When there are 3 or more conductors in a raceway What is the allowable fill percentage according to Chapter 9 Table 1?

Raceways must be large enough to hold conductors without overheating and avoid insulation damage during conductor pulls. Therefore, we must determine the maximum number and size of conductors to put in a given raceway. That limit is what we mean by raceway fill.

Chapter 3 of the NEC contains Articles for a dozen different types of raceway, including conduit, EMT, and PVC. Chapter 9 and Annex C are the primary references for determining the correct raceway fill for a given application. Annex C contains a different raceway fill table for each raceway type, because each differs in its ability to dissipate heat.

Because different conductor types (THW, TW, THHN, etc.) have different thicknesses of insulation, raceway fill also varies with the type of conductor you use. In fact, each Annex C Table for a given raceway type contains sections for specific conductor types. For example, Table C.8 (Rigid Metal Conduit) contains three sections, each of which lists specific conductor types.

Table 1 of Chapter 9 lists the maximum percentage of allowable conductor fill. This number is based on common conditions where the length of the conductor and number of raceway bends are within reasonable limits [Chapter 9, Table 1, FPN No. 1] and how many conductors or cables are to be installed in the raceway.

Number of conductors

Sometimes, you have a specific raceway being run, and you must know how many conductors you can pull into it.

Where multiple conductors of the same size are installed together in a raceway, you can determine the maximum number of conductors from the Annex C tables. Where conductors of different sizes are in a raceway, Chapter 9 contains the information necessary to calculate the required raceway size.

Raceway calculation tips

Tip 1: Take your time.

Tip 2: Use a straightedge when using tables.

Tip 3: Watch out for different types of raceways and conductor insulations, particularly RHH/RHW with/without an outer cover.

Annex C

Using Annex C is easy. You just locate the table for the raceway type you're using, find the conductor type on that table, and look up the size and quantity. For example, if you want to know how many 14 RHH conductors (without cover) you can install in trade size 1 EMT, just take a look at Annex C, Table C.1. The answer is 16 conductors.

Each table in Annex C has an alternative version, denoted as C.1(A), C.2(A), and so forth. These alternatives apply where you are using compact conductors. In the manufacturing process, these conductors are compressed so much that the spaces between strands are nearly eliminated. If you compare the standard tables to their compact conductor versions, you will see the variations are significant.

Note 2 at the end of Annex C, Table C.1 indicates that an asterisk (*) with certain conductor insulation means these types don't have an outer covering. This covering increases the dimensions of the conductor more than the thin nylon cover on conductors such as THHN.

Calculating raceway fill

You can't use Annex C tables to determine raceway fill for conductors of different sizes in the same raceway. To do so, use the following steps:

Determine the cross-sectional area of each conductor. Use Chapter 9, Table 5 for insulated conductors and Chapter 9, Table 8 for bare conductors.

Determine the total cross-sectional area for all conductors.

Size the raceway per the percent fill listed in Chapter 9, Table 1. Apply Chapter 9, Table 4 — this includes the various raceway types with columns representing the allowable fills. When using Chapter 9, Table 4, choose the correct section of the table for the raceway type.

Let's work through an example to show how this calculation is performed.

What is the minimum size Schedule 40 PVC raceway required for three 500kcmil THHN conductors, one 250kcmil THHN conductor, and one 3 THHN conductor

Determine the cross-sectional area of the conductors [Chapter 9, Table 5].

500 THHN [0.7073 sq in. × 3 wires = 2.1219 sq in.]

250 THHN [0.3970 sq in. × 1 wire = 0.3970 sq in.]

3 THHN [0.0973 sq in. × 1 wire = 0.0973 sq in.]

Total cross-sectional area of all conductors = 2.6162 sq in.

Size the conduit at 40% fill [Chapter 9, Table 1] using Chapter 9, Table 4 (select the table for PVC Schedule 40).

Trade size 3 Schedule 40 PVC has an allowable cross-sectional area of 2.907 sq in. for over two conductors in the 40% column.

