Conduit Fill Calculations

In Article " Electrical Boxes Volume and Fill Calculations ", I explained the following items:

• NEC 314.16 Part (A): Box Volume Calculations,
• NEC 314.16 Part (B): Box Fill Calculations,
• NEC 314.16 Part (C): Conduit Bodies.

Today, I will explain Conduit Fill Calculations as follows.

You can review the following articles in the same course for more information:

•  Types of Electrical Conduits

•  Conduit Fittings and Supports
•  Electrical Boxes - Part One
•  Electrical Boxes – Part Two

Conduit Fill Calculations

Conduit Sizes Designations The conduits have two size designations as follows: Metric designator, Trade size. Table 300.1(C) identifies a distinct metric designator for each circular raceway trade size.

Tables used for Conduit Fill Calculations First: Chapter 9  which includes the following tables: TABLE 1 Percent of Cross Section of Conduit and Tubing for Conductors, TABLE 2 Radius of Conduit and Tubing Bends, 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), TABLE 5 Dimensions of Insulated Conductors and Fixture Wires, TABLE 5A Compact Copper and Aluminum Building Wire Nominal Dimensions* and Areas, TABLE 8 Conductor Properties. You can download a PDF copy of Chapter 9 Tables by click on the link. Second: Annex C Tables Informative Annex C contains conductor fill tables for each of 12 types of conduit and tubing. The Informative Annex C tables — which are based on the dimensions given in Tables 1 and 4 of Chapter 9 for conduit and tubing fill and on the dimensions for conductors in Table 5 of Chapter 9 — provide conductor fill information based on the specific conduit or tubing and on the conductor insulation type, size, and stranding characteristics. Examples of how to use these tables are included in the commentary both here and in Informative Annex C. You can download a PDF copy of Annex C Tables by click on the link.

Chapter 9 - Table 1 Table 1 establishes the maximum fill permitted for the circular conduit and tubing types. It is the basis for Table 4 and for the information on conduit and tubing fill provided in the Informative Annex C tables. Informational Note No. 1: The installation of conductors in a conduit can face some difficulties due to: Long length of the run or Many numbers and total radius of bends. So, it is recommended that where a difficult installation is anticipated due to above reasons, the available solutions will be as follows: The maximum number of conductors permitted not be installed, or The size of the conduit or tubing be increased by at least one trade size larger than the minimum required by the Code. Informational Note No. 2: Conductor jamming may occur during the installation (pulling) of conductors into a conduit even if fill allowances of 40 percent are observed. During the installation of three conductors or cables into the raceway, one conductor could slip between the other two conductors. This is more likely to take place at bends, where the raceway may be slightly oval. The jam ratio is calculated as follows: Jam ratio = ID of raceway / OD of conductor  To avoid difficult conductor installations and potential conductor insulation damage due to jamming within the conduit or tubing, a jam ratio between 2.8 and 3.2 should be avoided. As an example: Table C.1 in Informative Annex C permits three 8 AWG conductors in trade size 1⁄2 electrical metallic tubing (EMT). An 8 AWG conductor has an outside diameter (OD) of 0.216 in. (from Table 5), and a 1⁄2 in. EMT has an internal diameter (ID) of 0.622 in. (from Table 4). The jam ratio is calculated as follows: Jam ratio = ID of raceway / OD of conductor = 0.622 / 0.216 = 2.88 So, Jamming of conductors will occur, use the next larger trade size conduit. A 3⁄4 in. EMT has an internal diameter (ID) of 0.824 in. (from Table 4). So, Jam ratio = ID of raceway / OD of conductor = 0.824 / 0.216 = 3.815

Chapter 9 - Table 4 Because conduits and tubing from different manufacturers have different internal diameters for the same trade size, Table 4 provides the diameter and the actual area of different conduit and tubing types at fill percentages of 100, 60, 53 (one wire), 31 (two wires), and 40 (more than two wires). The 60 percent fill is provided in Table 4 to correlate with Note 4 (found in the Notes to Tables section of this chapter) to the conduit and tubing fill tables, which permits conduit or tubing nipples 24 in. or less in length to have a conductor fill of up to 60 percent. Separate sections in Table 4 cover metal, nonmetallic, rigid, and flexible conduit and tubing types.

Notes to chapter 9 Tables (1) See Informative Annex C for the maximum number of conductors and fixture wires, all of the same size (total cross-sectional area including insulation) permitted in trade sizes of the applicable conduit or tubing. (2) Table 1 applies only to complete conduit or tubing systems and is not intended to apply to sections of conduit or tubing used to protect exposed wiring from physical damage. (3) Equipment grounding or bonding conductors, where installed, shall be included when calculating conduit or tubing fill. The actual dimensions of the equipment grounding or bonding conductor (insulated or bare) shall be used in the calculation. The dimensions of bare conductors are given in Table 8. (4) Where conduit or tubing nipples having a maximum length not to exceed 600 mm (24 in.) are installed between boxes, cabinets, and similar enclosures, the nipples shall be permitted to be filled to 60 percent of their total cross-sectional area, and 310.15(B)(3)(a) adjustment factors need not apply to this condition. (5) For conductors not included in Chapter 9, such as multi-conductor cables, high voltage Cables and optical fiber cables, the actual dimensions shall be used. The cross-sectional area can be calculated in the following manner, using the actual dimensions of each conductor: Cross-sectional area = d 2 cmil Where: d = outside diameter of a conductor (including insulation) 1 in. = 1000 mil 1 cmil (circular mil) = π/4 (3.1416/4) square mil =0.7854 square mil. Conversion from square millimeters to circular mils: To convert from square millimeters to circular mils (approximately) follows: k = 1973.53 circular mils / mm2 (6) For combinations of conductors of different sizes, use Table 5 and Table 5A for dimensions of conductors and Table 4 for the applicable conduit or tubing dimensions. (7) When calculating the maximum number of conductors permitted in a conduit or tubing, all of the same size (total cross-sectional area including insulation), the next higher whole number shall be used to determine the maximum number of conductors permitted when the calculation results in a decimal of 0.8 or larger. (8) Where bare conductors are permitted by other sections of this Code, the dimensions for bare conductors in Table 8 shall be permitted. (9) A multi-conductor cable or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.

