When used in
conjunction with UL 508A and NFPA 79, NEC Art. 409 serves as a guide for approval
of both new and field-constructed industrial control panels
Covering industrial control panels intended for general use and operating at 600V or less, including those without an appropriate third-party listing, the new Art. 409 in the 2005 NEC provides requirements that designers, builders, installers, and regulatory authorities (i.e. inspectors) can reference and apply for all new installations as well as modifications for existing installations. This addition to the Code establishes a minimum level of safety for all control panel installations. A key benefit of this new Article is getting all parties in the design, construction, and inspection phase of a project “on the same page” to reduce or eliminate confusion over interpretation of NEC rules on this important topic. The change will also help prevent the misuse of industrial control panels and control panel components in new and retrofit installations.
The requirements of Art. 409 complement two well-established industry standards: UL 508A, “Standard for Industrial Control Panels,” and NFPA 79, “Electrical Standard for Industrial Machinery.” Released in 2001 to assist those designing, evaluating, and testing control panels, UL 508A dictates requirements for the safe design, assembly, and installation of industrial control panels. It's also referenced in the Fine Print Note (FPN) of 409.1.
NFPA 79 was developed in the '60s and provides detailed information on the application of electrical/electronic equipment, apparatus, or systems supplied as part of industrial machines (including control panels). Often, the components and wiring systems of industrial machinery are installed in enclosures, which, in effect, allows them to be classified as industrial control panels. As you'll notice throughout this article, the similarities between Art. 409, UL 508A, and NFPA 79 are quite noticeable.
For any item not specifically addressed in Art. 409, it's a good idea to follow the requirements in NFPA 79. For example, NFPA 79 prohibits the use of Class H fuses with 10kAIR (Amps Interrupting Rating) ratings. It also includes very specific requirements for the identification (color coding) of conductors. Other key sections of NFPA 79 worth noting are listed in the Sidebar below.
A closer look inside Art. 409 reveals the following installation requirements and construction specifications.
Installation requirements. As per 409.20, you must size the industrial control panel supply conductor so its ampacity is not less than 125% of the full-load current rating of all resistance heating loads, plus 125% of the full-load rating of the highest rated motor, plus the sum of the full-load current ratings of all other connected motors and apparatus based on their duty cycle that may be in operation at the same time. If the information related to motors sounds familiar, that's good. This requirement is the same as noted in 430.24. The wording as related to heating loads should also sound familiar, as this is the same approach outlined in 215.2(A)(1) for the sizing of typical feeders.
When it comes to figuring out your overcurrent protection needs, you've basically got two choices. Install an overcurrent protective device ahead of the industrial control panel or directly within the industrial control panel. Then calculate the rating or setting of the overcurrent protective device for the circuit supplying the industrial control panel by adding these three figures:
Largest rating or setting of the branch-circuit and ground-fault protective device installed in the industrial control panel.
Sum of the full-load currents of all other motors and apparatus that could operate simultaneously.
125% of the full-load current rating of all resistance heating loads.
Where the control panel does not provide short-circuit and ground-fault protection for the motor(s) it controls, you must size the overcurrent device on the line side of the control panel in accordance with 430.52 (for a single motor) or 430.53 (for a group of motors).
The grounding requirements for industrial control panels are pretty straightforward. Just be aware of some specific rules when installing multi-section panels. You must bond together multi-section industrial control panels with an approved equipment grounding conductor or equivalent ground bus sized in accordance with Table 250.122. Terminate equipment-grounding conductors on this ground bus or to a grounding termination point provided in a single-section industrial control panel.
Construction specifications. The construction specification sections of Art. 409 focus on the enclosure, bus bars, wiring space, and marking requirements of industrial control panels. Here is a summary of the basic requirements:
Refer to Table 430.91 when selecting an enclosure for use in a space other than a hazardous (classified) location.
Protect bus bars from physical damage, and make sure they're designed to be held firmly in place. Ensure their phase arrangement complies with the requirements of 409.102(B).
Where used as service equipment, each industrial control panel shall be of the type suitable for use as service equipment.
Do not allow the control panel to be used as a junction box, auxiliary gutter, or raceway for conductors feeding through or tapping off to other switches or overcurrent devices, unless you provide adequate space for this purpose.
Allow for proper wire bending space and adhere to the conductor fill limitations as noted in 409.104(A) and (B).
The most challenging section of this new Article deals with the marking of the panel. As per 409.110, you must clearly mark an industrial control panel based on the following six criteria.
