4 How To Inspect Electrical Conduits and Boxes


In Article " Conduit Fill Calculations ", I explained the following items:

  • Conduit Sizes Designations,
  • Tables used for Conduit Fill Calculations.

Also, 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 How to Inspect Electrical Conduits and Boxes as follows.



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





Area Classification Requirements



  • As an Electrical Inspector, you must verify that all electrical equipment located in the classified areas meet the requirements of the NEC for use in that particular hazardous location.

  • Rigid metal conduit and type MI (mineral-insulated) cable are used in Class I, Division 1 locations.  Thread all conduits.  Termination fittings must meet approval if used with Type MI cable.  All boxes and fittings must be explosion-proof and have threaded openings.

  • Lighting fixtures used in these locations must meet approval for use by one of the approved testing laboratories.  Mounting boxes must be explosion-proof and approved for hanging and mounting fixtures.  Pendant fixture wiring must be in metal conduit and suspended from the ceiling.






Flame-Proof Junction Boxes


  • Seal conduit systems in Class I, Division 1 and 2 locations if sparks, arcs, or high temperatures could be present in enclosures containing electrical equipment.  The seals will minimize the spread of gases, vapors, and prevent the passage of flames from one part of the electrical system to another.  They will also prevent an explosion in the enclosure from traveling through the conduit or cables to other locations, causing additional fires or explosions.  If vapors in the conduit can condense into a liquid, provide drainage.






How to Inspect Conduit



Use this work Procedure as a guide to help you inspect conduit as follows:





  • Verify the fill percentage.
  • Verify that the conduit is properly supported.
  • Verify PVC coating, threaded lubricant is applied.  PVC patching compound must be on sleeves. 
  • Verify that the conduit connections are wrench tight.  Use a strap wrench for PVC-coated conduit.
  • Verify that the expansion joints are installed and bonded where needed
  • Verify that locknuts are installed and installed correctly.
  • Verify the installation of weatherproof hubs on outside installations.
  •  Verify that bushings are installed at boxes.
  • Verify that there is no more than 360 degrees of bend between pull points.
  • Verify that conduit is buried at the proper depth in underground installations.
  • Verify that conduit was reamed after cutting.
  • Verify that conduit threads are properly cut.
  • Verify that conduits are fireproofed where needed.
  • Verify that conduit bodies containing splices are completed in accordance with the NEC, Article 314-16(c).






How to Inspect Junction Boxes and Pull Boxes



Use this Work Procedure as a guide to help you inspect junction boxes and pull boxes as follows:





  • Knockout seals must close all unused openings in boxes or fittings.
  • All metal boxes are usually required to be grounded.
  • At least 6 inches of free conductors must be left at each outlet box. Three inches must extend from the edge of a box less than 8 inches in any dimension.
  • Conduits must not be connected to the sides of round boxes, only to square smooth sides.
  • Boxes may be recessed 1/4 inch or less for noncombustible material.
  • Each outlet box must have a cover, faceplate, or fixture canopy to complete installation.
  • Volume of original box may be increased by the cubic inches marked on the plaster ring.
  • All boxes used to install lighting fixtures must be designed so that a lighting fixture may be attached.
  • Boxes must be rigidly fastened to the surface upon which they are mounted.
  • Boxes shall not use suspended ceiling wire as a sole supporting means unless associated with electrical equipment designed for suspended ceilings.
  • Boxes mounted in a wall of combustible material must be flush with surface or project from it.
  • Junction boxes must be accessible without it being necessary to remove or disturb any part of the building. However, lift-out panels in suspended ceilings are considered accessible.





In the next Article, I will make a Review for Course WR-2: Inspect conduits, junction boxes, and pull boxes . Please, keep following.







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0 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:



Conduit Fill Calculations






Conduit Sizes Designations

The conduits have two size designations as follows:

  1. Metric designator,
  2. 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:

  1. Long length of the run or
  2. 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:

  1. The maximum number of conductors permitted not be installed, or
  2. 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.






