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How to size conduit for cable

clock 6 minutes | 10 Sep 2019

Man in lab examining cable protection

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.

Material Application Why
HDPE conduit Typically houses and protects electrical power and telecommunications cables, e.g. an outdoor utility cabinet or an outdoor telecom equipment cabinet Excellent corrosion, chemical and UV resistance
Highly flexible cable protection
High-impact strength
Nylon conduit Typically used in general machine building and automotive manufacturing Highly flexible cable conduit
High fatigue life
Self-extinguishing
Abrasion resistant
Highly resistant to solvents and oils
Good weather resistance
PVC-coated metal conduit Typically, general factory wiring and connections to machines High mechanical strength
Highly flexible cable protector


Factory operator installing cable protection in an electrical application

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.

Conductor Characteristics Typical applications
THHW Rated 167°F for wet locations and 90°C for dry locations
No outer coverings on its insulation
Service entrance, feeders and branch circuits for permanent installations
THHN Rated 194°F for dry and damp locations
Nylon jacket over its insulation
Machine tools
Control circuits
Appliances
THWN Rated 167°F for dry and wet locations
Nylon jacket over its insulation
Conduit
Machine tools
Controlled circuits
General-purpose wiring
XHHW Rated 167°F for wet locations and 194°F for dry and damp locations
No outer covering on its insulation
Building wiring
Conduit
Feeder and circuit wiring
THW Rated 167°F for dry and wet locations Building wiring
Feeder and branch circuits
Internal secondary industrial distribution


Cross section of electrical wires with flexible wire covering

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:

Number of wires Insulation type Gauge
4 THHN 8 AWG
2 THW 4 AWG
  • 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.

Conduit cable fill chart

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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Cable Management - The Design Engineers Application Guide​