Example#1:

Three 15-kV single conductors are to be installed in rigid metal conduit (RMC). The outside diameter of each conductor measures 15⁄8 in., or 1.625 in. What size RMC will accommodate the three conductors?



Solution:

Step 1: Find the cross-sectional area within the conduit to be displaced by the three conductors:

1.625 in. x 1.625 in. x 0.7854 x 3 = 6.2218 in.2 or 6.222 in.2


Step 2: Determine the correct conduit size to accommodate the three conductors. Table 1 allows 40 percent conduit fill for three or more conductors, and Table 4 indicates that 40 percent of trade size 5 RMC is 8.085 in.2.

Thus, trade size 5 RMC will accommodate three 15-kV single conductors.



Example#2:

What traditional wire size does the size 125 mm2 represent (approximately)?


Solution:

Circular mil area = wire size (mm2) x conversion factor = 125 mm2 x 1973.53 circular mils / mm2 = 246,691 circular mils or 246.691 kcmil

Therefore, the 125 mm2 wire is larger than 4/0 AWG (211.6 kcmil) but smaller than a 250-kcmil conductor.


Notes to example#2:

• If a 125 mm2 wire is determined to be the minimum or recommended size conductor, it is important to understand that size 250 kcmil would be the only Table 8 conductor with equivalent cross-sectional area because 4/0 AWG is simply not enough metal.

• It is important, however, to note that the 250-kcmil conductor ampacity could not be used for a 125 mm2 conductor, because the metric conductor size is smaller. The 4/0 AWG ampacity can be used, or the ampacity can be calculated under engineering supervision.

Example#3:

A 200-ampere feeder is routed in various wiring methods [electrical metallic tubing (EMT); rigid polyvinyl chloride conduit (PVC), Schedule 40; and rigid metal conduit (RMC)] from the main switchboard in one building to a distribution panelboard in another building. The circuit consists of four 4/0 AWG XHHW copper conductors and one 6 AWG XHHW copper conductor. Select the proper trade size for the various types of conduit and tubing to be used for the feeder.



Solution:


The used tables are:

• Table 1
• Table 1, Note 6 refers to Table 5 for the area required for each insulated conductor. 
• Note 6 also refers to Table 4 for selection of the appropriate trade size conduit or tubing. 
• Table 4 contains the allowable cross sectional area for conduit and tubing based on conductor occupied space (40 percent maximum in this example).

Step 1: assign the fill percentage from table 1

All the raceways for this example require conduit fill to be calculated according to Table 1 in Chapter 9, which chapter 9 table 1 permits conduit fill to a maximum of 40 percent where more than two conductors are installed.


Step 2: Calculate the total area occupied by the conductors, using the approximate areas listed in Table 5:
Four 4/0 AWG XHHW: 4 x 0.3197 in.2 = 1.2788 in.2

One 6 AWG XHHW: 1 x 0.0590 in.2 = 0.0590 in.2
Total area = 1.3378 in.2 or 1.338 in.2


Step 3: Determine the proper trade size EMT, RMC, and PVC (Schedule 40) from Table 4.

The portion of this feeder installed in EMT requires a minimum trade size 2, which has 1.342 in.2 of available space for over two conductors. The minimum required space is 1.338 in.2, which is less than the trade size 2 EMT 40 percent fill.

RMC also requires a minimum trade size 2, because trade size 2 RMC has 1.363 in.2 of available space for over two conductors. PVC (Schedule 40), however, requires a minimum trade size 2 1⁄2.

Trade size 2 PVC has 1.316 in.2 allowable space for over two conductors and is less than the 1.338 in.2 required for this combination of conductors. Therefore, it is necessary to increase the PVC size to 2 1⁄2 trade size, the next standard size increment.




Example#4:

Determine how many 10 AWG THHN conductors are permitted in a trade size 1 1⁄4 rigid metal conduit (RMC).



Solution:

Table 1 permits 40 percent fill for over two conductors.

From Table 4, 40 percent fill for trade size 11⁄4 RMC is 0.610 in., and from Table 5, the cross-sectional area of a 10 AWG THHN conductor is 0.0211 in.2.
The number of conductors permitted is calculated as follows:

0.610 in.2 / 0.0211 in.2 per conductor = 28.910 conductors

Based on the maximum allowable fill, the number of 10 AWG THHN conductors in trade size 1 1⁄4 RMC cannot exceed 28. However, in accordance with Note 7, an increase to the next whole number of 29 conductors is permitted in this case, because 0.910 is greater than 0.8.
In this case the number of conductors permitted = 29 conductors


Note to example#4:
Although increasing the total to 29 conductors results in the raceway fill exceeding 40 percent, the amount by which it is exceeded is a fraction of 1 percent and will not adversely affect the installation of the conductors.





In the next Article, I will explain how to verify Area Classification and Service Requirements of Conduit, Junction Boxes and Pull Boxes. Please, keep following.

Back to Course WR-2: Inspect conduits, junction boxes, and pull boxes