Considering the fact that it has nine parts, at first glance, Art. 424 might seem like overkill for a device as simple as a heater. Walk into any hardware store or one of the big retail variety stores, and look at their space heaters. What can be so complicated about plugging in these units?
Nothing, really. The reason Art. 424 has so much text is it addresses fixed space heaters — not the portable 120V plug-in models you can stick next to your desk or use to take the chill off your workshop. Fixed electric space-heating equipment includes such items as heating cable, unit heaters, boilers, and central systems [424.1].
Fixed space heaters are common in utility buildings and other small structures. One common application is a fire pump building, which is typically situated away from the main building and doesn't share the central heating system. The fire pump, jockey pump, and related equipment could freeze up, and they also require periodic inspection and maintenance; therefore, the structure is often heated in cold weather areas.
Fixed space heaters are also common in some larger structures, where central heat can't do the job alone. They may be wall-mounted, ceiling-mounted, or freestanding. When used to heat floors, space-heating cables address the thermal layering problem typical of forced-air systems.
Not all of Art. 424 is for the electrician in the field, however. Part IV provides requirements that manufacturers must meet. For electricians, the most relevant Parts are III, V, and VI.
Article 424 does not address wiring for fossil-fuel heating equipment, such as gas, oil, or coal central furnaces. Installation requirements for the wiring of those systems are found in Art. 422, specifically 422.12.
Electric space-heating equipment
These heaters aren't incidental loads. The heating units are permanently mounted and have a permanent power supply, and they usually provide a substantial load. The fixed electric space heaters used in that earlier fire pump building example might be 50A each (or even larger) and might be mounted on brackets that are bolted to the walls.
Obviously, these don't plug into a 15A convenience receptacle. Instead, the heaters are on a branch circuit that is typically dedicated to fixed space heating. Because of this arrangement, you must provide a disconnecting means when installing the equipment. The disconnecting means must 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 remain in place with or without the lock installed.
If the fixed electric space-heating equipment has supplementary overcurrent protection, the disconnecting means must be within sight from the supplementary overcurrent device. “Within sight” means the specific equipment is visible and not more than 50 feet from one to the other [Art. 100].
If the fixed electric space-heating equipment doesn't have supplementary overcurrent protection, then you can use the branch-circuit circuit breaker as the disconnecting means [424.19(B)]. That circuit breaker must be within sight of the heater or be capable of being locked in the open position.
You can also use the switch on the heater unit itself as a disconnect, if it has a marked “off” position that is an integral part of the equipment, and the switch disconnects all ungrounded conductors of the circuit [424.19(C)] (Fig. 1).
Baseboard heaters
Some models of permanently installed electric baseboard heaters have factory-installed receptacle outlets. That's a nice feature, because it allows you to reduce the work involved in providing convenience receptacles at the same location. Just make sure you don't connect the receptacles to the heater circuits [424.9].
Another advantage of using baseboard heaters with factory-installed receptacles is that you avoid heater location violations. The FPN in 424.9 says that listed baseboard heaters include instructions that prohibit their installation below receptacle outlets. Placing a heater under the receptacles would create two problems:
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The wiring and receptacles would be operating at higher temperatures, possibly above their rated ampacity ratings.
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The cord plugged into those receptacles would have a very good chance of drying out due to the heat, creating a shock or fire hazard.
Space-heating cables
If you use electric space-heating cables that are embedded in concrete (or poured masonry) floors of bathrooms or hydromassage bathtub locations, you must wire them with GFCI protection [424.44(G)] (Fig. 2).
See 680.27(C)(3) for restrictions on the installation of radiant heating cables for spas and hot tubs installed outdoors.
Duct heaters
In large offices and educational buildings, duct heaters are often the only source of local heat. 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. If you do much commercial work, you will undoubtedly encounter duct heaters if you haven't done so already.
You must provide a means to disconnect the heater, motor controller, and supplementary overcurrent devices from all ungrounded conductors of the circuit [424.65]. The disconnecting means must be within sight of 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 remain in place with or without the lock installed. A locking means that is portable doesn't meet this requirement (Fig. 3).
The disconnecting means for a duct heater doesn't have to be readily accessible; therefore, it can be located within a suspended ceiling adjacent to the duct heater, as long as it's accessible by portable means [240.24(A)(4) and 404.8(A) Ex 2].
Branch circuits
If you are sizing branch-circuit conductors for fixed electric space-heating equipment, you must consider these to be 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)].
Here's a short practice exercise that demonstrates how to size the branch circuit correctly.
Question: What size conductor and overcurrent device with 75°C terminals is required for a 10kW, 240V fixed electric space heater that has a 3A blower motor (Fig. 4)?
Here are the steps you should take to find the answer.
Step 1. Determine the total load.
I = VA ÷ E
I = 10,000VA ÷ 240V = 41.67A
I = 41.67A + 3A = 44.67A, round up to 45A [220.5(B)]
Step 2. Size the conductors at 125% of the load [110.14(C), 210.19(A)(1), and Table 310.16].
Conductor = 45A × 1.25 = 56A
A 6 AWG conductor is rated 65A at 75°C
Step 3. Size the overcurrent device at 125% of the load [210.20(A), 240.4(B) and 240.6(A)].
Overcurrent device = 45A × 1.25 = 56A
Choose the next standard size up, which is 60A [240.4(B)]
Think beyond heaters
When you are thinking about a project that involves heating a space with a fixed heater, think beyond simply adding a heater. Consider what the space is used for and who will be in that space.
Determine whether you need one big heater mounted at one end of the space or several units distributed throughout. Does this application work best with ceiling-mounted, wall-mounted, or baseboard heaters — or perhaps some combination of these? How big will each heater be? What kinds of controls will you use? Will you use a thermostat with an integral sensor or distributed sensors? Where is the best location for sensing the temperature?
Once you've weighed these considerations, think about the wiring. One branch circuit or two? What kind of disconnect(s), and what about location and mounting? Then, select the heater that best meets your application. Review the manufacturer's instructions, and install per Art. 424.
