Don't underestimate the NEC's requirements for flexible cords and fixture wires — they're actually important enough that the standard covers them in two separate articles. Specifically, you'll find flexible cord requirements in Art. 400 and fixture wire requirements in Art. 402.
Flexible cords. The first thing you need to understand about flexible cords is that the NEC doesn't consider them “a wiring method.” Article 400 applies to the cords and cables in Table 400.4, but it doesn't apply to the various types of cables in Chapter 3, such as NM, AC, or MC cable.
Next, it's important to remember that you must use the right cord and fittings for the application. For example, when you're working in a wet location, you need to use a cord approved for that purpose. This 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.
Take a few minutes to look at Table 400.4. If you think this is overkill for an extension cord, you're right. But it's not just about extension cords. Its scope ranges from lamp cords to vacuum cords — three of the entries are for elevator cords alone. It's still 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. Follow the requirements for 240.5 to protect cords.
You'll find a list of 11 permitted uses for flexible cords in 400.7. It helps to understand the rationale behind these uses. From time to time, equipment like pendants, cranes, and elevators must be moved and therefore need a flexible cord. In other cases, a short flexible cord provides for ease of installation and maintenance, as is the case with luminaires.
Art. 400 also includes a list of six uses not permitted for flexible cords. Consider the first item: you can't use cords to substitute for the fixed wiring of a structure. Requirements like these demonstrate the fact that flexible cords can't be used to get around Chapter 3 wiring methods.
You can't run cords in suspended ceilings or other “out-of-sight areas” (Fig. 1 above). Why is it OK to wire a luminaire with a cord when it's in the open, but not OK to use that same cord in a suspended ceiling? When a cord isn't in a concealed space, you can inspect it for damage caused by insects and rodents. Inside a suspended ceiling, these pests can chew through the cord and create a fire hazard you may not discover until it's too late. However, you can put cords within a raised floor not used for environmental air because this isn't considered a concealed space. See Art. 100 for the definition of “exposed.”
The concept of not using flexible cords in place of Chapter 3 wiring methods sets the tone for 400.7 and 400.8. 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 $1 million per month.
The subsequent investigation into these problems revealed 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 more than 200 feet away. Those loads included 10-hp motors and 120/208V transformers.
Redoing this installation to conform to Art. 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 their intended purpose and using Chapter 3 wiring methods where required.
Flexible cords need support. Per 400.10, be sure to install them 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 Code isn't a design guide — a higher level of stress relief than the 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 (Fig. 2).
Fixture wires. As with flexible cords, the NEC doesn't consider fixture wires to be “a wiring method.” And just as the NEC provides a large table that lists flexible cords, it also provides Table 402.3, which lists the various types of fixture wires, and Table 402.5, which specifies their allowable ampacities. If you look at that table, you'll see the smallest wire size is 18 AWG (Table). The NEC doesn't 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.
Per 402.10 and 402.11, you can use fixture wires to connect luminaires, but you can't use them as branch circuit conductors (Fig. 3 and Fig. 4 above). 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 overcurrent per 240.5. If you're 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 minimal amount of time, but adhering to them can provide big benefits by eliminating preventable problems.