What part of the NEC is a reference for calculating conduit fill where all conductors are the same size?

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Where installing Class 2 or Class 3 remote control, signaling, or power limited circuits in a raceway or where installing power-limited fire alarm circuits in a raceway, is it necessary to figure conduit fill? Will using cable rather than individual conductors change the method of calculating raceway fill? Is de-rating of the allowable ampacity of the conductors necessary based on the number of conductors that are installed in the raceway? Should there be concern over heating effects that these conductors and their insulation may be subjected to during the operation of the system? These are all questions that seem to plague contractors and engineers as they start designing low-voltage and fire alarm systems. Remembering the basics of the National Electrical Code (NEC) is probably the best place to start when attacking the problem as previously outlined. Section 90-3 states that Chapters 1-4 generally apply to all electrical wiring systems. These first four chapters provide wiring methods, grounding and bonding for systems, ampacity tables, and other general requirements necessary for any electrical installation. Chapters 5, 6, and 7 are devoted to special occupancies, special equipment, or special conditions, and these three chapters supplement or modify the first four general chapters of the NEC since they deal with special electrical installations. With that in mind, Section 725-3 for Class 2 or 3 circuits and Section 760-3 only require those sections of Article 300 that are specifically referenced within Articles 725 or 760 to apply to low-voltage or power-limited fire alarm systems. For example, Section 300-17 provides information on the number and size of conductors. However, this particular section is not referenced within either Article 725 or 760. Therefore, it does not apply. Section 300-17 basically states that the number and size of conductors must not be more than will permit dissipation of heat from the conductors and ready installation or removal of the conductors without damage, either to the conductors or the their insulation. There is also a Fine Print Note (FPN) in Section 300-17 that provides specific section numbers in raceway articles for conduit fill. Fine Print Notes are not enforceable in the NEC but provide extra information. The Class 2 or Class 3 low-voltage system and the power-limited fire alarm circuits may be installed using wiring methods suitable for nonpower-limited or power circuit wiring. For example, electrical metallic tubing (EMT) or flexible metal conduit, with individual conductors or sets of cables, would be acceptable. These same circuits may also be installed using power-limited cables without a raceway. If cables are used without a raceway, care must be taken as to the type of cable used. If a raceway is used to enclose the conductors or cables, the Article that applies to the type of raceway selected would apply to the installation. Remember! Sections 725-3 and 760-3 only gave us permission to disregard Article 300, unless specifically referenced. They did not give us permission to disregard all of Chapter 3. Since raceway articles are located in Chapter 3, if we use one of these raceways, we must comply with the requirements in that specific article. We must provide the proper support, number of maximum bends, proper reaming and threading of the raceway where necessary, and proper raceway fill. Each raceway article has a specific section that references the number of conductors that can be installed in that specific type of raceway. For example, electrical metallic tubing (EMT) is covered by Article 348 and has Section 348-8 covering the number of conductors that may be installed. It states that the number of conductors installed in a single EMT cannot exceed the percentage fill specified in Table 1, Chapter 9. Since only Article 300 is exempted from applying, the EMT fill must be calculated for even low voltage or fire alarm circuits. Table 1 of Chapter 9 allows 53 percent fill for one conductor, 31 percent fill for two conductors, and 40 percent fill for more than two conductors. The Notes to Table 1 provide some directions for using this table in any calculation and should be reviewed thoroughly before applying the table. Note 1 allows Appendix C tables to be used for raceway fill where conductors are all the same size. Note 5 allows multiconductor cables to be installed using the actual dimensions (diameter of the cable) to be inserted into the formula for the area of a circle =1/4 times p times diameter squared or 0.7854 X d2. Note 9 is an extremely important note that allows a multiconductor cable of two or more conductors to be treated as a single conductor for calculating the percentage of conduit fill. If it is an elliptical cable, the cross-sectional diameter used is based upon the major diameter of the ellipse. Now using all of this information as a basis, the answers to the questions in the first paragraph become easy to provide. Based upon each raceway used for the installation, the fill of the raceway must be calculated. If individual conductors are being used, either Table 5, Chapter 9 conductor dimensions can be used or Appendix C tables may be used. Using Table 5, Chapter 9, if 22 #18 TFN conductors are to be installed, then either the actual diameter of the conductor could be inserted into the area of a circle formula previously given, or, since it is already provided in Table 5, the following calculation would apply: 22 X 0.0055 in.2 per conductor = 0.121 in.2 total for all conductors Using that value in Table 4 of Chapter 9 for EMT, the 40 percent fill for a * inch EMT calculates to 0.122 in.2 of conductors (without being overfilled). Therefore, * inch EMT would be the answer. Using EMT Table C1 in Appendix C, if 22 #18 TFN conductors are to be installed, * EMT would be permitted. If cable is employed rather than individual conductors, the cable diameter must be determined. This information is normally available from the cable manufacturer. The diameter of each cable (if it is a multiconductor cable, remember to use the major diameter of the overall cable and treat it as a single conductor) must be inserted into the area of a circle formula to determine the square inch size of each cable. Once that is determined, treat the fill calculation the same as the calculation that was done for the individual conductors. # of cables X [0.7854 X d2] = in.2 total for all cables Use the square inch total of all of these cables in Table 4 of Chapter9 to determine the size of EMT necessary to hold the cables. The last two questions posed in the first paragraph are dealing with the same issue; de-rating the conductors based upon the amount of current that will be applied to each conductor and the effect overheating would have on the insulation of the conductors. The answer is yes, there should be concern for the amount of current that is placed on these conductors since excessive heating could adversely affect the insulation of any or all of the conductors. However, the normal de-rating associated with power and lighting conductors would not apply since these are remote-control, signaling, or power-limited conductors. However, these Class 2, Class 3, or power-limited fire alarm circuits conductors could be treated the same as Class 1 conductors in accordance with Section 725-28(a) or Section 760-28(a) for nonpower-limited fire alarm circuits, if there is sufficient concern about heating effects on the insulation. Similar to the methods used for Class 1 or nonpower-limited fire alarm circuits, the de-rating factors of Section 310-15(b)(2)(a) would apply only where the conductors carry a continuous load in excess of 10 percent of the ampacity of each conductor. Calculations dealing with conduit fill and conductor ampacity for Class 2, Class 3, or power-limited fire alarm circuits, as you can see, are easy if you follow the few simple rules in the NEC. Keep this in mind the next time you do an installation. The more often you use the calculations, the easier they become. ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at .

