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Conduit Fill Calculation 
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To find the maximum number of conductors allowed in a conduit with all of the conductors of the same size and type of insulation. You must refer to the NEC Appendix “C” in the back of the book. You must refer to the table representing the type of conduit that you are using to find the maximum number of conductors allowed within the conduit you are using.
To discover where you can find the chart pertaining to the certain conduit that you are using with all the same type insulation and conductor size in that conduit, then jump down to the last section of this article where it shows and list each conduit, and where the chart for that conduit fill can be found. Remember that the list of charts in the last section of this article pertains only to conduit fill of all the same type insulation and conductor size contained in that certain conduit.
Special Notes:
In real life installation you will find it much better if you install only 1/2 of the maximum number of conductors allowed by the NEC Code in a certain conduit. You can run two conduits parallel to each other much easier and faster than you can if you were trying to force the maximum number of wires into the conduit that the NEC Code allows. You will find 1/2 of the maximum allowed to be faster, easier, and cheaper in the long run. If you considerNEC Table 310.15.B.2.Aconcerning conductor ampacity deration requirements of more than 4 conductors in a raceway or conduit you will also lose ampacity of that conductor to a level of being detrimental to your design purposes if you go much more than four conductors in a raceway. This is true unless you are dealing with a lightly loaded circuit concerning low load amps such as motor control relay circuits, or a signaling relay circuit, that will pull such a minor amp draw that the loss of ampacity will not affect your design purposes.
Conduit fill calculations are not intended to apply to short sections of conduit that are used only to protect exposed wires [such as Romex] from physical damage. Conduit fill calculations pertain only to certain wiring designed sections that is substantially conduit in design. NEC Chapter 9 Notes to tables Note 2
Grounding {bonding} and grounded conductors whether bare or insulated shall be counted in your conduit fill calculations. Also often times when installing metal conduit and metal boxes the conduit is commonly used as the equipment grounding conductor as allowed by the NEC. NEC Chapter 9 Notes to tables Note 3 and NEC Article 250.118
When bare wires are within your calculations of different types insulation and different sizes of conductors you must calculate those bare wires separately from all others in their own special calculation. See circular mill in NEC Chapter 9 Table 8 then add this calculation into your summation because a bare wire takes up less area of fill than an insulated conductor does yet each individual bare or green grounding conductor still must be counted in your calculation. NEC Chapter 9 Notes to tables Note 3
NEC APPENDIX “C” is not intended to be used with nipples less than 24”long. A nipple less than 24”long shall be calculated at 60% fill allowed instead of the usual 40% fill allowed for more than 2 conductors in a conduit whether current carrying or not. NEC Chapter 9 Notes to tables Note 4
When installing multi  conductor cable such as Romex in a conduit you must calculate the actual dimensions of the cable at it’s widest point, and calculate this Cable as a circular assembly same as a single conductor . Then compare this area required to the 40% fill allowed for the conduit being used.NEC Chapter 9 Notes to tables Note 9
When your calculations results land in a decimal of point .8 or larger you must adjust to the next higher number of conductors. NEC Chapter 9 Notes to tables Note 7
This is the only time you must adjust up from .8 or larger instead of .5 or larger such as in resistance calculations. This is the one ringer I was telling you about that you will probably find on a test. The answer you can bet will land on .5 for resistance or .8 for conductor fill.
NEC Chapter 9 Table 4 and NEC Chapter 9
Notes to tables Note 7
Remember it is .5 for resistance or .8 for conduit fill requirements, and
when you land on the number, adjust up.
Special Notes:
Remember 40% fill allowed in a conduit with more than two conductors in a raceway. This is the column you will most commonly use to calculate conduit fill allowed for conductors of different types and sizes in the same conduit. NEC Chapter 9 Table 4
When calculating conductors of different sizes and types of insulation in a conduit fill calculation you must useNEC Chapter 9 Table 4 forfinding the dimensions and percent of area allowed. You must useNECChapter 9 Table 5for dimensions of insulated conductors for both copper and aluminum and,NECChapter 9 Table 5Afor dimensions of insulated conductors for compact aluminum wires now being found on the market.
When calculating number of conductors with all of the same size and insulation useNEC Appendix “C”. found at the bottom of this article
Conduit fill does not apply to those pieces of conduit used only for a form of protection and is not a part of a complete conduit or tubing system. NEC Chapter 9 Notes to tables Note 2
In conduit fill calculations you must count the grounding or bonding conductors. Remember that if that grounding conductor is bare useNECChapter 9 Table 8using the column for area of square inch of bare conductors.
