**Conduit Fill**

In conduit fill we are dealing with the age old question...

How Many Wires Can We Fit In A Pipe ?

The National Electrical Code states...

In this situation all of our conductors are going to have to be counted; hot conductors, neutrals, switch-legs and even grounding or bonding conductors (insulated or bare)...

*300.17: Number and Size of Conductors in Raceway. The number and size of conductors in any raceway shall not be more than will permit dissipation of the heat and ready installation or withdrawal of the conductors without damage to the conductors or to their insulation.*In this situation all of our conductors are going to have to be counted; hot conductors, neutrals, switch-legs and even grounding or bonding conductors (insulated or bare)...

*Chapter 9, Note 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 Easy Way

There is one easy way to solve this problem. It’s called Informational Annex ‘C’ (page 70-703) in the Code and it lists commonly used insulated conductor types and how many are allowed in different types of conduit. There are some restrictions though, all of the conductors involved have to be the same size (wiring gauge) and insulation rating.

For example; According to Annex ‘C1’ we can pull as many as eight #14 TW conductors in a 1/2” EMT conduit, or we could pull as many as eight #12 THW conductors in a 3/4” EMT raceway. Annex ‘C’ is a great help but it might cause severe health problems to any one who takes it too seriously. Just try pulling twelve #14 THHN conductors in a long run of 1/2” EMT !

Now, what if we have to pull different wire sizes and different insulation ratings in the same raceway ? There is a way of dealing with this situation, of course, it’s a little more complicated. It’s done by measuring conduits and conductors into what space they take up, which is of course space. The Code calls it "approximate area in square inches" or "cross-sectional area."

Now, what if we have to pull different wire sizes and different insulation ratings in the same raceway ? There is a way of dealing with this situation, of course, it’s a little more complicated. It’s done by measuring conduits and conductors into what space they take up, which is of course space. The Code calls it "approximate area in square inches" or "cross-sectional area."

According to this Table 1/2” EMT conduit has a total (100%) cross-sectional area of 0.304 square inches. Don’t worry, we’re not going to try and fill this conduit to 100% of it’s capacity.

Chapter 9, Table 1: shows us the percent of cross-section of conduit and tubing allowed to be filled based on number of conductors used.

Chapter 9, Table 1: shows us the percent of cross-section of conduit and tubing allowed to be filled based on number of conductors used.

Most of the time we’ll be dealing with more than two conductors so we’ll be using 40%. In fact it would be a good idea to highlight all of the 40% fill columns of Table 4 seeing as that’s where we’ll be spending most of our time.

There is one exception to the 40% rule that should be noted....

*Chapter 9, Note 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)(2)(a) adjustment factors need not apply to this condition..*Notice that the conduit must be 24” or less to meet this requirement. Also, Table 310.15(B)(2)(a) does not apply. This table deals with ampacity adjustments for number of conductors. There’s a 60% fill column in Chapter 9, Table 4.

Conductor Area

Chapter 9, Table 5 gives us the dimensions of insulated conductors. Of course, we’ll be using the approximate area in square inches (cross-sectional area) column. This is consistent with our conduit cross-sectional area of allowable fill. In fact, it would be a good idea to highlight all the area in square inch columns (Area In.2) in Table 5.

A small ruler would be useful in looking up table requirements. Try looking up the area of a #12 THW. Does it take up .0181 area in square inches ? How about a #8 TW ? Takes up .0437 square inches, right ?. It would also be a good idea to highlight the approximate area in square inch columns in Table 5. For the most part we’ll rarely use conduit or conductor diameters in conduit fill.

Rubber insulated conductors like RHH, RHW and RHW-2 normally come with an outer-covering. This is a protective layer on the outside of the insulation similar to THHN conductors. These R-Type conductors also come without outer-covering. In that situation you’ll see an asterisk (*) after the insulation (RHH*, RHW*, RHW-2*) and a different cross-sectional area.

There is also a special Table ‘5A’ for Compact Aluminum Building Wire dimensions. Compact conductors are molded and compressed by the manufacturer so that voids between strands are eliminated.

Square inches areas (Area In.2) for bare conductors, solid or stranded, can be found in Chapter 9, Table 8. Keep in mind that Article 310-3 requires that conductors #8 and larger installed in raceways must be stranded.

Conduit Fill

Chapter 9, Table 1, Note 6 states that 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.Keep in mind that the conduit and conductor dimensions that they’re referring to are primarily cross-sectional areas, or (area in square inches).

Here’s an example: What size EMT conduit is required when installing three #14 TW and four #12 THW conductors ?

The cross-sectional areas of the three #14 TW .0139 x 3 = .0417

The cross-sectional areas of the four #12 THW .0181 x 4 =

The total cross-sectional areas of all of the conductors = .1141

The cross-sectional area of 1/2” EMT conduit at 40% fill = .1220

Yes, we can fit 3 #14 TW and 4 #12 THW ( a total of .1141 area in square inches) in a 1/2” EMT raceway (.1220 area in square inches allowed).

What if we already have a conduit installed and want to find out how many conductors, of different sizes, we can fit into it ? For example: Will six #12 THHW and twelve #10 THWN conductors fit into a 1” EMT conduit ?

