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Calculations Conduit Fill
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 Rules That Apply to
Conduit Fill Calculations
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 de-ration 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 use NEC Chapter 9 Table 5 for dimensions of insulated
conductors for both copper and aluminum and NEC Chapter 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 use NEC 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 use NEC 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.
Calculation for Conduit Fill
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.
More on Calculations Conduit Fill
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 wall--you 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 wall--for (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.
Calculating Maximum Number of
Conductors and Fixture Wires
ENT: To find the
maximum number of conductors and fixture wires in electrical
non-metallic tubing (ENT)--blue smurf pipe--you 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 non-metallic tubing (ENT)--blue smurf pipe--for (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 aluminum--you
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 aluminum--for (compact) wires you must refer to NEC Table
"C4A" in the NEC Appendix "C" in the back of the book.
FNMC-B: To find
the maximum number of conductors and fixture wires in electrical
liquidtight flexible non-metallic conduit (Type FNMC-B )--This type
of flexible tubing may be used as a building wiring method (NEC
351-22 (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
non-metallic tubing (Type FNMC-B) for (compact) wires you must refer
to NEC Table "C5A" in the NEC Appendix "C" in the back of the book.
FNMC-A: To find
the maximum number of conductors and fixture wires in electrical
liquidtight flexible non-metallic conduit (Type FNMC-A)--This type
of flexible tubing is limited to lengths of 6' and may not be used
as a building wiring method (NEC 351-22 (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 non-metallic tubing (Type FNMC-A 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 conduit--threaded rigid heavy wall
steel--you 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 conduit--threaded
rigid heavy wall steel--for (compact) wires you must refer to NEC
Table "C8A" in the NEC Appendix "C" in the back of the book.
Rigid Non-Metallic Conduit (Heavy
Wall): To find the maximum number of conductors and
fixture wires in electrical rigid non-metallic 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 non-metallic
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 Non-Metallic Conduit (Light
Wall): To find the maximum number of conductors and
fixture wires in electrical rigid non-metallic 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 non-metallic
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 multi-page 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
External Website (use at your own risk)...
http://conduitfillcalculator.com/
