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3 phase power requirements

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  • 3 phase power requirements

    I am adding some 3 phase power equipment to a shop (240 volt). I am adding two to three motors. Two are 5 hp and one is 1 hp. I will be using a variable drive on one of the motors.

    My questions are as follows:

    Would it be better to set up a sub-panel from the main panel with say a 60 amp feed? I assume that I can only run one three phase circuit through a single conduit and hence would choose the subpanel approach. If I used number 8 wire within 50 feet would this suffice?

    If I could run several circuits I would be using #2 to my 17 amp 5 hp motors. What would be the best recommended way. By the way, I plan to use disconnect switches at each motor.


    Jim Marshall

  • #2
    Re: 3 phase power requirements


    Take a look around this web site. I think you'll find it helpful. Also, you would do well to call their technical support department.


    • #3
      Re: 3 phase power requirements

      Not sure if this will help or not,

      5 HP Motor @ 230(240) 3ph = 15.2a 14ga min wire 30a breaker trip (I would use 12 ga. for what its worth)
      1 HP Motor @ 230(240) 3ph = 4.2a 14ga min wire 15a breaker trip

      Amperages are based on 1750(1800) rpm motors @ 60hz. Always check the motor ID plate for additional information/current draw.

      Also, if you are not an electrician or have never worked with 3ph, you may want to consult a licensed electrician. You can get the rotation reversed on 3ph motors very easily.

      Always consult local building codes prior to work if it will be inspected. Some districts have their own rules above/beyond the NEC.

      Last edited by biscuit; 01-26-2007, 08:52 AM.


      • #4
        Re: 3 phase power requirements

        I just wondering of the 3 phase requirement you have in your shop. this will get little twister in here for a sec so bear with me a min .

        do your shop do have true 240 volt all 3 hot legs ??

        if so just watch out with netural leg on this set up what we called Delta service

        if you are reading 208 volt between all hot legs then you got Wye service [this is more common for small commercal area to use currentlly ]

        I am not sure if you were well aware about disconnect switch and mangatic switch for overload protection on 3 phase motor ?

        If not let us know i can able post some idea on this forum just a quick referince what to use in proper way related to the codes.

        if other question you need please do post it here

        Merci , Marc


        • #5
          Re: 3 phase power requirements

          I wonder if Jimmar has 3 phase power available or if he will need to use a converter. Most non commercial areas of the US only have single phase. I think it would be well to start as his main service entrance and find out the Ampere, Voltage(s) and if he has single or three phase electric service.


          • #6
            Re: 3 phase power requirements

            It is a 3 phase delta main. I have found that when I do my homework with electricians that I get a better job and hence now I am doing my homework. The more that I know, it seems, the better job I get.

            On my last building I laid out a map and produced a checklist of my past experiences and current and future potential requirements for electrical power. I asked the electrician for his inputs. We put extra conduits in the wall and ran empty conduits from the box. After the electrician did the job the inspector said it was one of the nicest conduit jobs he had seen.

            Many electricians and other tradesman only do the minimum required or easiest way out. Not so much electricians, but I have found that if I leave a job very long and come back I find that the job has been done in the easiest and fastest way without consulting me. I remember a case where the electrician needed more clearance for his boxes.

            The carpenter bitched and moaned and walked off so I said, "Fine, I will do it myself." I grabbed my skilsaw and my framing nailer and moved a stud and headered it off. I didn't say a thing to the carpenter. He was watching at the time and was a better person to deal with after that.

            I remember two situations that really ticked me off. In one the electrician wired the 120 volt access on a rooftop HVAC unit the wrong voltage. I don't know whether he grabbed the wrong leg of the three phase or what, but his work caused the burnout of the HVAC fellow's drill. I called and the company sent out a workman in a flash and replaced the HVAC guy's drill. In another case the electrian installed a temporary box with similar wrong power. It burned out the carpenter's skilsaw.


            • #7
              Re: 3 phase power requirements

              Sorry, I missed the question. I planned to use fused disconnects at the compressor and fan. One of the units is going to be using a speed control (drive). This unit calls for a 25 amp fuse before the drive. As I understand it the drive also functions as a magnetic starter and hence would eliminate the need for a starter. Since the drive control box is about 25 feet from the fan, I planned to put a non-fused disconnect at the fan. The other unit is a compressor which I planned to use a fused disconnect near the unit. I read in one manual that a heavier breaker was recommended at the box to (more than double the requirements) to assure adequate starting power.

              By the way, thanks to all that have replied.