When calculating conductors of different size useNEC Tables 5 to apply the dimensions of the conductor for normal non compact style conductors ,and 5A in Chapter 9to apply the dimensions of the compacted style conductors, and then useNECTable 4 in Chapter 9to apply dimensions of the conduit.
Example #1:
What size Electrical Metallic Tubing is required for the following conductors ?
5 # 14 THW 11  # 12 TW 8  # 10 THHN 7  # 8 THWN
To find the area in square in. of conductors, we must go to NEC Chapter 9 Table 5. Then multiply the areas of square inch by the number of conductors. The areas of square inch of the above conductors are as follows;
# 14 THW = .0209 x 5 CONDUCTORS = .1045 SQUARE INCH
# 12 TW = .0181 x 11 CONDUCTORS = .1991 SQUARE INCH
# 10 THHN = .0211 x 8 CONDUCTORS = .1688 SQUARE INCH
# 8 THWN = .0366 x 7 CONDUCTORS = .2562 SQUARE INCH
Total square inch of conductors used .7286 SQUARE INCH
Now you must compare that to the area of square inch for over 2 wires found in
NEC Chapter 9 Table 4for an Electrical Metallic Tubing at the allowable 40% fill, and you will find that a 1 1/2”Electrical Metallic Tubing will be required, because the next size smaller conduit is only .598 square inch, which is smaller than the required .7286 square inch we calculated as being required for the wires we intend to use.
Example #2:
What size Rigid Nipple is required for the following conductors ?
5 # 14 THW 11  # 12 TW 8  # 10 THHN 7  # 8 THWN
# 14 THW = .0209 x 5 CONDUCTORS = .1045 SQUARE INCH
# 12 TW = .0181 x 11 CONDUCTORS = .1991 SQUARE INCH
# 10 THHN = .0211 x 8 CONDUCTORS = .1688 SQUARE INCH
# 8 THWN = .0366 x 7 CONDUCTORS = .2562 SQUARE INCH
Total square inch of conductors used .7286 SQUARE INCH
Special Notes: You should remember that a nipple may be filled to 60% of the total area 100% square inch column. This is found in the NEC Chapter 9 Table 4 for a rigid conduit = 1 1/4”nipple = 1.526 x 60 % = .9156 square inch which is larger than the total square inch of conductors used .7286. A 1”nipple = .888 x 60%=.5328 square inch which is too small. The answer would be 1 1/4”.
Example #3
What size Rigid Conduit is required for the following conductors ?
5 # 14 BARE 11  # 12 TW 8  # 10 THHN 7  # 8 THWN
Special Notes: Remember for bare wires you must refer toNEC Chapter 9 Table 8 look for area in.2
# 14 BARE = .003 x 5 CONDUCTORS = .0150 SQUARE INCH
# 12 TW = .0181 x 11 CONDUCTORS = .1991 SQUARE INCH
# 10 THHN = .0211 x 8 CONDUCTORS = .1688 SQUARE INCH
# 8 THWN = .0366 x 7 CONDUCTORS = .2562 SQUARE INCH
Total square inch of conductors used .6391 SQUARE INCH
Now you must compare that to the area of square inch for over 2 wires in a rigid metallic conduit at the allowable 40% fill and you will find that a 1 1/2”with .829 square inch for a rigid metallic conduit will be required because the next size smaller 1 1/4”is only .610 square inch which is smaller that the required .6391 square inch we received from our calculation.
Special Notes:
The rounding to the next size larger did not come up in our calculations to this point. Remember the .08 or larger must be rounded up to the next size higher whole number.
Using all the same types insulation and all the same size conductors within that certain conduit you may use the following charts found in appendix "C."
Step One
To find the maximum number of conductors and fixture wires in electrical
metallic tubing (EMT)thin wallyou must refer to the NEC Table "C1," in
the NEC Appendix "C" in the back of the book, and to find the maximum
number of conductors and fixture wires in electrical metallic tubing (EMT)thin
wallfor (compact) wires you must refer to NEC Table "C1A" in the NEC
Appendix "C" in the back of the book.
Step Two
You must then refer to the type of insulation your conductors has THW /
THHN / THWN / XHHW / etc.
Step Three
You must then refer to the conductor size in AWG. or KCMIL or MCM.
Step Four
You must then refer to the column that refers to the number of conductors
you want to install.
Final Step
Once you find the number of conductors corresponding with the insulation
type and conductor size then look up at the top of the chart in that
column corresponding with the number of conductors that you want and it
will tell you the minimum trade size EMT conduit that you must use.
