Learn How to Calculate Conduit Size for Cables Fast

I’ll teach you how to calculate conduit size for cables and wires fast and easy. It’s a great way to verify your grueling conduit sizing hand calculations.

Also, convenient for calculating conduit sizes on the fly if you’re at a job site or standing in Home Depot. Our free-to-use conduit sizing calculator will do all the hard work for you.

Before we start, it’s important we learn the difference between a wire and a cable.

Difference between wires and cables

These two terms people often confuse together. But in fact, they’re quite different.

I’m going to use both of these terms throughout this article. So, we need to understand the difference.

Wire: is a single conductor. It’s usually made of copper or aluminum.

Cable: is a group of conductors. In other words, two or more insulated wires make a cable. Think of several insulated wires wrapped in one jacket.

HVDC submarine cable cross section from New Zealand Inter island scheme
HVDC submarine cable cross-section from New Zealand (Photo Credit: Marshelec)

Conventional way on how to calculate conduit size for cables and wires

For this example, we want to size a Schedule 40 PVC conduit for the below-listed wires. We’ll do our sizing per the National Electrical Code (NEC).

Circuit #Number of wiresInsulation TypeGauge
13THHN1/0 KCMIL
22THHN4 AWG
32THHN8 AWG
41THW10 AWG
51XHHW12 AWG

STEP #1: Calculate the wire cross-section area

Using NEC Chapter 9 Table 5, we calculate the cross-section area of each wire.

To do this, we match the insulation type to the wire gauge in the NEC table.

Circuit #Area (in²)Number of wiresTotal area (in²)
10.185530.5565
20.082420.1648
30.036620.0732
40.024310.0243
50.018110.0181

Next, we add all the wire cross-section areas together: 0.5565 + 0.1648 + 0.0732 + 0.0243 + 0.0181 = 0.8369 in²

STEP #2: Determine the minimum available conduit space

We need to find how much space our wires can take up inside of a conduit. NEC Chapter 9 Table 1 tells us the following:

  • 1 wire: maximum fill is 53% of the total space inside of a conduit
  • 2 wires: maximum fill is 31% of the total space inside of a conduit
  • Over 2 wires: maximum fill is 40% of the total space inside of a conduit

In our example, we have a total of 9 wires. So, per the NEC we can’t exceed 40% fill in our conduit.

STEP #3: Determine the conduit size

Using NEC Chapter 9 Table 1, we choose the conduit size that’ll meet the NEC 40% fill requirement.

We use our total cross-section wire area in the Schedule 40 PVC conduit table.

The NEC table shows a 2-inch conduit can have 1.316 in² of fill while remaining below the 40% requirement. This works since our total wire cross-section area is 0.8459 in².

On the flip side, one conduit size smaller is 1-1/2″. This conduit size can only have 0.794 in² of fill while remaining below the 40% requirement. Thus, this conduit size falls short of our total wire cross-section area of 0.8459 in².

The fast and easy way on how to calculate conduit size for cables and wires

We’ll now learn how to use our conduit sizing calculator using our original example. You’ll see how much faster and easier your calculation becomes.

Plus, I’ll show you how to edit your conduit fill on the fly. You can instantly test and calculate various conduit fill scenarios.

STEP #1: The conduit sizing calculator

Go to our conduit sizing calculator to get started.

how to calculate conduit size for cables - step 1

STEP #2: Initial inputs

Select your conduit type from the dropdown menu.

Then select your first conductor type and size. Next, input in the conductor count for your first conductor.

In the screenshot below, I’ve entered our Circuit #1 values from our example. The calculator will then automatically find and use the appropriate wire cross-section area.

Calculator tip: do you have a custom cable or conductor you want to use? If yes, in the “Select Cable / Conductor Type” field, choose “Other / Custom”. Then enter your cable or conductor cross-section area in square inches.

Many times, you’ll need to use a cable or conductor that’s not documented in the NEC. So, you wouldn’t find it in our database.

how to calculate conduit size for cables - step 2

STEP #3: Enter in your additional wires

Hit “Add Row” for each additional wire type you want to place in your conduit.

In our example, we have 5 total wire types. So, you’ll need to hit the “Add Row” button 4 times.

how to calculate conduit size for cables - step 3

STEP #4: Complete the wire inputs

Repeat STEP #2 for each wire type. Enter all your wire information one by one.

Calculator tip: if you add an accidental extra row, don’t worry. At the rightmost end of each row, hit the “remove” button.

how to calculate conduit size for cables - step 4

STEP #5: Calculate the conduit fill

The easiest part of it all. Hit the “Calculate” button to calculate your conduit size.

Calculator tip: even after you hit “Calculate” you can go back and modify your inputs. Then hit “Calculate” again to get instantly updated results.

how to calculate conduit size for cables - step 5

STEP #6: Output results

Scroll down to see your output results after you hit “Calculate”. Here you’ll find all your calculated conduit and wire information.

The output results include the following fields:

  • Total Conductor Area: the calculated total conductor cross-section area of all your inputted wires.
  • Conduit Size: the calculated minimum conduit size allowed by the NEC.
  • Total Conduit Area: the total cross-section area of your selected conduit size.
  • Total Conduit Fill: the percent fill of your selected conduit size per your total wire cross-section area. In our example, the conduit fill calculates to 25.43%. Much less than the required 40%. In other words, all the wires combined only take up 25.43% of the total 2-inch conduit cross-section area.

how to calculate conduit size for cables - step 6

Comparing our output results

We find our total wire cross-section area of 0.8369 in² matches in both calculation methods. Also, the 2-inch conduit size we calculated matches too.

In short, our calculator simplifies the conduit sizing calculation. It’s a great way to check your hand calculations too.

Also, it gives you much more flexibility. You can easily edit your conduit fill in the office, at home, or out on the go. All the while staying compliant with the NEC.

Even more, play around and add or subtract wires. See how your conduit instantly changes in size.

Conclusion

To sum up, if you see any features you’d like added just let us know. Also, if you see anything in our calculator that needs improvement just give us a holler.

All in all, we want to make calculating conduit sizes for cables and wires easy for everyone.

How do you typically calculate conduit size for cables?

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