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Thread: De-rating 90deg C rated cable

  1. #1
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    De-rating 90deg C rated cable

    When using 90deg C rated cable for de-rating purposes I understand that you have to correct temperature ampacity for such things as ambient temperature and conduit fill etc.. When looking at only ambient temperature and conduit fill you use these two multipliers to figure out the new reduced ampacity of the 90dec C rated cable. Once this de-rating is done does it need to be compared to the 75deg ampacity of the cable to make sure it is less? Do you copare it to the standard rating of the 75deg cable or a temperature corrected rating of the 75deg rated cable.

    In other words I have always understood that you can use the 90deg ampacity for de-rating, but after de-rating the ampacity must be less than the 75 deg rating.

  2. #2
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    The conductor should be equal to or greater than the 75 degree ampacity after derating or you have to go to a larger wire.
    "Electricity is really just organized lightning." George Carlin


    Derék

  3. #3
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    Quote Originally Posted by mull982 View Post
    When using 90deg C rated cable for de-rating purposes I understand that you have to correct temperature ampacity for such things as ambient temperature and conduit fill etc.. When looking at only ambient temperature and conduit fill you use these two multipliers to figure out the new reduced ampacity of the 90dec C rated cable. Once this de-rating is done does it need to be compared to the 75deg ampacity of the cable to make sure it is less? Do you copare it to the standard rating of the 75deg cable or a temperature corrected rating of the 75deg rated cable.

    In other words I have always understood that you can use the 90deg ampacity for de-rating, but after de-rating the ampacity must be less than the 75 deg rating.
    What Jumper said.


    Quote Originally Posted by jumper View Post
    The conductor should be equal to or greater than the 75 degree ampacity after derating or you have to go to a larger wire.
    True, but make sure of the terminal temperature ratings, they could be only 60 deg. Or could be SE cable (depending on code cycle) and that would be 60 deg. Both of which would have to be equal to or greater than the ampacity of the 60 deg. column after derating.
    Last edited by Little Bill; 08-24-11 at 03:19 PM.
    If you aim at nothing, you will hit it every time!

  4. #4
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    Quote Originally Posted by jumper View Post
    The conductor should be equal to or greater than the 75 degree ampacity after derating or you have to go to a larger wire.
    I always thought the de-rated value had to be less than the 75deg C rating?

    Do you compare the de-rated 90deg rating to the standard 75deg rating or the de-rated 75 degree rating if there is a temperature correctin factor needed for the 75deg C

  5. #5
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    Let me try it this way. Let's presume we are dealing with wire that has a 90C rating, such as THHN, and with terminations that have a 75C rating. You start with the ampacity listed in the 90C column, and apply the correction and adjustment factors related to ambient temperature and the number of current-carrying conductors. That gives you a new "calculated ampacity" value.
    • If the value you calculated is equal to, or lower than, the value in the 75C column, then you use the calculated value.
    • If the value you calculated is higher than the value in the 75C column, then you use the value in the 75C column.
    Does that clarify things for you?
    Charles E. Beck, P.E., Seattle
    Comments based on 2008 NEC unless otherwise noted.

  6. #6
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    Long before Section 110.14(C), the basic content below, extracted from the UL White Book, was around and enforceable through NEC Section 110.3(B) and its predecessors. Section 110.14(C) was originally introduced to “summarize” the termination temperature limitation requirements. A special rule for motors had to be added because 1) NEMA motors aren’t generally listed and 2) NEMA motors have 75°C rated terminations.

    UL Category Code AALZ generally applies to all UL listed product categories or products listed or labeled by other NRTLs to UL standards. Virtually all other UL Category Codes ultimately reference back to AALZ.

    I don’t know if this actually clarifies anything immediately, but it should be read and understood.

    From the latest (2011) UL White Book:



    ELECTRICAL EQUIPMENT FOR USE IN ORDINARY LOCATIONS (AALZ):



    Appliance and Utilization Equipment Terminations





    Except as noted in the general Guide Information for some product categories, most terminals, unless marked otherwise, are for use only with copper wire. If aluminum or copper-clad aluminum wire can be used, marking to indicate this fact is provided. Such marking is required to be independent of any marking on terminal connectors, such as on a wiring diagram or other visible location. The marking may be in an abbreviated form, such as ‘‘AL-CU.’’

