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Thread: Ampacity of extra flexible power cable

  1. #1
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    Ampacity of extra flexible power cable

    I'm looking at a cable spec sheet for 1/C 2kV extra flexible power cable that will be used in a temporary installation. The ampacity table gives the ampacity for the different cable sizes based on 40deg C ambient temperature or 90deg C conductor temperature.

    Even though these cables are rated at 90deg C can you use this rating if they will be connected to lugs/equipment rated for 75 deg C? If not than how do you go about getting the 75deg value for the same cable if not published by manufacturer?

  2. #2
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    I just use the closest smaller size in Table 310.15(B)(16).

    The 90°C rating can only be used for ampacity adjustment and or correction. The equipment, in almost all cases will limit you to the 75°C ampacities.

    Note that to be used in an NEC application, the conductor insulation must be one of those listed in 310.104(A). Some flexible cables are dual listed to one of the types in the NEC and other flexible cables are not so listed.
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

  3. #3
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    Whats temporary? One day or 6 weeks?

    For load test, we push 4/0 to 400 amps, for longer term temporary we are a tad more conservative than the tables
    Brian John
    Leesburg, VA

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    You don't mention stranding, but recognize that fine stranded cable (yeah, it may or may not be) requires suitable terminations. Temporary ... maybe you'll ignore that.

  5. #5
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    Quote Originally Posted by don_resqcapt19 View Post
    I just use the closest smaller size in Table 310.15(B)(16). I'm not sure I follow your logic here. The particular cable that I am looking at is a 4/0 cable which on the cable datasheet it shows a 90deg rating of 405A. 310.15(B)(16) shows 4/0 as having 230A in the 75deg column and 260A in the 90 deg column.
    How are you suggesting using the closest smaller size?

    Quote Originally Posted by brian john View Post
    Whats temporary? One day or 6 weeks?

    For load test, we push 4/0 to 400 amps, for longer term temporary we are a tad more conservative than the tables
    This is for a project where utility power is down and it is unknown how long before it is back. So temporary in this case will be at least a few months but could be as long as a year before things are restored to normal.

  6. #6
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    Quote Originally Posted by philly View Post
    Quote Originally Posted by don_resqcapt19 View Post
    I just use the closest smaller size in Table 310.15(B)(16).
    I'm not sure I follow your logic here. The particular cable that I am looking at is a 4/0 cable which on the cable datasheet it shows a 90deg rating of 405A. 310.15(B)(16) shows 4/0 as having 230A in the 75deg column and 260A in the 90 deg column.How are you suggesting using the closest smaller size?
    There are several determinations to establishing a circuit rating. Don is referring to the one for terminal temperature limitations as stipulated in 110.14(C). He probably said the closest smaller size because he thought you had a flexible hard use cable with wire sized in mm², rather than one with AWG sizing.

    As Code goes, the terminal temperature limitation for 4/0@75°C is 230A. To exceed this amperage is to say the terminal temperature would exceed 75°C. So as you can see, 230A caps the ampacity regardless of what its insulative properties will permit for the conditions of use in between terminals.

    Here's a good, but dated paper on the matter: http://www2.schneider-electric.com/r...B9901R2-02.pdf
    Note in particular Figure 9 and the text which describes its implementation.
    I will have achieved my life's goal if I die with a smile on my face.

  7. #7
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    In my opinion, a good approximation it is according to IEEE 242/2001 ch. 9.5.2.2 Cable current and temperature:
    TX = Ta + (TN – Ta) (IX / IN)2
    where Tx=75oC Ta=40 TN=90 IN=rated ampacity at 90oC IX=rated ampacity at 75oC
    Ix/In=sqrt((75-40)/(90-40))=0.866
    For ambient correction factor see NEC [2014] 310.15 Ampacities for Conductors Rated 0–2000 Volts.
    (B) Tables. (2) Ambient Temperature Correction Factors.
    I’=SQRT((Tc-Ta’)/(Tc-Ta))
    If Tc=75 Ta=40 and Ta’=30 then I’/I=SQRT((75-30)/(75-40))=1.134
    So the new ampacity for Tc=75 and 30oC ambient is 0.98 of the previous.

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    Quote Originally Posted by Smart $ View Post
    ... He probably said the closest smaller size because he thought you had a flexible hard use cable with wire sized in mm², rather than one with AWG sizing. ...
    Actually I was thinking of DLO that uses sizes like 262.6 and 313.3 kcmil. For those I would use the table values shown for 250 and 300 kcmil.
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

  9. #9
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    Quote Originally Posted by don_resqcapt19 View Post
    Actually I was thinking of DLO that uses sizes like 262.6 and 313.3 kcmil. For those I would use the table values shown for 250 and 300 kcmil.
    In any case, an NEC recognized cable type (not DLO, perhaps dual rated... RHW?) having an unrecognized size.
    I will have achieved my life's goal if I die with a smile on my face.

  10. #10
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    Quote Originally Posted by Smart $ View Post
    In any case, an NEC recognized cable type (not DLO, perhaps dual rated... RHW?) having an unrecognized size.
    Even the dual rated DLO use those odd sizes once you are larger than 4/0.
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

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