Conduit fill, also known as raceway fill, is the amount of a conduit's cross-sectional area occupied, or filled, by a cable or multiple cables. The fill is based on the cable outside diameter (O.D.) and the conduit inside diameter (I.D.).

Determining cable conduit fill is critical in order to comply with the requirements of the National Electrical Code (NEC). Failing to do this correctly can lead to expensive and time-consuming rewiring at the very least, and at most, an electrical installation that’s dangerous.

Don’t have access to the NEC book?

You’ll need the NEC book to calculate conduit size for cable. If you’re outside of the U.S. and don’t have access to the book, you may find this conduit cable fill chart helpful.

Getting started

First, it helps to have an idea of the type of cable conduits you should use, so let’s start there.

1. Which conduit material?

Conduits are a form of cable protection, so you need to make sure you choose the right material for your application. You can go with flexible plastic conduit for cables or one with a metal base. Here are three popular options to consider.

2. Which insulated conductor?

Insulated conductors – or insulated wires – are your cable conduit fill. Make sure you use the right wires for your application. For instance, don’t use THHN in wet conditions; it’s rated only for dry and damp locations. Here are the most common types used.

Conduit size for cable

A word before we get started: you need to consider three factors when doing your calculations:

1. Number of cables in your conduit
2. Cross-section area of your cables
3. Number of bends in your conduit

You need: NEC book

You’ll use NEC tables to find wire-type diameters, fill amounts and conduit diameters.

Step 1: Open your NEC book to chapter 9

You need to choose your fill table. This will depend on the type of conduit and wire that you’re using.

• Read down the first column on the fill table to find the wire gauge
• Across from the wire gauge, you’ll find the maximum number of wires that can be placed inside your conduit diameter
• Choose a number equal to or greater than the number of wires you’ll put inside the conduit

Step 2: Calculate the wire cross-sectional area

You know the number of wires you need and the insulation type. The NEC book will tell you the gauge. Now you just need to determine each wire’s cross-sectional area and total these up.

Example:

Let’s say you have these wire types and amounts:

• An 8AWG THHN wire has a cross section of 23.61 square mm (0.03659 square in)
• A 4 AWG THW has a cross section of 62.77 square mm (0.09729 square in)

Therefore, the wires’ total cross-sectional area is:

(23.61 sq. mm) x 4 + (62.77 sq. mm) x 2 = 219.98 sq. mm

Step 3: Find the conduit’s minimum space available

The NEC specifications are:

• One wire: maximum fill is 53% of the space inside a conduit
• Two wires: maximum fill is 31%
• Three wires or more: maximum fill is 40% of the conduit’s total available space

Using the wire cross-sectional areas you’ve already calculated, you can now determine the minimum conduit size that you need.

Example:

Returning to the example in Step 2, you’re using a total of 6 wires. This means that your maximum fill percentage is 40%. You already have your total wire area, so you can now calculate the minimum conduit area:

219.98 sq. mm / 0.4 = 549.95 sq. mm

Step 4: Find your conduit fill

Back to your NEC book. Find the type of conduit that you want to use in table 4.

Example:

If you’re using electrical metal tubing (EMT) conduit, you’ll see that the closest size that you need is a 1 in conduit, which gives you a 39% fill.

Conduit cable fill chart

This chart for conduit size for cable is based on the 2017 NEC and uses common conduit types and wires. If you don’t have access to the NEC book, you may find it helpful in determining how many wires you can safely place in conduit.

• The rows going across illustrate the size of the conduit and the type
• The columns going down gives the gauge of wire that you’re using

The results are the numbers of wires of that gauge, that can be run through that size, of that kind of conduit.

The information in this table is referenced from tables C1, C4, and C8 in the National Electric Code of 2017. The NEC is updated every three years.

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