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Thread: Correct conduit size calculation?

  1. #1
    Join Date
    Sep 2006
    Location
    Riverside, California
    Posts
    108

    Correct conduit size calculation?

    I'm trying to figure out if I can use a 1 1/2" EMT instead of a 2" EMT for 4 #1/0 and 1 #6 ground based on these calculations. Instead of using NEC 2008 "Table C.1" I have calculated the maximum capacity of the1 1-1/2" conduit based on "Table 4". This table shows the maximum allowable percent fill on that particular conduit size. The following are my calculations. Please confirm if I am doing this correctly.
    "Table C.1 Maximum Number of Conductors or Fixture Wires in Electrical Metallic Tubing (EMT)" on page #70-705 for THHN conductors it allows to install (4)#1/0 in a 1-1/2" or (7)#1/0 in a 2" conduit. Now refer to "Table 4 Dimensions and Percent Area of Conduit and Tubing (Areas of Conduit or Tubing for the Combinations of Wires Permitted in Table 1, Chapter 9)" on page #70-672. "Table 1" states that you are allowed up to 40% of the cross section of conduit and tubing for more than 2 conductors. Now referring back to "Table 4", for trade size 1-1/2" conduit we are allowed up to 0.814 square inch of fill in EMT. "Table 5 Dimensions of Insulated Conductors and Fixture Wires" on page #70-678 lists the approximate area of #1/0 THHN as 0.1885 square inch and #6 THHN as 0.0507 square inch. Using these approximate areas, we get the total approximate area of the #1/0 as 0.742 square inch for four conductors. The total approximate area of the #1/0 THHN plus the approximate area of the #6 THHN equals 0.7927 square inch. This total is below the 0.814 square inch as listed in "Table 4". Based on this calculation, am I still within code and can use 1 1/2" EMT instead of 2" EMT?
    The reason why I ask if that I have stacked electrical rooms with very limited space and by using 1 1/2" EMT it should be easier to work with bends, etc.

  2. #2
    Join Date
    Jan 2007
    Location
    Yakima, Wash.
    Posts
    102
    Your numbers, calculations and reasoning appear correct

  3. #3
    Join Date
    Apr 2009
    Posts
    1

    A littel guidens

    AN EXISTING 3/4 TRADE SIZE RMC NIPPEL HAS 4 THHN CONDUCTORS AND ONE AWG 10 BARE GROUNDING WIRE HOW MANY ADDITIONAL AWG 10 THHN CONDUCTORS CAN BE INSTALLED? OKAY HERE IS MY QUESTION HOW DO I FIGUARE OUT HOW MANY COULD BE INSTALLED I KNOW HOW TO SIZE THE CONDUIT I UNDERSTAND THE PERCENT IS 60% FILL BECAUSE ITS A NIPPEL HOW DO I FIGURE HOW MANY MORE WIRES COULD BE INSTALLED WHAT THE STEP I WILL BE SO APPRICATE THE LITTEL GUIDENS:confused::confused:

  4. #4
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
    Posts
    25,983
    Quote Originally Posted by GERM80A View Post
    AN EXISTING 3/4 TRADE SIZE RMC NIPPEL HAS 4 THHN CONDUCTORS AND ONE AWG 10 BARE GROUNDING WIRE HOW MANY ADDITIONAL AWG 10 THHN CONDUCTORS CAN BE INSTALLED? OKAY HERE IS MY QUESTION HOW DO I FIGUARE OUT HOW MANY COULD BE INSTALLED I KNOW HOW TO SIZE THE CONDUIT I UNDERSTAND THE PERCENT IS 60% FILL BECAUSE ITS A NIPPEL HOW DO I FIGURE HOW MANY MORE WIRES COULD BE INSTALLED WHAT THE STEP I WILL BE SO APPRICATE THE LITTEL GUIDENS:confused::confused:
    You should start another thread but here goes--- Go to the back of the NEC (Chapter 9-- Table 4) and look for the info on RMC conduit. Look up the sq. inches of the 3/4" size conduit at the 60% column. Now go to Table 5 and look up the area of #10 thhn. Now divide the area of the conduit by the area of the 10 THHN. This will tell you the total number of wires that would be legal at 60%.

  5. #5
    Join Date
    Jan 2008
    Location
    Northeastern PA
    Posts
    284
    What size are the existing 4 THHN conductors?

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