motor. Its an appliance as far as the NEC goes, not a motor.

                OP. Your issue sounds like an overload due to other loads being plugged into that circuit. Also, the male plugs you picture are not a cord and plug assembly. Do you have a cord that is molded to the male plug? If not, someone put the plug on it. Separate the motor circuit from the other receptacles and use a 15 or 20 amp breaker. Try the 15 amp first.
                Keep in mind, my example is maximum, not minimum.

                I hope its more clear now. If not let me know.
                Licensed Electrician

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