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0 Electrical Boxes Volume and Fill Calculations


In Article " Electrical Boxes – Part Two ", I explained the following items:

  • Device boxes, 
  • Pull and junction boxes,
  • Sizing of Junction and pull boxes according to NEC Section 314-28.

Today, I will explain Electrical Boxes Volume and Fill Calculations  as follows.



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





Electrical Boxes Volume and Fill Calculations




  
NEC Section 314.16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies

This section include three parts as follows:

  • Part (A), “Box Volume Calculations,” defines the volume of a wiring enclosure or box. The calculations must take into account the volume of the box as well as the volume of any extensions such as domed covers or extension rings.
  • Part (B), “Box Fill Calculations,” describes the method for determining how much volume (fill) may be occupied by conductors, clamps, support fittings, devices (switches or receptacles) or equipment, and equipment grounding conductors.
  • Part (C), “Conduit Bodies,” covers enclosing No. 6 AWG or smaller conductors and requires that the maximum number of conductors be computed.


This section states that:

" Boxes and conduit bodies shall be of sufficient size to provide free space for all enclosed conductors. In no case shall the volume of the box, as calculated in 314.16(A), be less than the fill calculation as calculated in 314.16(B). The minimum volume for conduit bodies shall be as calculated in 314.16(C)".


Notes to section NEC 314.16:

  • The provisions of this section shall not apply to terminal housings supplied with motors or generators.
  • Boxes and conduit bodies enclosing conductors 4 AWG or larger shall also comply with the provisions of 314.28.






NEC 314.16 Part (A): Box Volume Calculations

The volume of a box is the total volume in cubic inches of the assembled sections and, where used, the space provided by plaster rings, domed covers and extension rings that are marked with their volume in cubic inches or are made from metal boxes that are included in Table 314-16(a) in the NEC.






Notes for NEC Table 314-16(a):

  • The volumes of standard boxes that are not marked with their volume shall be as given in Table 314.16(A).
  • Boxes 1650 cm3 (100 in.3) or less, other than those described in Table 314.16(A), and nonmetallic boxes shall be durably and legibly marked by the manufacturer with their volume. Boxes described in Table 314.16(A) that have a volume larger than is designated in the table shall be permitted to have their volume marked as required by this section.
  • The total volume determines the number and size of conductors and wiring devices that are permitted to be contained in the box. The cubic inch area required for each wire, clamp, support fitting, device and equipment ground is added together. The box must have a cubic-inch capacity that equals or exceeds the total of the contained items.
  • Sometimes more conductors end up in boxes than were originally intended. Where practicable, an extension ring that is the same shape as the box can be installed that will add adequate space so the original box does not have to be replaced.
  • The NEC Table 314-16(a) covers the maximum number of conductors permitted within a standard metal box. A “standard” box is one that is included in the Table. The minimum cubic inch capacity for each size is given along with the maximum number of conductors of sizes #18 through #6 that are permitted in the box.
  • This number of conductors permitted in various boxes, as shown in the Table, applies only where all conductors are the same size.  A calculation must be made of the cubic inch capacity that is required where conductors of different sizes are installed as in section 314.16(b) Box Fill Calculations.






NEC 314.16 Part (B): Box Fill Calculations

  • Section 314-16(b) includes the requirements and method for determining the minimum cubic inch area that are required in boxes that have different size conductors or any equipment installed in them.
  • The method is to add up all the allowances required for various items, and this becomes the minimum area box that is required. At that point, a standard box with a cubic area that equals or exceeds the volume required can be used.


