Mike Holt, NEC Consultant

Article 424 contains the installation requirements for fixed electrical equipment used for space heating, such as heating cables, unit heaters, boilers, or central systems.

Article 424 is a nine-part article on fixed electric space heating. Why so much text for what seems to be a simple application? The answer is that Article 424 covers a variety of applications—heaters come in various configurations for various uses. Not all of these parts are for the electrician in the field—the requirements in Part IV are for manufacturers

Article 424 does not cover the wiring for fossil-fuel heating equipment, such as gas, oil, or coal central furnaces. Install these per Article 422 (specifically, 422.12).

Fixed space heaters (wall-mounted, ceiling-mounted, or free-standing) are common in many utility buildings and other small structures, and in some larger structures. When used to heat floors, space-heating cables address the thermal layering problem typical of forced-air systems—so it’s likely you’ll encounter them.

Duct heaters are common in large office and educational buildings. These provide a distributed heating scheme. Locating the heater in the ductwork, but close to the occupied space, eliminates the waste of transporting heated air through sheet metal routed in unheated spaces, so it’s likely you’ll encounter those also.

Branch circuits
When sizing branch-circuit conductors, always consider fixed electric space-heating equipment and motor(s) to be continuous loads [424.3(B)].

The branch-circuit conductors and overcurrent devices for fixed electric space-heating equipment must have an ampacity not less than 125 percent of the total heating load [210.19(A)(1) and 210.20(A)].

Question: What size conductor and overcurrent device are required for a 9.60 kW, 240V fixed electric space heater that has a 3A blower motor with 60°C terminals?
(a) 10 AWG, 30A      (b) 8 AWG, 40A    (c) 6 AWG, 60A     (d) 4 AWG, 80A

Answer: (c) 6 AWG, 60A
Step 1: Determine the total load:
I = W/E
9.60 kW x 1,000 = 9,600W
I = 9,6000W/240V = 40A
I = 40A + 3A blower motor = 43A total amps
Step 2: Size the conductors at 125 percent of the total current load [110.14(C)(1), 210.19(A)(1), and Table 310.15(B)(16)]:
Conductor = 43A x 1.25
Conductor = 53.75A, 6 AWG, rated 55A at 60°C
Step 3: Size the overcurrent device at 125 percent of the total current load [210.20(A), 240.4(B), and 240.6(A)]:
Overcurrent Protection = 43A x 1.25
Overcurrent Protection = 53.75A, next size up is 60A

Permanently installed electric baseboard heaters with receptacles
If a permanently installed electric baseboard heater has factory-installed receptacle outlets, don’t connect the receptacles to the heater circuits [424.9]. Note that listed baseboard heaters include instructions that prohibit their installation below receptacle outlets.

Electric space-heating equipment disconnecting means
You must provide a means to simultaneously disconnect the heater, motor controller, and supplementary overcurrent devices of all fixed electric space-heating equipment from all ungrounded conductors [424.19].

The disconnecting means must be capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means must be on the switch or circuit breaker, and it must remain in place with or without the lock installed.

The requirements for the disconnecting means for fixed electric space-heating equipment depend on whether it has supplementary overcurrent protection.

The disconnect for equipment with supplementary overcurrent protection must be within sight from the supplementary overcurrent devices. According to Article 100, within sight means that it’s visible and not more than 50 ft from one to the other. The disconnecting means for fixed electric space-heating equipment with a motor(s) rated more than 1⁄8 horsepower and a single unit switch can have a disconnecting means that isn’t within sight from the equipment [424.19(A)(2)(2). That disconnect must be capable of being locked in the open position, and the locking means must remain in place with or without the lock installed [424.19].

And if the equipment does not have supplementary overcurrent protection? In that case, the branch-circuit circuit breaker may serve as the disconnecting means if the circuit breaker is within sight from the heater or is capable of being locked in the open position [424.19(B)].

A unit switch with a marked “off” position that’s an integral part of the equipment can serve as the heater disconnecting means, if it disconnects all ungrounded conductors of the circuit [424.19(C)].

Electric space-heating cables
Wiring located above heated ceilings must be spaced not less than 2 in. above the heated ceiling and is considered as operating at an ambient temperature of 50°C [424.36].

Some installation restrictions apply to heating cables. First, heating cables must not extend beyond the room or area in which they originate [424.38(A)].

And you can't install them [424.38(B)]

• In closets.
• Over walls.
• Over partitions that extend to the ceiling.
• Over cabinets whose clearance from the ceiling is less than the minimum horizontal dimension of the cabinet to the nearest cabinet edge that is open to the room or area.

Clearance from other objects and openings
Heating elements of cables must be separated at least 8 in. from the edge of outlet boxes (and junction boxes) used for mounting surface luminaires and 2 in. from recessed luminaires and their trims [424.39].

Installing in concrete or poured masonry floors

GFCI protection is required for electric space-heating cables that are embedded in concrete or poured masonry floors of bathrooms, kitchens, and hydromassage bathtub locations [424.44 (G)]. See 680.27(C)(3) for restrictions on the installation of radiant-heating cables for spas and hot tubs installed outdoors.

Duct Heaters
Duct heater controller equipment must be either accessible with the disconnecting means installed at or within sight of the disconnecting means or as permitted by 424.19(A) [424.65].

The disconnecting means must be within sight from the equipment, or it must be capable of being locked in the open position [424.19(A)]. The provision for locking or adding a lock to the disconnecting means must be on the switch or circuit breaker, and must remain in place with or without the lock installed. A portable locking means doesn’t meet the “locked in the open position” requirement.

The disconnecting means for a duct heater isn’t required to be readily accessible. Therefore, it can be located within a suspended ceiling area adjacent to the duct heater as long as it’s accessible by portable means [240.24(A)(4) and 404.8(A) Ex 2].

Where a duct heater is located above a suspended ceiling, all of the following must apply [424.66(B)]:
(1)        The duct heater must be accessible through the lay-in type ceiling or access panel(s).
(2)        The width of the working space for the duct heater must be no less than 30 in.
(3)        Doors or hinged panels must open at least 90 degrees.
(4)        The space in front of the duct heater must comply with Table 110.26(A)(1), yet a horizontal ceiling T-bar can be in this space.

Is your heater installation really Code-compliant?
With many types of equipment, it’s possible to meet the letter of the Code but not the intent of it. Heating equipment is perhaps a classic case of this. For example, you must install this equipment to provide the required spacing between the equipment and adjacent combustible material (unless the heater is listed for direct contact with combustible materials) [424.13].

You look at the place on the wall where you’re supposed to install the heater, and there are cardboard boxes stacked there and all around it. The heater isn’t listed for direct contact with combustible materials. Suppose you solve this problem by moving the boxes away and then installing the heater. Technically, you have a Code-compliant installation.

But the next day, the people who stacked those boxes against that wall notice you apparently didn’t clean up behind yourself and put things back. They solve that problem by stacking the boxes next to the heater.

When the place burns down, whose fault is that?
To truly satisfy 424.13, you need to provide some means of reasonably ensuring the required spacing will be maintained. Don’t assume the means is entirely your decision. Before installing the heater, discuss the problem with the person in charge of the area and reach agreement on a solution (e.g., install a barrier such as bollards or rails, or perhaps post a sign, or maybe locate the heater in another spot).

The Code can't anticipate every application problem. Always evaluate the application for how well the intent of the Code is being achieved, and remember that meeting Code is merely the minimum requirement for any installation.