Code Q&A: Properly Serving Fixed Electric Space-Heating Equipment

Nov. 20, 2014

NEC rules on branch circuit sizing for fixed electric space-heating equipment and motors

Q. How are the branch circuit conductors and overcurrent protection sized for fixed electric space-heating equipment? Please provide an example.

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Find the Answer

A. For the purpose of sizing branch circuit conductors, fixed electric space-heating equipment is considered a continuous load [424.3(B)]. The branch circuit conductors and overcurrent devices for fixed electric space-heating equipment must have an ampacity not less than 125% of the total heating load [210.19(A)(1) and 210.20(A)]. Let’s run through an example problem to show you exactly how to make these calculations.

What size conductor and overcurrent device are required for a 9.6kW, 240V fixed electric space heater that has a 3A blower motor with 75°C terminals?

Step 1: Determine the total load.
I = VA ÷ E = 9,600VA ÷ 240V = 40A
I = 40A + 3A = 43A

Step 2: Size the conductors at 125% of the total current load [110.14(C)(1), 210.19(A)(1), and Table 310.15(B)(16)].
Conductor = 43A x 1.25 = 53.75A
A 6 AWG conductor is rated 65A at 75°C

Step 3: Size the overcurrent device at 125% of the total current load [210.20(A), 240.4(B), and 240.6(A)].
Overcurrent protection = 43A x 1.25 = 53.75A
The next size up is 60A.

Therefore, the answer is to use a 6 AWG conductor and a 60A overcurrent protection device.

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.