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NEC Code Question

Discussion in 'Electrical Engineering' started by Max, Mar 28, 2008.

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  1. Max

    Max Guest

    Planning a new addition to my current shop. This addition will require
    running 4 new circuits from my main gutter.

    A 60a 480v, 40a 480v and 2 - 60a 240v.

    I have to route these about 100' before they need to branch. Does code
    prohibit running all of these circuits in one conduit?
     
  2. Guest

    | On Mar 28, 2:49?pm, Gerald Newton <>
    | wrote:
    |>
    |> > Planning a new addition to my current shop. This addition will require
    |> > running 4 new circuits from my main gutter.
    |>
    |> > A 60a 480v, 40a 480v and 2 - 60a 240v.
    |>
    |> > I have to route these about 100' before they need to branch. Does code
    |> > prohibit running all of these circuits in one conduit?
    |>
    |> When the number of current carrying conductors in the same raceway
    |> exceeds 9, the derating factor is 50 percent or less. ?Generally, this
    |> is not acceptable and additonal raceways are used.
    |> Neutral conductors that carry only the unbalanced current do not count
    |> as current carrying, but if the majority of the load is non-linear the
    |> neutral is counted as current carrying.
    |
    | I should make a correction here. Instead of not acceptable I should
    | have said not practical. When conductors are derated by 50 percent or
    | more the conductor size usually has to be increased by at at least one
    | size. This in turn requires a larger diameter raceway. The total
    | cost for this usually exceeds the cost of running more than one
    | raceway and not increasing the size. For 7 to 9 current carrying
    | conductors in the same raceway the derating factor is 70 percent. By
    | using 90 degree C insulations the 90 degree C ampacity is used for
    | derating and ususally this does not require upsizing the conductors.
    | For example, for a 90 degree C No. 12 conductor the ampacity is 30
    | amperes and the derated ampacity is 21 amperes where 7 to 9 conductors
    | are in the same raceway..

    How is the derating done for bus bars that don't have circular cross
    sections (and thus have been heat transfer through larger surface area)?
     
  3. Max

    Max Guest

    Thanks for the help, Gerald, Paul and gfretwell.

    One more question, how about plastic conduit?

    Can I use the grey plastic pipe sold in Lowe's and Home Depot in my shop? Is
    it considered HDPE?

    I looked at an online copy of the NEC and couldn't be sure. It mentions
    several different types of nonmetallic conduit, and makes it sound as though
    plastic should not be used inside my shop. But I do have some larger plastic
    in isolated areas the have been subject to previous city inspections and
    passed. Is there any simple way do define where it can be used in my
    industrial shop? My environment is dusty and sometimes damp, but not
    necessarily corrosive. It's a small foundry and machine shop.
     



  4. They also sell schedule 80, EMT, rigid, and flex types of conduit at The
    Home Depot...


    A
     
  5. Guest

    | [Crap! Wrong button!]

    They should label that button better :)
     
  6. Guest

    | "Paul Hovnanian P.E." wrote:
    |>
    |> wrote:
    |> >
    |> > | On Mar 28, 2:49?pm, Gerald Newton <>
    |> > | wrote:
    |> > |>
    |> > |> > Planning a new addition to my current shop. This addition will require
    |> > |> > running 4 new circuits from my main gutter.
    |> > |>
    |> > |> > A 60a 480v, 40a 480v and 2 - 60a 240v.
    |> > |>
    |> > |> > I have to route these about 100' before they need to branch. Does code
    |> > |> > prohibit running all of these circuits in one conduit?
    |> > |>
    |> > |> When the number of current carrying conductors in the same raceway
    |> > |> exceeds 9, the derating factor is 50 percent or less. ?Generally, this
    |> > |> is not acceptable and additonal raceways are used.
    |> > |> Neutral conductors that carry only the unbalanced current do not count
    |> > |> as current carrying, but if the majority of the load is non-linear the
    |> > |> neutral is counted as current carrying.
    |> > |
    |> > | I should make a correction here. Instead of not acceptable I should
    |> > | have said not practical. When conductors are derated by 50 percent or
    |> > | more the conductor size usually has to be increased by at at least one
    |> > | size. This in turn requires a larger diameter raceway. The total
    |> > | cost for this usually exceeds the cost of running more than one
    |> > | raceway and not increasing the size. For 7 to 9 current carrying
    |> > | conductors in the same raceway the derating factor is 70 percent. By
    |> > | using 90 degree C insulations the 90 degree C ampacity is used for
    |> > | derating and ususally this does not require upsizing the conductors.
    |> > | For example, for a 90 degree C No. 12 conductor the ampacity is 30
    |> > | amperes and the derated ampacity is 21 amperes where 7 to 9 conductors
    |> > | are in the same raceway..
    |> >
    |> > How is the derating done for bus bars that don't have circular cross
    |> > sections (and thus have been heat transfer through larger surface area)?
    |>
    |> The reasoning behind derating (for # of conductors in a raceway) is for
    |> the need to dissipate the heat from the I^2R losses of more conductors.
    |>
    |> Since bus bars aren't field installed into raceways, this situation
    |> doesn't apply. Bus ducts are designed as
    |
    | [Crap! Wrong button!]
    |
    | Bus ducts are factory designed and built, with an ampacity based upon
    | the assembly. The heat dissipation is already taken into consideration
    | in the design.

    Still, it would seem that such a construction would be able to do better
    than ordinary wire in heat dissipation. One just has to go about a very
    different way to design the installation to use them, and select them,
    and have them installed.
     
  7. Max

    Max Guest

    Thanks, I see that now also. Seems to be a lot more permissive than
    prohibitive. I appreciate the help.
     
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