NEC Code Question

Yes. From the 2008 NEC:

"300.3(C) Conductors of Different Systems.

300.3(C)(1) 600 Volts, Nominal, or Less. Conductors of ac and dc circuits, rated 600 volts, nominal, or less, shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway."

There are conduit fill requirements, conductor ampacity derating, and grounding conductor considerations that apply. But it can be done

The only issues are conduit fill and derating. Once you get over 9 current carrying conductors in a raceway you will have to upsize THHN. (up until then derating from the 90c column usually covers you) That may come back and raise the size of the raceway. It is really going to end up being a money issue. Steel pipe may be cheaper than larger copper wire. Out beyond 100' you may also have a voltage drop issue so you may be upsizing anyway.

This ends up being a pencil and paper question to figure out the way to go. I would start with voltage drop issue and see where you are after that.

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..

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)?

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.

Which raceway you can use depends on the classification of the areas where it is installed. For instance some areas may be classified as hazardous, wet, and/or corrosive. Rigid Metal conduit is generally acceptable for all areas. Nonmetallic conduit has limitations. Plastic coated rigid conduit with an assortment of plastic coated fittings is used in highly corrosive areas. You should get advice on this from someone that can inspect the locations. Your local fire department may help or try your building department, or better yet, hire an engineer or licensed electrical contractor. Some engineers will do the inspection for an hourly fee. I have found that paying a few hundred dollars for advice from a professional is well worth the investment.

You can probably use sch 40 (the stuff sold at Home Depot) or sch 80 a little tougher but it all depends on whether it is going to be sublect to physical damage and then whether it is "severe" physical damage. These are all judgement calls. There is no precise definition of physical damage. You also have the option of providing supplimental protection in the vulnerable areas.

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.

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

A

| [Crap! Wrong button!]

They should label that button better :)

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.

Better or worse dissipation isn't the issue. Bus ducts are factory assembled and, as such, have a fixed configuration w.r.t. the numbers and size of the conductors within the duct.

Wire in conduit is field installed. As such, there is no way to easily predict the combinations of sizes and number of conductors. The NEC tables and derating factors are an easy to use method to allow electricians and designers to account for these combinations. There are better methods for calculating conductor dissipation for the purpose of sizing. But these require many more parameters and (per the NEC) are only allowed under engineering supervision.

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