2008 nec question.

Discussion in 'Electrical Forum discussion & Blog' started by tig welder, May 9, 2009.

  1. tig welder

    tig welder New Member

    Messages:
    4
    in january i was promoted to a apprentice election and i am in my 2nd class residential wiring 2 at the school.my work started me late ( with the school class schedule) so i missed the code book class and i will take it in the fall.

    i have been tring to understand the code book by looking up codes and thinking of questions and finding the answers and i found a question that i can not find the answer that was given to me in school.

    #1. i am told by everyone that you can not put romex in coduit for a continous run,because of heat and #2. you can not put 1 raceway inside of another . ( i can not find these 2 things spelled out in the nec book)

    i found you can run it inside conduit for protection of nonmetallic cable in a basement. nec 300.15(c) nec 334.15(b)
    i know i have missed something

    i want to understand how i should be reading or understanding what i have read to make it make sence . if that makes sence .

    any help would be greatly appreciated
    thanks in advance.
    Last edited: May 9, 2009
  2. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    My 2¢

    50' of #14 AWG copper Romex carrying 15A will dissipate 59w, about 1.1w/ft. With a straight wire in free air this is not a problem.

    Temp. rise due to heat dissipation from Romex inside a conduit can be pretty messy to calculate.

    Beyond reverse engineering this
    http://www.jhlarson.com/ind_tables/conductor/table310-16.htm
    I'm pretty much stuck, and this is with two degrees in EE.

    The NEC is probably the results of lab measurements, elec. theory, predictable heat loss by conduction and convection, calculations, prediction and observation of human behavior, electrocutions, explosions and fires. So far I haven't done the last three but I did melt a wall outlet timer (due to high contact impedance due to the timer being half out of the wall socket).
    Last edited: May 9, 2009
  3. tig welder

    tig welder New Member

    Messages:
    4
    hi thatguy.
    i have been looking at the n.e.c. book hard since tuesday.
    a teacher told me to look at 310.8 (a)(b)(c) and table 310.13(a).
    the table is like the one you posted. but i am not seeing how these get me to my answer.
    you and a teacher showed about the same table that tells me something is there.
    i keep reading 310.10 temperature limitation of conductors which says to see table 310.13 (a) & (c). but i can not put everything together yet.
    thank you for your help.
  4. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    There is nothing that expressly forbids, or allows, NM cable in a conduit system, as long as conductor fill is met.

    The old "You can't put a raceway/cable in a raceway" myth is just that, a myth.

    I will say one thing for sure, it is NOT done by any professional that I know of. It is simply bad practice.
  5. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    I have done commercial renovation where we pulled MC cable thru existing EMT which was installed in a 12th story floor. It was easier than setting extra boxes and adapting to MC for the rest of the circuit. I didn't see anything wrong with it.
  6. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    A situation like that is understandable.

    I am talking about running NM cable in conduit when other options (IE: doing it the right way) are available.
  7. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    I have to defer to the electricians on this site, but if you can post links to the these two tables I'll take a whack at them. I only have the '99 NEC paper copy.

    The table I posted relates ampacity to conductor size to insulation type. Since heating is proportional to I²R, the temp. rise above ambient is proportional to I²R. It helps if you add a column to the table showing resistance of each AWG size.
    #14 copper is ~2.6 milliohms/ft, and #12 is 1.6, and resistance halves for each 3 increasing gauge numbers.

    Heat loss is by conduction, convection and radiation. At these temps. heat loss by radiation is probably negligible, which leaves conduction and convection.
    If heat in is more than heat out, the temp. will rise until heat in does equal heat out. Hopefully this final temp. is within the temp. rating of the wire insulation.
    Last edited: May 10, 2009
  8. Billy_Bob

    Billy_Bob In the Trades

    Messages:
    422
    Might try searching googe.com for the words...

    2008 nec 310.8 table

    or

    nec 310.8 table
  9. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Last edited: May 10, 2009
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