    Except as noted in the general Guide Information for some product categories, the termination provisions are based on the use of 60°C insulated conductors in circuits rated 100 A or less, and the use of 75°C insulated conductors in higher rated circuits as specified in Table 310.16 of the NEC. If the termination provisions on equipment are based on the use of other conductors, the equipment is either marked with both the size and temperature rating of the conductors to be used or with only the temperature rating of the conductors to be used. If the equipment is only marked for use with conductors having a higher (75 or 90°C) temperature rating (wire size not specified), the 60°C ampacities (for circuits rated 100 A or less) and 75°C ampacities (for circuits rated over 100 A) should be used to determine wire size. Conductors having a temperature rating higher than specified may be used, though not required, if the size of the conductors is determined on the basis of the 60°C ampacity (circuits rated 100 A or less) or 75°C ampacity (circuits rated over 100 A).

    Distribution and Control Equipment Terminations




    Most terminals are suitable for use only with copper wire. Where aluminum or copper-clad aluminum wire can or shall be used (some crimp terminals may be Listed only for aluminum wire), there is marking to indicate this. Such marking is required to be independent of any marking on terminal connectors, such as on a wiring diagram or other visible location.The marking may be in an abbreviated form, such as ‘‘AL-CU.’’

    Except as noted in the following paragraphs or in the general Guide Information for some product categories, the termination provisions are based on the use of 60°C ampacities for wire size Nos. 14-1 AWG, and 75°C ampacities for wire size Nos. 1/0 AWG and larger, as specified in Table 310.16 of the NEC.

    Some distribution and control equipment is marked to indicate the required temperature rating of each field-installed conductor. If the equipment, normally intended for connection by wire sizes within the range 14-1 AWG, is marked ‘‘75C’’ or ‘‘60/75C,’’ it is intended that 75°C insulated wire may be used at full 75°C ampacity. Where the connection is made to a circuit breaker or switch within the equipment, such a circuit breaker or switch must also be marked for the temperature rating of the conductor.


    A 75°C conductor temperature marking on a circuit breaker or switch normally intended for wire sizes 14-1 AWG does not in itself indicate that 75°C insulated wire can be used unless 1) the circuit breaker or switch is used by itself, such as in a separate enclosure, or 2) the equipment in which the circuit breaker or switch is installed is also so marked.


    A 75 or 90°C temperature marking on a terminal (e.g., AL7, CU7AL, AL7CU or AL9, CU9AL, AL9CU) does not in itself indicate that 75 or 90°C insulated wire can be used unless the equipment in which the terminals are installed is marked for 75 or 90°C.
    "Bob"
    Robert B. Alexander, P.E.
    "I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)

  7. #7
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    Let's try some actual numbers. Let's say you have a 50A circuit and you plan on running #8 Thhn dual rated wire. The wire will be run through 110 deg F ambient temperature in a raceway, there will be (4) CC conductors in the raceway. First you would derate for the ambient temp. which would be a 0.87 correction factor. Under the 90 deg. column the #8 is rated at 55A. So 55A x 0.87=47.85A. The #8 is already lower than the 75 deg. rating of 50A. However, now you must derate for the (4) CCC in the raceway. The previous derated value of 47.85 is now multiplied by .80 for 4-6 conductors. 47.85 x .80=38.28. Now your #8 is rated less than the needed 50A. This would be the case in either the 60 or 75 deg. column. You now would have to go to the next size wire of #6. Which would give you a rating of 52.20A after doing the derating. #6 @ 90deg. is 75, 75 x0.87=65.25, 65.25 x .80=52.20A.
    Hope this helps and didn't "muddy the waters!"
    Last edited by Little Bill; 08-24-11 at 09:43 PM. Reason: being dumb
    If you aim at nothing, you will hit it every time!

  8. #8
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    Here is a thought about 90 C wire ratings that I haven't applied, yet. If a run of wire is landed in or on a 90 C rated item: split bolt, compression coupling, terminal block, lug, etc. and spliced or extended to a larger piece of wire that would meet load ampacity requirement rating at 75 c rating, then one might be able to use that 90 c chart for the main wire run. One might land the larger wire on a 75 c breaker, provided the loadcenter/panelboard is also rated for that temp.

    There are other considerations such as voltage drop and heat transfer beyond the splice point. Labor verses material (cu is up there) verses NEC.

  9. #9
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    Quote Originally Posted by thunder15j View Post
    Here is a thought about 90 C wire ratings that I haven't applied, yet. If a run of wire is landed in or on a 90 C rated item: split bolt, compression coupling, terminal block, lug, etc. and spliced or extended to a larger piece of wire that would meet load ampacity requirement rating at 75 c rating, then one might be able to use that 90 c chart for the main wire run. One might land the larger wire on a 75 c breaker, provided the loadcenter/panelboard is also rated for that temp.

    There are other considerations such as voltage drop and heat transfer beyond the splice point. Labor verses material (cu is up there) verses NEC.
    Are you talking about this?

    "Electricity is really just organized lightning." George Carlin


    Derék

  10. #10
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    10-4 Yes. Nice drawing!

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