Here's another example to help drive this process home.

What size RMC nipple is required for three 3/0 THHN conductors, one 1 THHN conductor, and one 6 THHN conductor?

Determine the cross-sectional area of the conductors [Chapter 9, Table 5].

3/0 THHN [0.2679 sq in. × 3 wires = 0.8037 sq in.]

1 THHN [0.1562 sq in. × 1 wire = 0.1562 sq in.]

6 THHN [0.0507 sq in. × 1 wire = 0.0507 sq in.]

Total cross-sectional area of the conductors = 1.0106 sq in.

Size the conduit at 60% fill [Chapter 9, Table 1, Note 4] using Chapter 9, Table 4.

Trade size 1¼ nipple = 0.0916 sq in. [too small]

Trade size 1½ nipple = 1.243 sq in. [just right]

Trade size 2 nipple = 2.045 sq in. [larger than required]

A metal wireway is a sheet metal raceway with hinged or removable covers for housing conductors [376.2]. Metal wireways (and nonmetallic wireways) are often called “troughs” or “gutters” in the field.

One common application for these troughs is where installers or maintenance personnel need access for making terminations, splices, or taps to several devices at a single location. The high cost of wireways precludes using them for other than short distances, except in some commercial or industrial occupancies where the wiring is frequently revised.

• The maximum size conductor in a wireway must not be larger than that for which the wireway is designed [376.21].

• The maximum number of conductors in a wireway is limited to 20% of the crosssectional area of the wireway [376.22].

• Splices and taps must not fill more than 75% of the wiring space at any cross section [376.56].

When installing more than 30 currentcarrying conductors in any crosssectional area of the wireway, adjust the conductor ampacity (as listed in Table 310.16) per Table 310.15(B)(2)(a). For this adjustment, don't count the signaling and motor control conductors between a motor and its starter (if used only for starting duty).

Where conductors are bent within a metal wireway, size the wireway to meet the bending radius requirements in Table 312.6(A), based on one wire per terminal [376.23].

Where insulated conductors 4 AWG or larger are pulled through a metal wireway, the distance between raceway and cable entries enclosing the same conductor must not be less than required by 314.28(A)(1), 314.28(A)(2), and 376.23(B) [ Fig. 4 ].

The type of pull determines what those requirements are:

• Straight pulls

  The distance from where the conductors enter to the opposite wall must be at least eight times the trade size of the largest raceway [314.28(A)(1)].

• Angle pulls

  The distance from the raceway entry to the opposite wall must be at least six times the trade diameter of the largest raceway, plus the sum of the trade sizes of the remaining raceways on the same wall [314.28(A)(2)].

• U pulls

  When a conductor enters and leaves from the same wall, the distance from where the raceways enter to the opposite wall must be at least six times the trade size of the largest raceway, plus the sum of the trade sizes of the remaining raceways on the same wall and row [314.28(A)(2)].

Also, the distance between raceways enclosing the same conductor must not be less than six times the trade size of the largest raceway [314.28(A)(2)].

Some people refer to EMT and other raceways as “conduit” instead of “raceway.” Is this really a problem?

Glance through the Annex C tables, and you'll see significant variations in raceway fill. A large number of underfilled raceways can mean cost overruns, but a single overfilled raceway can lead to a failed inspection and expensive rework. Even worse, the error won't be caught until a forensic inspection traces the cause of a horrific fire back to your incorrect raceway fill. To prevent disasters and keep costs down, correctly identify the raceway type from the 12 distinctions made in the NEC. Then, determine the raceway fill.

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.

Free CADs are available for most solutions, which you can download. You can also request free samples to make sure you’ve chosen exactly what you need. If you’re not quite sure which solution will work best for your application, our experts are always happy to advise you.

Whatever your requirements, you can depend on fast despatch. Request your free samples or download free CADs now.

Questions?

Email us at or speak to one of our experts for further information on the ideal solution for your application 800-847-0486.