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Thread: Multiconductor Cable Ampacity

  1. #1
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    Multiconductor Cable Ampacity

    Existing facility has plans to replace a 40 HP pump with a 100 HP pump. I am told the conduit from power source to motor controller is 1-1/4”. To me it looks like RGS. There is one multi-conductor cable in the conduit, and I am told it is #4. If I replace the cable with a larger one, how do I figure ampacity? Am I right in believing that the NEC tables don’t apply to other than single conductors? How do I interpret the top of Table 310.15(B)(16): "Not more than three current-carrying conductors in . . . cable. . ."? Do I look to manufacturer data for ampacity? I am looking at one web site that tells me their #2 cable (3 conductor plus ground) has an ampacity of 130, and I need 124 for a 100 HP motor. Their table also tells me that this cable has a cross-sectional area that is below 53% of the area of a 1-1/4” RGS. So that barely satisfies the project requirements.

    Voltage drop may complicate matters a bit. But is this a realistic approach to this upgrade project? I strongly suspect that replacement of the 1-1/4” conduit will be physically impossible.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  2. #2
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    The not more than 3 current carrying conductor means the ampacities are based on that however if you have 4-6 then you have to de-rate 80%. Table 310.15(B)(3)

    You will have to use note 9 from Table 9 for fill

    A multiconductor cable, optical fiber cable, or flexible
    cord of two or more conductors shall be treated as a
    single conductor for calculating percentage conduit or
    tubing fill area. For cables that have elliptical cross
    sections, the cross-sectional area calculation shall be
    based on using the major diameter of the ellipse as a
    circle diameter. Assemblies of single insulated conductors
    without an overall covering shall not be considered
    a cable when determining conduit or tubing fill area.
    The conduit or tubing fill for the assemblies shall be
    calculated based upon the individual conductors
    For fill the cable is considered one conductor but for ampacity you will need to use the Table mentioned for the number of current carrying conductor's in the cable.
    They say I shot a man named Gray and took his wife to Italy
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  3. #3
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    The cable will serve one motor. So there will be exactly 3 CCCs. For conduit fill purposes, there is only one cable, so I can use up to 53% of the internal area. If I have to use Table 310.15(B)(16) for ampacity, I will need a #1 (not yet addressing voltage drop). Per the only one manufacturer's data I have looked at so far, their 3/c + G #1 will take up more than 53% of the interior area of a 1-1/4" RGS. So I am looking for alternatives. Is there a reason I cannot use the manufacturer's published ampacity values?

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

  4. #4
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    You are asking if the manufacturer's ampacity supercedes the NEC? Idk

    Have you looked at Annex B and see how those ampacities compare. Since you are an engineer you may be okay using those values
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  5. #5
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    Quote Originally Posted by charlie b View Post
    The cable will serve one motor. So there will be exactly 3 CCCs. For conduit fill purposes, there is only one cable, so I can use up to 53% of the internal area. If I have to use Table 310.15(B)(16) for ampacity, I will need a #1 (not yet addressing voltage drop). Per the only one manufacturer's data I have looked at so far, their 3/c + G #1 will take up more than 53% of the interior area of a 1-1/4" RGS. So I am looking for alternatives. Is there a reason I cannot use the manufacturer's published ampacity values?

    Signed,
    Desperately hoping.
    IMO you can't use them because the code requires you to use the tables.

    Is there some reason you cannot use (4) THW/THWN conductors instead of a cable?
    Bob

  6. #6
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    Annex B might be tricky. There is no dirt nearby. I don't think the tables and formulas related to underground ducts would apply. The "facility" is a dam on the Columbia River. The conduit is likely to be surrounded by concrete (thus my guess that replacement would be impossible). The tables would allow up to 4 #1 AWG THHN conductors in the 1-1/4" RGS. I might be able to pencil in a set of 1/0 with a smaller ground wire.

    I haven't completed a VD calculation yet, as I don't know the distances involved. The spillway is about 1000 feet long, and there is a unit substation "somewhat close" to each end. But how far the farthest of these is from the location of the motor controller is something I have asked the locals to find out for me.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  7. #7
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    You are required to use the NEC tables for determining ampacities, partly so that future projects to replace a failed motor can do so without having to validate whatever particular motor is available at that time.

  8. #8
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    I am sorry to disappoint you, but after NEC the 3 * #1 cable with a diameter of 1.09”[0.933 square inch] can not pass through the pipe of 1-1/4”, which for 53% only allows 0.809 square inch. In any case, NEC art.430.22 requires 1.25 times the current as per Table 430-250, that means 155 A -3*2/0 copper [1.26” diameter 1.257 square inch] and you need 2” RMC

  9. #9
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    Quote Originally Posted by Julius Right View Post
    I am sorry to disappoint you, but after NEC the 3 * #1 cable with a diameter of 1.09”[0.933 square inch] can not pass through the pipe of 1-1/4”, which for 53% only allows 0.809 square inch. In any case, NEC art.430.22 requires 1.25 times the current as per Table 430-250, that means 155 A -3*2/0 copper [1.26” diameter 1.257 square inch] and you need 2” RMC
    What about cable with compact conductors? Any possibility?

  10. #10
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    Don't forget that you can use the 90 degree tables if the terminations on both end of the wire are rated for 90 degrees i e Polaris connectors or the like. Also it seems to me that jumping from a 40 horsepower to a 100 horsepower pump maybe a bit suspect or certainly worth reevaluating, especially for a dam.... final thought on the matter is that if the conduit is buried under several feet of concrete, you may get a ampacity rating adjustment upward for a lower ambient temperature.

    Eta: why run a ground wire at all? If the existing wire can be removed and new pulled in, the existing RGS is in good enough shape to serve as the EGC. No need to blow precious fill with a wire EGC.
    Last edited by JFletcher; 05-16-18 at 07:11 AM.
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