The following two tables can be used in calculating box fill:






Conductor, Device or Type of Fitting
Counted as Conductors
Based on
Each conductor, originating outside and terminating inside the box
1
Conductor size
Each conductor passing through unbroken
1
Conductor size
Conductor that does not leave the box
0
Not Applicable
Maximum of four fixture wires smaller than #14 plus ground from fixture canopy. Must terminate in box.
0
Not Applicable
Cable clamps, one or more (internal)
1
Largest size conductor present
Support fittings, e.g. fixture studs or hickeys (per type)
1
Largest size conductor present
Device or equipment yoke, e.g. switch, receptacle, pilot light, etc.
2
Largest size connected to device
Equipment grounding conductors, all except isolated ground
1
Largest EGC present
Additional equipment grounding conductors for isolated grounding
1
Largest EGC present
Each loop or coil of unbroken conductor not less than twice the minimum length required for free conductors in 300.14
2
Conductor size
Commentary Table 314.1 Summary of Items Contributing to Box Fill


Let’s take a closer look at some of the requirements that are listed on the above table as follows:

  • No allowance is required for small fittings like locknuts and bushings.
  • Where one or more fixture studs or hickeys are present in the box, a single volume allowance is required to be made for each type of fitting in the box. A fixture stud is a fitting that mounts to the top of the box, usually inserts through the knockout of a metal box and is threaded to accommodate the fixture stem. A hickey is a fitting that can be described as a coupling that has threads the same size as the fixture stem and has an oval-shaped hole on one or more sides for the fixture wires to exit inside the box. The hickey is no longer popular and has been replaced with hanger straps that are fastened to the box.
  • For each yoke or strap containing one or more devices or equipment, a double volume allowance is required for each yoke or strap. Each device or equipment is considered individually where more than one item is contained in the box. For example, if a switch has #14 wire connected to it, a volume allowance of 2 x 2.0 cubic inches or 4 cubic inches is required. If a receptacle has #12 wire connected to it, a volume allowance of 2 x 2.25 or 4.5 cubic inches must be made.
  • Where one or more equipment grounding conductors enters a box, a single volume allowance is required to be made. The allowance is based on the largest equipment grounding conductor. This applies to all equipment grounds except for an isolated equipment ground often installed on computer circuits.




Example#1:


A standard-sized box is used where all the conductors are the same size and, as shown in below figure, the box does not contain any cable clamps, support fittings, devices, or equipment grounding conductors. Determine whether the box in below figure is adequately sized or not.






Solution:


To determine the number of conductors permitted in the box, which is a standard 4 in. x 1.5 in. square box (21.0 in.3), count the conductors in the box and compare the total to the maximum number of conductors permitted by Table 314.16(A).

Each unspliced conductor running through the box is counted as one conductor, and each other conductor is counted as one conductor.

Therefore, the total conductor count for this box is nine conductors. Table 314.16(A) indicates that the maximum fill for this box is nine 12 AWG conductors, so the box is adequately sized.



Example#2:

Determine whether the box in below figure is adequately sized or not.






Solution:

The standard method for determining adequate box size first calculates the total box volume and then subtracts the total box fill to ensure compliance.

For a standard 3 in. x 2 in. x3.5 in. device box, Table 314.16(A) shows the minimum permitted box volume to be 18 in.3 and allows up to a maximum of nine 14 AWG conductors.



Total Box Fill
Items Contained Within Box
Volume Allowance
Unit Volume Based on Table 314.16(B) (in.3)
Total Box Fill (in.3)
4 conductors
4 volume allowances for 14 AWG conductors
2
8
1 clamp
1 volume allowance (based on 14 AWG conductors)
2
2
1 device
2 volume allowances (based on 14 AWG conductors)
2
4
Equipment grounding conductors (all)
1 volume allowance (based on 14 AWG conductors)
2
2
Total
16


The box fill for this situation as given in Commentary Table 314.2 is 16 in.3. Because the total box fill of 16 in.3 is less than the 18 in.3 total box volume permitted, the box is adequately sized.




Example#3:

Determine the adequacy of the device box illustrated in below figure, where two standard sized 3 in. x 2 in. x 3.5 in. device boxes have been ganged to form a single box.







Solution:

Table 314.16(A) shows that the permitted box volume for a single box is 18 in3. Thus, the total box volume for the ganged box is 36 in.3 (2 x 18 in.3).

The total box fill, based on Table 314.16(B), is determined as given in Commentary Table 314.3. With only 26 in.3 of the 36 in.3 filled, the box is adequately sized.





In the next Article, I will explain NEC 314.16 Part (C) and Conduit Fill Calculations. Please, keep following.






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