Ecmweb 2556 Fig1
Ecmweb 2556 Fig1
Ecmweb 2556 Fig1
Ecmweb 2556 Fig1
Ecmweb 2556 Fig1

Code Calculations

Aug. 24, 2004
When installing conductors of different sizes (or types of insulation) in a raceway, you can’t size the raceway based on Tables 1 through 12 in Annex C. You must instead follow this three-step process to determine the proper raceway and nipple size ...

When installing conductors of different sizes (or types of insulation) in a raceway, you can’t size the raceway based on Tables 1 through 12 in Annex C. You must instead follow this three-step process to determine the proper raceway and nipple size required for this type of installation.

Step 1: Determine the cross-sectional area (square inches) for each conductor from Table 5 of Chapter 9 for insulated conductors and Table 8 of Chapter 9 for bare conductors.
Step 2: Determine the total cross-sectional area for all conductors.
Step 3: Size the raceway according to the percent fill as listed in Table 1 of Chapter 9; 40% for three or more conductors and 60% for raceways 24 in. or less in length (nipples).

Example 1: A 400A feeder is installed in Schedule 40 rigid nonmetallic conduit. This raceway contains three 500 kcmil THHN conductors, one 250 kcmil THHN conductor, and one 3 AWG THHN conductor. What size raceway is required for these conductors?

Example 2: What size rigid metal nipple is required for three 3/0 AWG THHN conductors, one 1 AWG THHN conductor, and one 6 AWG THHN conductor?

Example 1 answer:
Step 1: Determine the cross-sectional area of the conductors per Table 5 of Chapter 9.
500 kcmil THHN = 0.7073 sq in. x 3 wires = 2.1219 sq in.
250 kcmil THHN = 0.3970 sq in. x 1 wire = 0.3970 sq in.
3 AWG THHN = 0.0973 sq in. x 1 wire = 0.0973 sq in.
Step 2: Calculate the total cross-sectional area of all conductors.
2.1219 + 0.3970 + 0.0973 = 2.6162 sq in.
Step 3: Size the conduit at 40% fill [Chapter 9, Table 1] using Table 4.
A 3-inch Schedule 40 PVC has a cross-sectional area of 2.907 sq in. for conductors, which is just the right size.

Example 2 answer:
Step 1: Determine the cross-sectional area of the conductors per Table 5 of Chapter 9.
3/0 AWG THHN = 0.2679 sq in. x 3 wires = 0.8037 sq in.
1 AWG THHN = 0.1562 sq in. x 1 wire = 0.1562 sq in.
6 AWG THHN = 0.0507 sq in. x 1 wire = 0.0507 sq in.
Step 2: Calculate the total cross-sectional area of all conductors.
0.8037 + 0.1562 + 0.0507 = 1.0106 sq in.
Step 3: Size the conduit at 60% fill [Table 1, Note 4 of Chapter 9] using Table 4.
A 1.5 inch nipple = 1.243 sq in., which is just the right size.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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.