Find Local Contractor Call: 844-251-6308
No Obligation, Free Quotes
Home > Home Wiring USA Archive: NEC 1999 > Definitions and Calculations > Calculations Conduit Fill (NEC 1999)

Calculations Conduit Fill (NEC 1999)

By Warren Goodrich
Calculations Conduit Fill (NEC 1999)

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 you are using to find maximum number of conductors allowed within the conduit you are using.

To find the maximum number of conductors and fixture wires in electrical metallic tubing {EMT } ( thin wall ) you must refer to the NEC Table “C 1”, 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 “C 1 A”, in the NEC Appendix “C” in the back of the book.

You must then refer to the type of insulation your conductors has THW / THHN / THWN / XHHW / ETC.

You must then refer to the conductor size in AWG. / KCMIL.

You must then refer to the column that refers to the number of conductors you want to install.

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.

Special Notes: There are more than just one page per chart. For an example NEC Table “C 1” 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.

Special Notes: In real life installation you will find it much better if you run only 1/2 of the numbers allowed by the Code. You can run two conduits parallel to each other much easier than you can force the maximum number of wires into the conduit that the Code allows. You will find 1/2 of the maximum allowed to be faster, easier, and cheaper in the long run. If you consider NEC Table 310 / 16 Note 8 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, unless you are dealing with a circuit 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.

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 “C 2”, 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 “C 2 A”, in the NEC Appendix “C” in the back of the book.

To find the maximum number of conductors and fixture wires in electrical flexible metallic tubing { flex } you must refer to NEC Table “C 3”, 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 “C 3 A”, in the NEC Appendix “C” in the back of the book.

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 “C 4”, 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 “C 4 A”, in the NEC Appendix “C” in the back of the book.

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 “C 5”, 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 “C 5 A”, in the NEC Appendix “C” in the back of the book.

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 “C 6”, 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 “C 6 A”, in the NEC Appendix “C” in the back of the book.

To find the maximum number of conductors and fixture wires in electrical liquidtight flexible metallic conduit { Sealtite } you must refer to NEC Table “C 7”, 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 “C 7 A”, in the NEC Appendix “C” in the back of the book.

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 “C 8”, 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 “C 8 A”, in the NEC Appendix “C” in the back of the book.

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 “C 9”, 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 “C 9 A”, in the NEC Appendix “C” in the back of the book.

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 “C 10”, 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 } (can not 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.

To find the maximum number of conductors and fixture wires in rigid PVC conduit Type A you must refer to NEC Table “C 11”, 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 “C 11 A”, in the NEC Appendix “C” in the back of the book.

To find the maximum number of conductors and fixture wires in rigid PVC conduit Type EB you must refer to NEC Table “C 12”, 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 “C 12 A”, in the NEC Appendix “C” in the back of the book.

Special Notes: 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.

Special Notes: Grounding {bonding} and grounded conductors whether bare or insulated shall be counted in your conduit fill calculations.

Special Notes: 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.

Special Notes: NEC APPENDIX “C” is not intended to be used with nipples less than 24” long. A nipple less than 24” long shall be calculated as 60% fill instead of the usual 40% fill.

Special Notes: 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 figured as a circular assembly. Then compare this area required to the 40% fill allowed for the conduit being used.

Special Notes: When your calculations results land in a decimal of point .8 or larger you must adjust to the next higher number of conductors.

Special Notes: 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.

Remember these .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.

Special Notes: When calculating conductors of different sizes and types of insulation in a conduit fill calculation you must use NEC Chapter 9 Table 4 for finding 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 5A for dimensions of insulated conductors for compact aluminum wires now being found on the market.

Special Notes: When calculating number of conductors with all of the same size and insulation use NEC Appendix “C”.

Special Notes: 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.

Special Notes: In conduit fill calculations you must count the grounding or bonding conductors. Remember that if that grounding conductor is bare use NEC Chapter 9 Table 8 using the column for area of square inch of bare conductors.

Special Notes: When calculating a conduit nipple with a maximum length of 24” use the NEC Table 4 using the column for 100 % cross sectional area of a conduit and multiply the figure in that column by 60% for the allowable fill of the nipple.

Special Notes: When using one multi - conductor cable, the actual dimension at it’s widest point must be calculated as it’s diameter, and you must consider that multi - conductor cable as one conductor in your conduit fill calculation regardless of the fact it may contain more than one conductor. You must use the column for one conductor being used in NEC Table 4 in Chapter 9.

Special Notes: When calculating conductors of different size use NEC Tables 5 and 5A in Chapter 9 to apply the dimensions of the conductor, and then use NEC Table 4 in Chapter 9 to apply dimensions of the conduit.

Example 1;

“ CALCULATION FOR CONDUIT FILL “

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 4 for 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;

“ CALCULATION FOR CONDUIT FILL “

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;

“ CALCULATION FOR CONDUIT FILL “

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 to NEC 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.

This document is based on the 1999 national electrical code and is designed to give you an option, as a self-help, that should pass minimum code requirements. While extreme care has been implemented in the preparation of this self-help document, the author and/or providers of this document assumes no responsibility for errors or omissions, nor is any liability assumed from the use of the information, contained in this document, by the author and / or provider.

Find a Pre-Screened Electrician
Enter Your Zip Code:


THIS ARTICLE IS PROVIDED 'AS IS' WITH NO WARRANTY OF ANY KIND. THE AUTHOR, THE SITE OWNER AND ITS AFFILIATES ASSUME NO LIABILITY FOR ERRORS OR OMISSIONS CONTAINED THEREIN OR FOR ANY USE OF THE INFORMATION CONTAINED IN THIS DOCUMENT. The article is for informational purposes only and is not a substitute for professional advice.