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 too.
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 lifting for you.
Before we get started, it’s important to discuss 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.
With this in mind, I’m going to use both of these terms throughout this article. So, we need to understand how they differ.
Wire: is a single conductor. It’s usually made of copper or aluminum.
Cable: is a group of conductors. In other words, a cable is made of two or more insulated wires. Think of several insulated wires wrapped in one jacket.
Now that we’ve discussed wires and cables, let’s get started. We’ll first go over the conventional way in how to calculate conduit size for cables and wires.
Conventional Way in Sizing Conduits
To begin with, for this entire article we’ll work off one example.
For this example, assume we want to place the below wires inside of a Schedule 40 PVC conduit. So, we need to calculate the size of this conduit. While also, staying compliant with the National Electrical Code (NEC).
|Circuit #||Number of wires||Insulation Type||Gauge|
STEP #1: Calculate the wire cross-section area
Use NEC Chapter 9 Table 5 to find the cross-section area of each of our listed wires. Match the insulation type to the wire gauge in the NEC table to find the cross-section area of each wire.
|Circuit #||Area (in²)||Number of wires||Total area (in²)|
After that, we add up all the wire cross-section areas: 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 out how much space our wires can take up inside of a conduit. Using NEC Chapter 9 Table 1 we find 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 figure out the conduit size we can use to meet the NEC 40% fill requirement.
Since we know our conduit type, we use the Schedule 40 PVC conduit table. Next, using our total cross-section wire area, we can choose the correct conduit size.
The 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 calculated to 0.8459 in².
Keep in mind, one conduit size smaller is 1-1/2″. This conduit size can have only 0.794 in² of fill while remaining below the 40% requirement. Thus, this conduit size falls just short of our total wire cross-section area of 0.8459 in².
Fast and Easy Way in Sizing Conduits
We’ll now learn how to use our conduit sizing calculator by solving our original example.
You’ll see how much faster and easier the calculation becomes. This will show you how you can edit your conduit fill on the fly. Thereafter, instantly test and calculate various conduit fill scenarios.
STEP #1: The calculator
Go to our conduit sizing calculator to get started.
STEP #2: Initial inputs
Select your conduit type and first conductor type. Then input in the conductor size and count for this first conductor.
In the screenshot below, I’ve entered in our Circuit #1 values from our example. The calculator will then automatically find and use the appropriate wire cross-section area.
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”.
Many times, you’ll need to use a cable or conductor that’s not commonly used. So, you wouldn’t find it in our database.
By selecting “Other / Custom”, you can enter in the cross-section area in square inches of your cable or wire.
STEP #3: Entering additional wires
Hit “Add Row” to enter in all the wire types you want to place in your conduit.
In our example, we have 5 total wire types. So, we need to hit the “Add Row” button 4 times.
STEP #4: Complete the wire inputs
As we did in STEP #2, enter in all the wire information one by one.
Tip: if you add an accidental extra row, don’t worry. At the rightmost end of each row, simply hit the “remove” button.
STEP #5: Calculate the conduit fill
The easiest part of it all. Hit “Calculate” and the conduit calculation will be made for you.
Tip: even after you hit “Calculate” you can go back and modify your inputs. Then hit “Calculate” again to get updated results.
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.
Now, I’ll go over each of the output fields below to explain what you’ll find.
- Total Conductor Area: the calculated total conductor cross-section area of all your inputted wires.
- Conduit Size: the calculated minimum conduit size you can use that will meet the NEC guidelines.
- Total Conduit Area: the total cross-section area of your calculated conduit size from the previous “Conduit Size” output field.
- Total Conduit Fill: the percent fill of your calculated conduit size using your total wire cross-section area. In our example, the conduit fill calculates to 25.43%. Much less than the required 40%. This means, all the wires combined only take up 25.43% of the total 2-inch conduit cross-section area.
Comparing the Output Results
We find the total wire cross-section area of 0.8369 in² matches in both calculation methods.
Also, the 2-inch conduit we calculated as compliant with the NEC matches too. In short, our calculator simplifies the conduit calculation process. Also, it gives you much more flexibility.
You can instantly edit your conduit fill in the office, at home, or out on the go.
Your Thoughts on How to Calculate Conduit Size for Cables
Summing up, if you see any features you would like added just let us know. Also, if you see something in the calculator that can be improved we’d love to know.
All in all, we want to make this the best conduit sizing calculator for everyone.
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Koosha started Engineer Calcs in 2019 to help people better understand the engineering and construction industry, and to discuss various science and engineering related topics to make people think. He has been working in the engineering and tech industry in California for over a decade now and is a licensed professional electrical engineer, and also has various entrepreneurial pursuits.
Koosha has an extensive background in the design and specification of electrical systems with areas of expertise including power generation, transmission, distribution, instrumentation and controls, and water distribution and pumping as well as alternative energy (wind, solar, geothermal, and storage).
Koosha is most interested in engineering innovations, the cosmos, and our history and future.