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Service Maintenance Company, Inc. Electrical Contractors |
| NEC Article 400 - Cords and Fixture Wires | 12/29/2003 |
| Correctly applying Articles 400 and 402 can prevent serious problems. By Mike Holt for EC&M Magazine Don't underestimate the requirements for flexible cords and fixture wires. They are important enough that the NEC covers them in two separate articles. Specifically, you'll find flexible cord requirements in Article 400 and fixture wire requirements in Article 402. Flexible cords The first thing to understand about flexible cords is the NEC does not consider them "a wiring method." Article 400 applies to the cords and cables in Table 400.4, but it does not apply to the cables in Chapter 3 (e.g., NM, AC, or MC cable). Perhaps the second thing to understand about flexible cords is you must use the right cord (and fittings) for the application. This means, for example, in a wet location use a cord approved for a wet location. The rule exists because the jacket material is tested to maintain its insulation properties and other characteristics only in the environment for which UL (or another certifying body) has approved it (Figure 400-1). Note: Graphics are not contained in this newsletter; they will be in the magazine. Take a few minutes to look at Table 400.4. If you are thinking this is overkill for an extension cord, you are right. But, it's not about extension cords. Three of the entries are for elevator cords alone. It's good to note the entries in this table. You'll find the allowable ampacities for flexible cords in Tables 400.5(A) and 400.5(B). You'll find the overcurrent protection requirements in [400.13]. Protect cords per [240.5] requirements. Uses permitted, not permitted In [400.7], you'll find a list of 11 permitted uses for flexible cords. It helps to understand the rationale behind these uses. The equipment supplied by flexible cords often requires some movement between the equipment and the power supply. Examples include pendants, cranes, and elevators. In other cases, a short flexible cord provides for ease of installation and maintenance-as with luminaires. There is logic behind the list, so think of the purpose of the cord before installing it. The purpose never includes "getting around" Chapter 3 wiring methods. In [400.8], you'll find a list of 6 uses not permitted for flexible cords. Consider the first item: you can't use cords to substitute for the fixed wiring of a structure-that's Chapter 3, again. You can't run cords in suspended ceilings and other "out of sight areas" (Figure 400-6). Why is it OK to wire a luminaire with a cord when it's in the open, but a Code violation to use that same cord in a suspended ceiling? When a cord is not in a concealed space, you can inspect it for damage from insects and rodents. Inside a suspended ceiling, rats or other vermin can chew through the cord and create a fire hazard you are completely unaware of. You can put cords within a raised floor not used for environmental air, because this is not concealed space (see Article 100 for the definition for "exposed"). The concept of not using flexible cords in place of Chapter 3 wiring methods sets the tone for [400.7] and [4008]. Consider the example of an appliance factory that violated this concept. Severe power quality problems in the finishing area resulted in unscheduled shutdowns and high scrap rates at a cost of nearly a million dollars per month. An associate of mine investigated these problems, and was amazed to see a 2-foot thick bundle of flexible cords (SO wire) held together by hundreds of tie-wraps. This bundle ran from several 480V breakers to loads over 200 feet away. Those loads included 10HP motors and 120/208V transformers! Redoing this installation to conform to Article 400 and Chapter 3 eliminated the power quality problems-and a major fire hazard. The payback period was just a few days, based on the power quality issues alone. While this is an extreme example, it illustrates the concept of using flexible cords for what they are intended to do and using Chapter 3 wiring methods where required. Installation Flexible cords need support. Per [400.10], be sure to install these in a way that prevents transmission of tension to the conductor terminals. The NEC allows you to knot the cord, wind it with tape, or use fittings designed for relieving stress. However, the NEC is not a design guide-a higher level of stress relief than Code minimum is often appropriate. For critical installations, it's best to use a factory-made stress-relieving listed device; not an old-timer's knot (Figure 400-7). Fixture wires Now let's turn our attention to fixture wires. As with flexible cords, the NEC does not consider these "a wiring method." And just as the NEC provides a large table listing out flexible cords, so it provides Table 402.3 for fixture wires. It also provides Table 402.5, which lists the allowable ampacities for fixture wires. If you look at that table, you'll see the smallest wire size is 18 AWG. The NEC does not allow using a smaller fixture wire. If you use 20 AWG for fixture wire, you'll have a Code violation. Raceways for fixture wires must be large enough to permit the installation and removal of conductors without damaging the insulation. Don't exceed the percentage fill specified in Table 1, Chapter 9. See [300.17] for additional details. If all conductors in a raceway are the same size and insulation, refer to Annex C for the maximum quantity per raceway type. Uses permitted, not permitted Per [402.10] and [402.11], you can use fixture wires to connect luminaires (Figure 402-2)-but you cannot use them as branch circuit conductors (Figure 402-3). You can also use them for elevators and escalators [620.11(C)], Class 1 control and power-limited circuits [725.27(B)], and nonpower limited fire alarm circuits [760.27(B)]. You must protect them against over overcurrent per [240.5]. If you are using fixture wires for motor control circuit taps, follow [430.72(A)]. For Class 1 control circuits, follow [725.23]. Understanding flexible cord and fixture wiring requirements requires a minimum of time. Adhering to them can provide big benefits by eliminating easily preventable problems. Copyright © 2003 Mike Holt Enterprises,Inc. |
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