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pump motor with 300ft run of 8awg voltage loss

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  • pump motor with 300ft run of 8awg voltage loss

    Hello, I recently upgraded my irrigation pump from my pond with a new one.
    I found out that this pump requires a Split-Phase power source. I ran a new 30amp duel breaker for the 240VAC Split Phase to my junction box just inside of my house before it runs to the pond.
    I measured the voltage at that box from the 2 hot leads at 245VAC and then checked the voltage at the pump and found it down to 128Vac from the 2 hot leads. Could the 2 hot split phase wire somehow being converted more two single phase in the 300ft run? The 8awg wire is in the grey direct burial cable. Or would there be that much voltage drop in 300ft?
    I didn't have any trouble with the 1 phase pump and wiring before both 2hp motors.
    thanks
    gary

  • #2
    the Split-Phase power source means that it is a three wire system that has two hot wires with a neutral that measure half of the voltage of the two hots, to the two how wires one will get 240 or so, and from a hot to the nuteral one should get 120 volts or so,
    http://www.boost-energy.com/faqs/wha...nd-split-phase
    are you sure you did not check the voltage between a hot and the neutral, as 128 +128= 256 you said your measurement was 245, if this was a direct no power use measurement, meaning the line on and no motors hooked up to it, you should have the same voltage, at the end of the wire as at the start, if you have a voltage drop my guess you have a cut wire or break some place in the line, if your actually measuring the two hot wires,

    according to this site
    https://www.engineeringtoolbox.com/e...nt-d_1499.html
    a two hp motor should be about 12 amps @ 240 volts,
    according to this voltage caculator

    https://www.calculator.net/voltage-d...s=24&x=52&y=17

    240 volts @12 amps @300 feet, of #8 copper, one 2 hp motor at 230 volts,

    Voltage drop: 4.52
    Voltage drop percentage: 1.88%
    Voltage at the end: 235.48

    240 volts @24 amps @ 300 feet, of # 8 copper, two 2 hp motors @ 230 volts,

    Voltage drop: 9.05
    Voltage drop percentage: 3.77%
    Voltage at the end: 230.95

    one should keep voltage drop to less than 3% if possible but your still above 220 volts at the calculated end of run, @ 24 running amps,

    from what I am understanding you should be OK, but if your are only reading about 150 volts at the end of the wire some thing is wrong,

    Last edited by BHD; 10-01-2018, 09:27 AM.
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    Comment


    • Mightyservant
      Mightyservant commented
      Editing a comment
      Engineering toolbox is a great source of information.

  • #3
    yes i suspected a bad wire, so i checked the continuity between the 2 hot wire and got 50ohms on my meter. so i've got a bad wire somewhere in the 300ft run. The line is down about 18-24in so i'm not sure trying to pull it up will work.

    Comment


    • johncameron
      johncameron commented
      Editing a comment
      I agree, you should have less than 1/2 ohm resistance at 300ft assuming #8 solid copper.

  • #4
    i installed a 50ft run of 10awg romex wire and after a few years i had a fault. after testing i found 240v at the panel and only 70+ volts at the end of the wires. turns out there was a crack in the copper on one of the hot leads that eventually opened. i talked to a Electrician and he said that is a problem with solid copper wire and thats why he uses stranded for his commercial wiring. i pulled a new romex cable thru the conduit, pulling out the old by connecting the new wire to the end of the old and sure enuf, i found the open in the wire. i learned something new. before that i never thought that would be possible in a protected conduit.
    HEY! What does this button do?

    Comment


    • PLUMBER RICK
      PLUMBER RICK commented
      Editing a comment
      I didn't know you're allowed to run romex through conduit the entire distance. Has to do with heat buildup and derating the max amp draw.

      Seems like stranded wire through conduit would have been the better choice for just slightly more money.

      Rick

  • #5

    plumber Rick.
    Good point.
    Just to clarify:
    There's no code that prohibits Romex (NM) cable in conduit per se, but there are a few code sections that make it prohibitive or impractical.

    Nm can't be used in wet or damp locations which includes the inside of conduit (raceway).
    This means it can't be used anywhere outside even if its in conduit.

    It has nothing to do with heat build up or derating, but derating can come into play for wire selection.

    For one thing, cable must usually be secured within 12in of a box or panel which would keep you from running nm through conduit into a box or panel

    Another thing is complying with conduit fill rules where no more than 53% of the id can be used based on the widest part of the cable.
    This means using conduit sized very large in order to comply.

    Where conduit usually comes into play with Romex, is sleeving short sections of it to protect from physical damage.

    Btw, Less than 18in of conduit is considered a nipple and doesn't have to comply with all the rules of conduit.

    Comment


    • #6
      John for discussions sake, why does it matter if you overfill a single run of romex your running in say a 3/4 or 1/2" run of EMT or PVC conduit. I understand romex is not listed for running through conduits and the individual romex conductor wires themselves are not listed for running through a conduit. Other than derating is there more?

      Comment


      • #7
        Originally posted by Mightyservant View Post
        John for discussions sake, why does it matter if you overfill a single run of romex your running in say a 3/4 or 1/2" run of EMT or PVC conduit. I understand romex is not listed for running through conduits and the individual romex conductor wires themselves are not listed for running through a conduit. Other than derating is there more?
        It matters because of the internal temperature of the pipe. There needs to be adequate air space around the wire(s) to dissipate this heat. That is why there are rules about the number of conductors that can be put in conduit. Aside from the insulation burning up, wires can sweat inside the pipe because of temperature differences. It is not unusual to have a bunch of water sitting inside conduit because of this condensation build up.

        Yes, derating factors for ambient temperature, the number of conductors bundled together causing hysteresis heat loss, etc.. are very important factors. All these things and more contribute to heat... and heat is the enemy.

        Again, There is no listing restrictions that I know of putting Romex in conduit. In fact, the conductors inside the outer jacket are thhn insulation rated a very high 90?c, but because of its outer jacket sheath it is only rated 60?c. So yes, it can be put in conduit in some circumstances.

        Another way to put it; Wires need room to breath.

        Edit; The ?= degree symbol
        Last edited by johncameron; 10-02-2018, 09:55 PM.

        Comment


        • #8
          10-4 thank you. I'm curious if when wire calcs are done in addition to the conductor count for the allowed conduit size, is the ambient temperture taken into consideration? Are there losses if the feeders are running through an attic in Pheonix or gains if your inch a trench in Alaska?

          Comment


          • #9
            Originally posted by Mightyservant View Post
            10-4 thank you. I'm curious if when wire calcs are done in addition to the conductor count for the allowed conduit size, is the ambient temperture taken into consideration? Are there losses if the feeders are running through an attic in Pheonix or gains if your inch a trench in Alaska?
            Yes, ambient must be taken into account.
            Location is critical.
            One example:The temp inside conduit 1/2" above the roof in Pheonix with direct sunlight has an ambient temperature of 170 degrees.
            In Anchorage it can reach 130 degrees.

            There are many charts and correction factors that can be applied to determine how many amps a conductor is allowed to safely carry.

            Comment


            • Mightyservant
              Mightyservant commented
              Editing a comment
              Excellent, thank you for taking the time out to teach !

          • #10
            Question, this is a "well pump" right? Anything more than a half horse requires a box to make it 3 phase, to run the pump. If you ran #8 you must be running a 50 horse or larger. If you have the pump box, check the caps and make sure they are connected right, 2 hots and a ground to the box 4 wires to the pump 3 hots 1 ground. Well pumps are not like any other pump when it comes to wiring it.

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