The cross-sectional areas of the six #12 THHW .0181 x 6 = .1086

The cross-sectional areas of the twelve #10 THWN .0211 x 12

The total cross-sectional areas of all of the conductors = .3618

The cross-sectional area of 1” EMT conduit at 40% fill = .3460

Guess what ? They don’t fit ! How about 1-1/4” EMT ?

Cross-sectional area of 1-1/4” EMT conduit (at 40% fill) = .5980

It looks like we have more than enough room in the 1-1/4” EMT for all eighteen conductors. What if we want to add some #12 THW wires ?

First, let’s see how much spare space is left over:

1-1/4” EMT (40% fill) = .5980

Spare Space = .2362 area in square inches

Here’s the formula for new wires that can be added to the spare ...

Number Of Conductors =

Area Of New Conductor

Spare Space = .2362

New Conductor (#12 TW) = .0181

Number Of onductors

New Conductor (#12 TW) = .0181 = 13.0497

We can’t pull 13.0497 conductors in a pipe so where going to have to drop the decimal and go with 13. There’s only one circumstance where you can round-up a fraction of a wire....

Chapter 9, Table 4 now contains a 60% fill column There is also a 60% fill table located in the “Conduit Fill Chart” at the beginning of this workbook.

Here’s an example: What size EMT conduit is required when installing three #14 TW and four #12 THW conductors ?

__From Chapter 9, Table 5:__The cross-sectional areas of the three #14 TW .0139 x 3 = .0417

The cross-sectional areas of the four #12 THW .0181 x 4 =

__.0724__The total cross-sectional areas of all of the conductors = .1141

__From Chapter 9, Table 4:__The cross-sectional area of 1/2” EMT conduit at 40% fill = .1220

Yes, we can fit 3 #14 TW and 4 #12 THW ( a total of .1141 area in square inches) in a 1/2” EMT raceway (.1220 area in square inches allowed).

What if we already have a conduit installed and want to find out how many conductors, of different sizes, we can fit into it ? For example: Will six #12 THHW and twelve #10 THWN conductors fit into a 1” EMT conduit ?

__From Chapter 9, Table 5:__The cross-sectional areas of the six #12 THHW .0181 x 6 = .1086

The cross-sectional areas of the twelve #10 THWN .0211 x 12

__= .2532__The total cross-sectional areas of all of the conductors = .3618

__From Chapter 9, Table 4:__The cross-sectional area of 1” EMT conduit at 40% fill = .3460

Guess what ? They don’t fit ! How about 1-1/4” EMT ?

__From Chapter 9, Table 4:__Cross-sectional area of 1-1/4” EMT conduit (at 40% fill) = .5980

It looks like we have more than enough room in the 1-1/4” EMT for all eighteen conductors. What if we want to add some #12 THW wires ?

First, let’s see how much spare space is left over:

1-1/4” EMT (40% fill) = .5980

__6 #12 THHW and 12 #10 THWN = .3618__Spare Space = .2362 area in square inches

Here’s the formula for new wires that can be added to the spare ...

Number Of Conductors =

__Spare Space__Area Of New Conductor

Spare Space = .2362

New Conductor (#12 TW) = .0181

Number Of onductors

__= .2362__New Conductor (#12 TW) = .0181 = 13.0497

We can’t pull 13.0497 conductors in a pipe so where going to have to drop the decimal and go with 13. There’s only one circumstance where you can round-up a fraction of a wire....

*Chapter 9, Note 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.*Chapter 9, Table 4 now contains a 60% fill column There is also a 60% fill table located in the “Conduit Fill Chart” at the beginning of this workbook.

Another Problem...

Keep in mind also that RHH, RHW and RHW-2 conductors normally come with an outer- covering. R-Type conductors without an outer-covering have an asterisk (*) after the insulation (RHH*, RHW*, RHW-2*) and a different cross-sectional area.

Another Example: There are four #4/0 THW conductors in a 2-1/2” IMC conduit. If one #4/0 THW conductor is removed, how many #8 THHN conductors will fit ?

Another Example: There are four #4/0 THW conductors in a 2-1/2” IMC conduit. If one #4/0 THW conductor is removed, how many #8 THHN conductors will fit ?

The Answer...

Chapter 9, Table 5: The approximate area in square inches of a #4/0 THW equals .3718. There are 3 conductors in the raceway (one is removed).

.3718 x 3 = 1.1154 area in square inches

Chapter 9, Table 4: The conduit must have room enough to allow for the 1.1154 area in square inches of the 3 #4/0 THW conductors plus the new #8 THHN conductors.

2-1/2” IMC conduit (@ 40% fill) = 2.0540 area in square inches

Area Of Spare Space

Chapter 9, Table 4 & 5: Subtract the total area in square inches of the 3 #4/0 conductors from the area in square inches of the conduit (1.92 square inches) to determine the area of spare space for future use.

2.0540

.9386 (Spare Space)

__-1.1154__.9386 (Spare Space)

Number Of New Conductors Allowed In Spare Space

Chapter 9, Table 4 & 5: Divide the area of spare space (.9386 area in square inches) by the square inch area of the new conductor (#8 THHN = .0366 square inches).

Number Of Conductors =

.0366 (Area of New Conductor)

__.9386 (Spare Space)__.0366 (Area of New Conductor)

Number Of Conductors = 25 (Whole numbers only, drop all decimals.)