Raceway Fill

Raceway Fill

By Mike Holt for EC&M Magazine

The correct raceway fill depends on the conductor type and raceway type.

Raceways must be large enough to hold conductors without overheating, and to avoid insulation damage during conductor pulls. Thus, 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 type has 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 or 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].

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 (see the Sidebar).

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.

Question: How many 14 RHH conductors (without cover) can you install in trade size 1 EMT?

(a) 13 conductors (b) 16 conductors

Answer: (b)

16 conductors [Annex C, Table C.1]

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. So, use the following steps to do that.

Step 1. Determine the cross-sectional area of each conductor. Use Chapter 9, Table 5 for insulated conductors and Chapter 9, Table 8 for bare conductors.

Step 2. Determine the total cross-sectional area for all conductors.

Step 3. 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.

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

(a) Trade size 2 (b) Trade size 2½

Answer: (c) Trade size 3

Step 1. Determine the cross-sectional area of the conductors [Chapter 9, Table 5].

500 THHN 0.7073 sq in. x 3 wires = 2.1219 sq in.

250 THHN 0.3970 sq in. x 1 wire = 0.3970 sq in.

3 THHN 0.0973 sq in. x 1 wire = 0.0973 sq in.

Step 2. Total cross-sectional area of all conductors = 2.6162 sq in.

Step 3. Size the conduit at 40 percent 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 percent column.

Another question: what size RMC nipple is required for three 3/0 THHN conductors, one 1 THHN conductor, and one 6 THHN conductor?

(a) Trade size ½ (b) Trade size 1

Answer: (c) Trade size 1½

Step 1. Determine the cross-sectional area of the conductors [Chapter 9, Table 5].

3/0 THHN 0.2679 sq in. x 3 wires = 0.8037 sq in.

1 THHN 0.1562 sq in. x 1 wire = 0.1562 sq in.

6 THHN 0.0507 sq in. x 1 wire = 0.0507 sq in.

Step 2. Total cross-sectional area of the conductors = 1.0106 sq in.

Step 3. Size the conduit at 60 percent 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

Metal wireways

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 percent of the cross-sectional area of the wireway [376.22].
Splices and taps must not fill more than 75 percent of the wiring space at any cross section [376.56].

When installing more than 30 current-carrying conductors in any cross-sectional 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) and 314.28(A)(2).

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)].

Preventing disaster

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 under-filled raceways can mean cost overruns. But a single overfilled raceway can lead to a failed inspection and expensive rework. Or, 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 twelve distinctions made in the NEC. Then, determine the raceway fill.

Sidebar: Tips for Raceway Calculations

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.