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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Earthing Systems Design Steps – Part Four


In Article " Earthing Systems Design steps – Part One ", I indicated the following points:




Earthing Systems Design Steps

A grounding system design process has (3) main steps:

  1. Data Collection,
  2. Data Analysis,
  3. Grounding Design Calculations.



In the above Article and Article " Earthing Systems Design steps – Part Two ", I explained the first step: Data Collection.

Also, in Article " Earthing Systems Design steps – Part Three ", I explained the second step: Data Analysis.


Today I will explain the third step of earthing system design which is Grounding Design Calculations as follows.



You can preview the following Articles for more info:






Third Step: Earthing System Design Calculations

Under this title, I will differentiate between two types of buildings as follows:

  1. Substations buildings,
  2. Other Non-Substation Buildings which can be called – in this course – General buildings.


 The design process of a substation grounding system requires many steps as established by the IEEE Standard 80, and it is more complicated than that of the general building (Non-substation buildings). However, the design process of a substation grounding system is not under the scope of this course.






What we are going to design for grounding system in any building?

Grounding system in any building can be broken down into several subdivisions:

  1. The building exterior grounds,
  2. The electrical service grounding,
  3. The building interior bonding,
  4. Equipment grounding and bonding,
  5. Lightning protection.






First: The Building Exterior Grounds

Generally, the building exterior grounds consists of:

  1. The building’s perimeter grounds,
  2. Fence grounds,
  3. Other grounds.






1- The Building’s Perimeter Grounds

The building’s perimeter grounds consist of the following components:

  1. A copper conductor that is directly buried in the earth and installed around the perimeter of the building.  The steel building columns are bonded to this conductor to complete the grounding system.
  2. The columns around the perimeter of the building are excellent grounding electrodes and provide a good path into the earth for any fault currents that may be imposed on the system.


Note:

In some cases, an electrical design requires ground rods to be installed in addition to the perimeter ground ring. The use of ground rods helps to minimize the effects of dry or frozen soil on the overall impedance of the perimeter ground system. This is because the ground rods can reach deeper into the earth where the soil moisture content may be higher or the soil may not have frozen.

Design Recommendations For Building’s Perimeter Grounds:

  • The electrical designer, based on the size and usage of the building, will determine whether every column or just some of the columns are bonded. However, at least one column every 50 feet shall be connected to the above described ground ring. (see Fig. 1)




Fig. 1


  • The size of the ground ring will depend upon the size of the electrical service but is seldom less than 1/0 AWG copper.
  • It is recommended that the ground ring and ground rods be copper or copperbonded steel and installed at least 24 inch from the foundation footer and 18 inch outside the roof drip line. This location will allow for the greatest use of the water coming off of the roof to maintain a good soil moisture content.
  • “Triad” ground rod arrangements (rods placed in a triangular configuration) are sometimes specified, usually at the corners of the building or structure. Figure 2 shows possible conductor/ground rod configurations. Triad arrangements are not recommended unless the spacing between the ground rods is equal to or greater than the individual ground rod length.




Fig. 2


  • Three rods in a straight line spaced at least equal to the length of the individual ground rods are more efficient and result in a lower overall system impedance.
  • When the required resistance is not achieved using the usual grounding layouts, a prefabricated wire mesh can be added to lower the overall grounding impedance (see Fig. 3). Prefabricated wire mesh products are existing in sizes ranging from No. 6 to No. 12 AWG solid conductors.



Fig. 3

  • Another method which can be used to lower the grounding system impedance is ground enhancement materials. These materials can be added around ground rods or other conductors to enhance system performance. (see Fig. 4)





Fig. 4






2- Fence Grounds

  • The National Electrical Safety Code (NESC) recommends that where fences are required to be grounded, such grounding shall be designed to limit touch, step and transferred voltages in accordance with industry practice.
  • The NESC requires that the grounding connection be made either to the grounding system of the enclosed equipment or to a separate ground. In addition, the NESC in Section 92E, lists six separate requirements for fences:


1- Where gates are installed, the fence shall be grounded at each side of the gate or similar opening (see Fig. 5).