Manufacturer's name, trademark, or other descriptive marking by which the organization responsible for the product can be identified.
Supply voltage, phase, frequency, and full-load current.
Short-circuit current rating of the industrial control panel based on one of the following:
a. Short-circuit current rating of a listed and labeled assembly.
b. Short-circuit current rating established using an approved method. (FPN: UL 508A-2001, Supplement SB, is an example of an approved method.)
If the industrial control panel is intended as service equipment, it shall be marked to identify it as being suitable for use as service equipment.
Electrical wiring diagram or the number of the index to the electrical drawings showing the electrical wiring diagram.
An enclosure type number shall be marked on the industrial control panel enclosure.
A key point to note with regard to item (3) above is that a change has been made as to the method used to determine the short-circuit current rating (SCCR) of the industrial control panel. Under the old method, you used the AIR of the main overcurrent protective device. The AIR is the interrupter rating as noted in 110.9. The SCCR is the withstand rating as noted in 110.10. Under the new method, you must include the entire combined power circuit in your calculations.
One way to calculate the SCCR is to use the least common denominator method. If there are numerous items within the panel, such as circuit breakers, motor controllers, and motor control circuits, the SCCR marked on the panel must be based on the lowest rating of any of the devices within the enclosure. For example, if the short-circuit ground-fault device (i.e. circuit breaker) has an SCCR of 42kA, the motor control has an SCCR of 25kA, and the motor controller has an SCCR of 10kA, then the overall panel SCCR is 10kA.
An alternate method for determining the SCCR of the industrial control panel is to use a group of power circuit components that have already been tested in combination and assigned a specific SCCR.
Testing promotes safety. As is the case with other types of electrical control and distribution equipment, one of the primary concerns associated with a group of components assembled in a common enclosure for the purposes of operation, control, and overcurrent protection is the ability of this equipment to limit and contain the effects of an internal fault. The goal should always be to contain the fault so that it doesn't pose an external ignition threat.
In many control panel installations, the available fault energy at the line terminals of components within the panel is significant, and the absence of an SCCR promotes the possibility of a failure that may extend beyond the control panel enclosure. Not only are there high levels of short-circuit current available at the line terminals of many industrial control panels, but there is also an interaction of the protection and control components under fault conditions. This is where independent, third-party testing of panels containing interrelated and interactive control and protective components proves so valuable.
However, if the installation consists of simple assemblies, such as control components, the authority having jurisdiction (AHJ) may not define the assembly as an industrial control panel. Nevertheless, it's important for them to be fully aware of the short-circuit current considerations of the particular installation and be confident there is no interaction between the assembled listed components, which may result in noncompliance with 110.10 of the 2005 NEC.
The bottom line. The NEC does not require industrial control panels to be listed, even though the installed control components and enclosures within them are required to be listed. This relates to the requirements of 90.7, which involves the AHJ in determining whether or not a specific control panel is safe to use as designed, assembled, and installed. The intent of Art. 409, in conjunction with UL 508A and NFPA 79, is to provide a set of requirements that can be used as a benchmark for approval of both new and field-constructed industrial control panels.
In addition to the requirements of Art. 409, industrial control panels that contain branch circuits for specific loads for components, or are for control of specific types of equipment addressed in other articles of this Code, shall be constructed and installed in accordance with the applicable requirements from the specific articles in Table 409.3.
Owen is the owner and president of National Code Seminars in Pelham, Ala.
Sidebar: Definition: Industrial Control Panel
As per 409.2 of the 2005 NEC, an industrial control panel is defined as “An assembly of a systematic and standard arrangement of two or more components such as motor controllers, overload relays, fused disconnect switches, and circuit breakers and related control devices such as pushbutton stations, selector switches, timers, switches, control relays, and the like with associated wiring, terminal bocks, pilot lights, and similar components. The industrial control panel does not include the controlled equipment.”
Sidebar: Key Features of NFPA 79
Chapter 4 — General Operating Conditions. This chapter includes general considerations for electrical components and devices, electrical supplies, physical environment and operating conditions, transportation and storage, provisions for handling, and installation and operating conditions.
Chapter 6 — Protection from Electric Shock. This chapter notes electric shock hazards that are not addressed in Art. 409 but are extremely important for personnel safety.
Chapter 8 — Grounding. This chapter offers more detailed information on grounding requirements for industrial control panels than does Art. 409.
Chapter 19 — Testing and Verification. In part, this chapter notes requirements for continuity of equipment grounding conductors, and discusses insulation resistance, voltage, and functional testing procedures, as well as retesting requirements.