On Sat, 29 Mar 2008 19:20:03 -0800 Paul Hovnanian P.E. wrote: | "Paul Hovnanian P.E." wrote: |> |> snipped-for-privacy@ipal.net wrote: |> >
|> > On Fri, 28 Mar 2008 18:24:14 -0700 (PDT) Gerald Newton wrote: |> > | On Mar 28, 2:49?pm, Gerald Newton |> > | wrote: |> > |> On Mar 28, 9:33?am, "Max" 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.