How to calculate what the required NM cable size for fixed electric space-heating equipment

Stumped by the Code? NEC Requirements for Disconnects and Conductors Serving Permanently Connected Appliances and Space-Heating Equipment

Feb. 7, 2022

Answering your NEC questions.

All questions and answers are based on the 2020 NEC.

Q. What is the Code rule regarding disconnects for permanently connected appliances?

A. For hard-wired appliances rated not more than 300VA or 1⁄8 hp, the branch-circuit overcurrent device is permitted to serve as the disconnecting means where the switch or circuit breaker is within sight from the appliance or is capable of being locked in the open position per Sec. 110.25 [Sec. 422.31(A)].

Hard-wired appliances rated over 300VA must have a disconnect located within sight of the appliance, or the disconnect must be capable of being locked in the open position per Sec. 110.25 [Sec. 422.31(B)].

Author’s comments:

If the appliance is hard-wired, then the appliance disconnect must be located within sight of the appliance or circuit breaker for the appliance and must be lockable with provisions for locking to remain in place whether the lock is installed or not [Sec. 422.31(A) or (B)].
According to Art. 100, “Within Sight” means that it is visible and not more than 50 ft from the location of the equipment.

Q. What is the NEC requirement for the minimum size of branch-circuit conductors for fixed electrical space heating equipment?

A. The branch-circuit conductors for fixed electric space-heating equipment and any associated motors must be sized not smaller than 125% of the load [424.4(B)].

Author’s comment: The branch-circuit conductors and overcurrent protection for fixed electric space-heating equipment must have an ampacity not less than 125% of the total heating load [Sec. 210.19(A)(1) and Sec. 210.20(A)].

Let’s clarify this rule through a practical example.

Situation: What size NM cable and overcurrent protection is required for a 9,600W, 240V fixed electric space heater that has a 3A, 240V blower motor? (Figure).

Solution:

Step 1: Determine the total load.

I = Watts ÷ Volts = 9,600W ÷ 240V = 40A

Total Amperes = 40A (heat) + 3A (blower) = 43A

Step 2: Size the conductors at 125% of the total current load [Sec. 424.4(B)].

Conductor = 43A × 125% = 53.75A

Based on this ampere total, use 6 AWG rated 55A at 60°C [Sec. 110.14(C)(1)(a)(2), Sec. 334.80, and Table 310.16].

Step 3: Size the overcurrent protection at 125% of the total current load [Sec. 210.20(A), Sec. 240.4(B), and Sec. 240.6(A)].

Overcurrent Protection = 43A × 125% = 53.75A

Therefore, use the next size up (60A).

Q. What is the Code rule for sizing disconnecting means for fixed electric space heating equipment?

A. Heating equipment must have a means to simultaneously disconnect all circuit phase conductors [Sec. 424.19].

(B) Heating Equipment Without Supplementary Overcurrent Protection. The disconnect for fixed heating equipment must be located within sight of the fixed heating equipment, or it must be capable of being locked in the open position with provisions for locking to remain in place whether the lock is installed or not per Sec. 110.25. Figure 424.19B0 01

(C) Unit Switch as Disconnecting Means. A unit switch on fixed electric heating equipment with a marked “off” position can serve as the required disconnect.

These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.

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.