The Basics Of Determining Raceway Fill

Feb. 1, 1998
You may have laughed at the electrical drawing the first time you saw it, but now you have to finish the installation, and you still don't have a good drawing. For one thing, the conduit fills are obviously all wrong. Nobody puts 45 No. 12 wires and 15 No. 14 wires in single half-inch raceway. You're out in the field, and you don't have a computer with you. How do you determine what size raceway you

You may have laughed at the electrical drawing the first time you saw it, but now you have to finish the installation, and you still don't have a good drawing. For one thing, the conduit fills are obviously all wrong. Nobody puts 45 No. 12 wires and 15 No. 14 wires in single half-inch raceway. You're out in the field, and you don't have a computer with you. How do you determine what size raceway you need?

The figure shows a flow chart of the general steps. Let's continue with our example of the No. 14 and No. 12 wires, and go through the steps in detail.

The first step is to calculate the size of the wires. Let's say we are using THHN. We go to the National Electrical Code, Table 5 and find that a No. 12 THHN has an area of 0.0133 sq in., and a No. 14 has an area of .0097 sq in. Our 45 No. 12 wires times 0.0133 sq in. give us an area of 0.5985 sq in. Our 15 No. 14 wires times 0.0097 sq in. give us an area of 0.1455 sq in. Our total, then, is 0.744 sq in. Don't forget that the derating penalties exacted by Note 8(a) to the ampacity tables effectively preclude large numbers of power conductors in a common raceway. In this case, the ampacity of the No. 12 conductors would fall to just over 10A (30A30.35510.5A).

What do we do with our 0.744 sq in. of wire? Let's look at the National Electrical Code, Table 4 to see what size raceway we need. We'll assume the raceway is EMT and runs overhead for far more than 24 in. (maximum nipple length). With more than two wires, we can max out at 40% fill.

How do you use the NEC's Table 4 for this type of calculation? The column you want to use is "Over 2 Wires," since we have some 60 wires and want a 40% fill. Accommodating all of our wire will require a 1 1/2 in. EMT.

Now that we've covered how to determine raceway fill, let's cover some basic concerns on conduit selection. Remember that the Code is not a design manual. You may decide to go with a larger raceway if you have a long wire pull or simply want an extra margin of safety and performance from your wiring. Web Kee, whose byline has appeared in EC&M, has done extensive thermography on hundreds of installations. From his perspective, it's almost always better to go with bigger raceway. In some cases, you may want to divide the run into two raceways. If you do this, keep circuits intact. Do not, for example, run two of a motor's phase conductors in one conduit and the third in the other.

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Fiberglass Electrical Conduit Chemical Resistance Chart

This information is provided solely as a guide since it is impossible to anticipate all individual site conditions. For specific applications which are not covered in this guide...

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.