Fig. 5


2- If a conducting gate is used, a buried bonding jumper must be installed across the opening (see Fig. 5).

3- Where gates are installed, they shall be bonded to the fence, grounding conductor or other bonding jumper (see Fig. 6).



Fig. 6
4- If the fence posts consist of a conducting material, the grounding conductor must be connected to the fence posts with a suitable connecting means (see Fig. 6).

5- If the fence contains sections of barbed wire, the barbed wire must also be bonded to the fence, grounding conductor or other bonding jumper (see Fig. 6).

6- If the fence posts consist of a nonconducting material, a bonding connection shall be made to the fence mesh strands and barbed wire strands at each grounding conductor point (see Fig. 6).

  • Any fence around a substation on the property should be grounded and tied into the substation ground system. If a facility fence meets the substation fence, it is recommended to isolate the two fences to prevent any fault in the substation from being transferred throughout the facility using the fence as the conductor (see Fig. 7).


Fig. 7







3- Other Grounds

Other grounds that are located on the outside of the building that should be considered are:

  1. Handhole, manhole and pull box covers,
  2. Metal poles,
  3. Lighting fixture standards,
  4. Rails.



A- Handhole, Manhole And Pull Box Covers

  • Handhole, manhole and pull box covers, if conductive, should be bonded to the grounding system using a flexible grounding conductor (see Fig. 8).


Fig. 8


  • The NEC Section 370-40 (d) requires that a means be provided in each metal box for the connection of an equipment grounding conductor. Metal covers for pull boxes, junction boxes or conduit bodies shall also be grounded if they are exposed and likely to become energized.



B- Metal Poles

  • The NEC in Section 410-15 (b) Exception, permits metal poles, less than 20 feet (6.4 m) in height to be installed without handholes if the pole is provided with a hinged base. Both parts of the hinged base are required to be bonded to ensure the required low impedance connection. (see Fig. 8)



C- Lighting Fixture Standards

Lighting standards in parking lots and other areas where the public may contact them should be grounded (see Fig. 8). Keep in mind that the earth cannot serve as the sole equipment grounding conductor. Light standards which are grounded by the use of a separate ground rod must also be grounded with an equipment grounding conductor to ensure that the overcurrent protective device will operate.


D- Rails

Rails or sidings into hazardous locations such as grain storage facilities, ammunition dumps, etc., should also be properly bonded and grounded (see Fig. 8). Designers and installers must not forget that distant lightning strikes can travel through the rails for many miles.








Installation Recommendation For Building Exterior Grounds

  • Installers of these perimeter ground systems need to provide a “water stop” for each grounding conductor that passes through a foundation wall. This is especially important when the grounding conductor passes through the foundation wall at a point that is below the water table. The water stop ensures that moisture will not enter the building by following the conductor strands and seeping into the building. A CADWELD Type SS (splice) in the un-spliced conductor and imbedded into the concrete wall provides the required water stop (see Fig. 9).


Fig. 9

  • When “inspection wells” are required to expose points from which to measure system resistance, several methods are available. Inspection wells are usually placed over a ground rod.
  • If the grounding conductors do not have to be disconnected from the rod, the conductors can be welded to the rod, and a plastic pipe, a clay pipe, or a commercial box. (see Fig. 9)., can be placed over the rod. The plastic pipe also works well when an existing connection must be repeatedly checked, since it can be custom made in the field to be installed over an existing connection. If the conductors must be removed from the rod to enable resistance measurements to be made, either a bolted connector or lug may be used (see Fig. 9).
  • Grounding conductors shall be protected against physical damage wherever they are accessible (see Fig. 10).


Fig. 10

  • Grounding conductors installed as separate conductors in metal raceways always must be bonded at both ends to ensure that current flow is not choked off by the inductive element of the circuit.




In the next Article, I will continue explaining Other Building’s Earthing System Divisions. Please, keep following.




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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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