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Thread: Multiconductor Cable Ampacity

  1. #21
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    Quote Originally Posted by charlie b View Post


    How's come we are so far apart on this calculation?
    The FLC is 124 Amps from the charts. The required ampacity of the conductors is 125% of the FLC.

    Are you trying to use the 90 deg C column?
    Bob

  2. #22
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    Quote Originally Posted by charlie b View Post
    Here is another approach: Table 430.22(E). The application is a sump pump, and the on/off cycles are driven by water level. I watched the existing system operate a couple weeks ago. It seemed to me that it took less than 2 minutes for the pump to bring the water level from the "on" setpoint level to the "off" setpoint level. Under worst case conditions, I am made to understand that the pump would cycle on every 11 minutes or so. We are looking at ways to extend that, so that the pump doesn't experience that frequent a start/stop cycle.

    With that said, I think I can use the "intermittent duty" 15 minute rated value of 85%. So my minimum ampacity would be 85% of 124, or 106 amps. A #2 would do that job. I would still have to deal with VD issues, but this could resolve the "conduit fill" problem.

    Any comments?
    using a larger or higher horsepower sump pump is only going to make the line I bolded worse not better. unless they are flooding the sump with the existing pump not keeping up, I am completely failing to understand why they want to increase the size so much
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  3. #23
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    Quote Originally Posted by petersonra View Post
    The FLC is 124 Amps from the charts. The required ampacity of the conductors is 125% of the FLC.
    But you don't use the 125% if you are applying table 430.22(E). It says that in the opening paragraph of 430.22.

    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  4. #24
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    Quote Originally Posted by charlie b View Post
    But you don't use the 125% if you are applying table 430.22(E). It says that in the opening paragraph of 430.22.

    note to table 430.22(E)

    Note: Any motor application shall be considered as continuous duty
    unless the nature of the apparatus it drives is such that the motor will
    not operate continuously with load under any condition of use
    .
    Is there any condition of use where the motor might run for 3 hours? Short of some control interlock preventing it, I don't see how you can claim it is not continuous. Will is a pretty strong word.

    would 310.15(C) help any?

    ETA: In the end I think you are going to be constrained by VD if you have to deal with that issue. maybe run a smaller set of conductors inside the conduit and splice on larger ones once you escape the conduit.
    Last edited by petersonra; 05-16-18 at 12:29 PM.
    Bob

  5. #25
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    Quote Originally Posted by charlie b View Post
    Aside: Is "RMC" the current and more appropriate name for what I have been calling "RGS"?

    The same--- rigid metal conduit and rigid galvinized steel. Code uses RMC so that is what I use
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  6. #26
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    Quote Originally Posted by petersonra View Post
    Is there any condition of use where the motor might run for 3 hours? Short of some control interlock preventing it, I don't see how you can claim it is not continuous. Will is a pretty strong word.
    I had noticed that as well. I will need to discuss this with the mechanical engineer. I think we can claim that the motor will never run more than a couple minutes at a time. That is based on the maximum rate at which the sump has ever taken in water, the rate at which the new 100 HP pump will remove water, and the fact that there are three other pumps that would automatically start if one pump alone cannot keep up with the inflow. But that is a calculation I would still have to formalize.

    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  7. #27
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    One more tidbit to consider: The power supply is at 480V and the motor is rated at 460V. So we have 20 volts, or about 4%, to throw away, before we even start talking about the impact of a 3 or 4% VD.

    Has anyone ever used this to your advantage?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  8. #28
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    Quote Originally Posted by charlie b View Post
    Here is another approach: Table 430.22(E). The application is a sump pump, and the on/off cycles are driven by water level. I watched the existing system operate a couple weeks ago. It seemed to me that it took less than 2 minutes for the pump to bring the water level from the "on" setpoint level to the "off" setpoint level. Under worst case conditions, I am made to understand that the pump would cycle on every 11 minutes or so. We are looking at ways to extend that, so that the pump doesn't experience that frequent a start/stop cycle.

    With that said, I think I can use the "intermittent duty" 15 minute rated value of 85%. So my minimum ampacity would be 85% of 124, or 106 amps. A #2 would do that job. I would still have to deal with VD issues, but this could resolve the "conduit fill" problem.

    Any comments?
    Look carefully at what that table says.

    If motor isn't identified as an intermittent duty motor, you actually need to use 140% or even 200% of the motor full load current for conductor ampacity if you are not calling it continuous duty, where a 5 or 15 minute duty rated motor can have 85%.

    This actually is something that maybe is overlooked a lot - I'm sure I have done it on motors that frequently start-stop, even reverse. Most time you get away with little or no trouble - but effects of that frequent starting is more noticeable on contactors, soft starters or even to VFD's.
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  9. #29
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    Quote Originally Posted by charlie b View Post
    One more tidbit to consider: The power supply is at 480V and the motor is rated at 460V. So we have 20 volts, or about 4%, to throw away, before we even start talking about the impact of a 3 or 4% VD.

    Has anyone ever used this to your advantage?
    Even a 10% VD is not a big issue with a pump application like this. The motor will work a little harder and maybe draw a little more current but it is not like it is going to matter all that much.
    Bob

  10. #30
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    In addition to other problems mentioned, the idea of having to re-pull a larger conductor in an existing conduit for that distance is a formidable task.
    Have you looked at the possibility of using the existing #4 and boosting your power on the supply end with a 480-600v transformer and using a 600v drive at the load end of reducing the power back to 480 ? (Yiou would have roughly 150 amps of 480 available based on the #4 at 125 amps.)
    With this method you might also use your transformer taps to address your voltage drop concern.
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