Code Calculations

May 7, 2004
The ampacity of a conductor as listed in Table 310.16 is based on the conductor operating at an ambient temperature of 86ºF (30ºC). When the ambient temperature is different than 86ºF (30ºC) for a prolonged period of time, the conductor ampacity listed in Table 310.16 must be adjusted. The temperature adjustment factors used to determine the new conductor ampacity are listed at the bottom of Table

There are times when we need to add conductors to an existing raceway. But do you know how do so without damaging either the existing or new conductors, or both? You can ensure a quality and Code-compliant installation by first determining the available spare space in the raceway and then calculating the number of conductors allowed in this spare space. Here’s a five-step process that will keep you on the straight and narrow.

Step 1: Determine the raceway’s cross-sectional area for conductor fill [Table 1 and Table 4 of Chapter 9].
Step 2: Determine the area of the existing conductors [Table 5 of Chapter 9].
Step 3: Subtract the cross-sectional area of the existing conductors (Step 2) from the area of permitted conductor fill (Step 1).
Step 4: Determine the cross-sectional area of the conductors to be added [Table 5 of Chapter 9 for insulated conductors and Table 8 of Chapter 9 for bare conductors].
Step 5: Divide the spare space area (Step 3) by the cross-sectional area of the conductors to be added (Step 4).

Now you should be ready to tackle a sample calculation.

Q. An existing 1-inch EMT contains two No. 12 THHN conductors, two No. 10 THHN conductors, and one No. 12 bare (stranded) conductor. How many additional No. 8 THHN conductors can be added to this raceway?

Answer:
Step 1: Cross-sectional area permitted for conductor fill
Raceway = 0.864 x 0.4 = 0.3456 sq in.
Nipple = 0.864 x 0.6 = 0.5184 sq in.

Step 2: Cross-sectional area of existing conductors
No. 10 THHN [0.0211 sq in. x 2 = 0.0422 sq in.]
No. 12 THHN [0.0133 sq in. x 2 = 0.0266 sq in.]
No. 12 bare [0.0060 sq in. x 1 = 0.0060 sq in.]
Total cross-sectional area of existing conductors = 0.0748 sq in.
Note: Ground wires must be counted for raceway fill.

Step 3: Subtract the area of the existing conductors from the permitted area of conductor fill.
Raceway more than 24 inches long: 0.3456 sq in. – 0.0748 sq in. = 0.2708 sq in.
Nipple (less than 24 inches long): 0.5184 sq in. – 0.0748 sq in. = 0.4436 sq in.

Step 4: Cross-sectional area of the conductors to be installed.
No. 8 THHN = 0.0366 sq in.

Step 5: Divide the spare space area (Step 3) by the conductor area.
Raceway = 0.2708 sq in. ÷ 0.0366 sq in. = 7.4 or 7 conductors
Nipple = 0.4436 sq in. ÷ 0.0366 sq in. = 12.1 or 12 conductors
We must round down to 18 conductors because Note 7 to Table 1 only applies if all of the conductors are the same size and same insulation.

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.