ENT: To find the maximum number of conductors and fixture wires in electrical nonmetallic tubing (ENT)blue smurf pipeyou must refer to NEC Table "C2" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in electrical nonmetallic tubing (ENT)blue smurf pipefor (compact) wires you must refer to NEC Table "C2A" in the NEC Appendix "C" in the back of the book.
FLEX: To find the maximum number of conductors and fixture wires in electrical flexible metallic tubing (flex) you must refer to NEC Table "C3" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in electrical flexible metallic tubing (flex) for (compact) wires you must refer to NEC Table "C3A" in the NEC Appendix "C" in the back of the book.
IMC: To find the maximum number of conductors and fixture wires in electrical intermediate metallic conduit (IMC)usually threaded aluminumyou must refer to NEC Table "C4" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in electrical intermediate metallic conduit (IMC)usually threaded aluminumfor (compact) wires you must refer to NEC Table "C4A" in the NEC Appendix "C" in the back of the book.
FNMCB: To find the maximum number of conductors and fixture wires in electrical liquidtight flexible nonmetallic conduit (Type FNMCB )This type of flexible tubing may be used as a building wiring method (NEC 35122 (2))you must refer to NEC Table "C5" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in electrical liquidtight flexible nonmetallic tubing (Type FNMCB) for (compact) wires you must refer to NEC Table "C5A" in the NEC Appendix "C" in the back of the book.
FNMCA: To find the maximum number of conductors and fixture wires in electrical liquidtight flexible nonmetallic conduit (Type FNMCA)This type of flexible tubing is limited to lengths of 6' and may not be used as a building wiring method (NEC 35122 (1))you must refer to NEC Table "C6" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in electrical liquidtight flexible nonmetallic tubing (Type FNMCA for (compact) wires you must refer to NEC Table "C6A" in the NEC Appendix "C" in the back of the book.
Sealtite: To find the maximum number of conductors and fixture wires in electrical liquidtight flexible metallic conduit (Sealtite) you must refer to NEC Table "C7" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in electrical liquidtight flexible metallic conduit (Sealtite) for (compact) wires you must refer to NEC Table "C7A" in the NEC Appendix "C" in the back of the book.
Rigid Metallic Conduit: To find the maximum number of conductors and fixture wires in electrical rigid metallic conduitthreaded rigid heavy wall steelyou must refer to NEC Table "C8" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in electrical rigid metallic conduitthreaded rigid heavy wall steelfor (compact) wires you must refer to NEC Table "C8A" in the NEC Appendix "C" in the back of the book.
Rigid NonMetallic Conduit (Heavy Wall): To find the maximum number of conductors and fixture wires in electrical rigid nonmetallic conduit schedule 80(heavy wall) (PVC Sch. 80) (can be used where subject to physical damage)you must refer to NEC Table "C9" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in electrical rigid nonmetallic conduit schedule 80(heavy wall) (PVC Sch. 80) (can be used where subject to physical damage)for (compact) wires you must refer to NEC Table "C9A" in the NEC Appendix "C" in the back of the book.
Rigid NonMetallic Conduit (Light Wall): To find the maximum number of conductors and fixture wires in electrical rigid nonmetallic conduit schedule 40(light wall) (PVC Sch. 40) (can not be used where subject to physical damage)you must refer to NEC Table "C10" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in electrical rigid nonmetallic conduit schedule 40(light wall) (PVC Sch. 40) (cannot be used where subject to physical damage)for (compact) wires you must refer to NEC Table "C 10 A", in the NEC Appendix "C" in the back of the book.
Rigid PVC Conduit Type A: To find the maximum number of conductors and fixture wires in rigid PVC conduit Type A you must refer to NEC Table "C11" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in rigid PVC conduit Type A for (compact) wires you must refer to NEC Table "C11A" in the NEC Appendix "C" in the back of the book.
Rigid PVC Conduit Type EB: To find the maximum number of conductors and fixture wires in rigid PVC conduit Type EB you must refer to NEC Table "C12" in the NEC Appendix "C" in the back of the book, and to find the maximum number of conductors and fixture wires in rigid PVC conduit type EB for (compact) wires you must refer to NEC Table "C12A" in the NEC Appendix "C" in the back of the book.
Final Special Note
There is more than just one page per chart. For an example NEC Table "C1"
contains five pages of tables just for electrical metallic tubing. These
multipage table charts are required just to list the different types of
conductor insulation that are normally used in the electrical industry.
Each Table contains several pages of tables that are still a part of that
one certain table section.
Originally Written By: Warren Goodrich
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