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  • Special Numbers

    Here are some special numbers that anyone seriously into electrical work should make note of? Can you tell me why they need to be remembered?

    1.732 (This was rounded off)

    120.0 & 360.0 degrees (not temperature) Think of a circle.

    80%

    Now for a question where one or more of the above numbers comes into the formula. Assume that you have a three phase heater that draws 10,000 Watts total at 208 Volts. How many Amperes of current will flow through each hot conductor? There are 3 matched elements connected in a delta configuration. Also, what is the Wattage rating of each element?

    If there were single phase power and just one element, I'm sure most of you would figure the answer quickly. I want to know the current based on the above, please.

    Now let's figure this out as well. What would be the minimum circuit breaker rating that's safe to use for this heater? Please assume that all wiring will be done with a good safety factor figured in and I'm not asking about wire gauge or insulation type here. The idea is to prevent tripping of the circuit breaker when the heater is running for hours at a time.

    There are 3 hot conductors and the case of the heater is grounded.

    Feel free to round off your answers some. Either you'll be pretty close or you'll have goofed.
    Last edited by Woussko; 04-15-2007, 02:41 AM.

  • #2
    Re: Special Numbers

    simply put, difference between 2 phases provides √3 or 173% of the single phase voltage


    phase separation of 120°


    80% is recognized as minimum standard PF



    Assuming a PF of .80 gives us 35A


    kW x 1000
    ----------
    1.73 x E x PF

    Comment


    • #3
      Re: Special Numbers

      Newman

      For the most part you have it, but have you ever heard of resistive heating elements with a 0.8 power factor? Let's assume a power factor of unity and a resistive load for this.

      Now back to the questions about this heater. What is the current in each hot conductor and what is the minimum circuit breaker to use for it?

      By the way everyone, it's 80% of some value and not 0.80 PF
      Last edited by Woussko; 04-15-2007, 07:20 PM. Reason: Correction

      Comment


      • #4
        Re: Special Numbers

        What Newman said, and ...
        360 is a full cycle

        PF on a purely resistive load is 1

        80% is the accepted design load of a circuit (15A breaker should only have 12A design load)

        I-line is 27.75 Amps

        breaker should be 35A

        Each element is 3333 W

        I-delta is 16 Amps

        Comment


        • #5
          Re: Special Numbers

          Woussko why don't you take all that special knowledge you have and apply it somewhere people need it and can use it.

          I feel your talents are wasted on us normal folks.

          Comment


          • #6
            Re: Special Numbers

            "By the way as Plumber Rick in another thread suggested and "The Dog" has bugged me about, I really do need a nice vacation from this forum. There is a standing rule of no more than 1000 posts per year. I'm gonna get the booting soon if I don't really show down."

            Don't take this the wrong way but ya keep yapping about this but I don't see you leaving. That "rule" as you call it is you own self-proclaimed bit of BS. There is no limit on the number of posts a person can post to satisfy some weird need to hear themselves talk or feel that they are the center of the discussion, or whatever other reason they may have to babble on about.
            ---------------
            Light is faster than sound. That's why some people seem really bright until you hear them speak.
            ---------------
            “If I had my life to live over again, I'd be a plumber.” - Albert Einstein
            ---------
            "Its a table saw.... Do you know where your fingers are?"
            ---------
            sigpic http://www.helmetstohardhats.com/

            Comment


            • #7
              Re: Special Numbers

              Originally posted by plumberscrack View Post
              Woussko why don't you take all that special knowledge you have and apply it somewhere people need it and can use it.

              I feel your talents are wasted on us normal folks.
              Thanks but I am the one that totally lacks knowledge so I make up for it by howling far too much.

              Bob D is correct about the number of posts. I do think it would be good if there was such a rule however.

              Comment


              • #8
                Re: Special Numbers

                Once again Wayne is correct and deserves the award.
                Last edited by Woussko; 04-15-2007, 07:22 PM.

                Comment


                • #9
                  Re: Special Numbers

                  Originally posted by wbrooks View Post
                  What Newman said, and ...
                  360 is a full cycle


                  80% is the accepted design load of a circuit (15A breaker should only have 12A design load)
                  80% applies only with continuous duty [heat is considered continuous load] - a load where maximum current is expected to continue for three hours or more [NEC Article 100 (I)].
                  Licensed Philadelphia electrician
                  Philadelphia emergency lighting certification

                  Comment

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