Understanding the wiring requirements of Article 400, including guidelines for flexible cords, cables, and fixture wires
Flexible cords and cabling are very useful, allowing us to do things that Chapter 3 wiring methods do not. But this flexibility comes at a cost, and that cost is increased vulnerability. For that reason, Art. 400 requirements are essential for safety.
We can see the various types of flexible cords and cables in Table 400.4, which takes up seven pages and comprises more than half of Art. 400. The first rule of working with flexible cords and cables is always use that table. Some other basic rules to always keep in mind include:
Tables 400.5(A) and 400.5(B) list the allowable ampacity for flexible cords and flexible cables with not more than three current-carrying conductors at an ambient temperature of 86°F. The tables assume copper conductors.
The ampacity from Table 400.5(A) or Table 400.5(B) may need to be adjusted as follows:
Wiring methods for flexible cords and cables aren't like those defined in Chapter 3. So, where can you use them? Per 400.7, you can use them in the following applications:
In 400.8, the NEC lists specific types of uses that are not permitted for flexible cords and cables [400.8]. Unless specifically allowed in 400.7, flexible cords must not be:
Some clarification is in order for the fifth bulleted item above. You can use flexible cords under a raised floor with removable panels used for environmental air, because this area isn't considered a concealed space. See the definition of "Exposed" in Art. 100.
You can install receptacles above suspended or dropped ceilings, but you can't install flexible cords up there to plug into them. So what's the point of installing the receptacle? The receptacle can supply power for portable lights and tools, but that receptacle can't be used to supply cord- and plug-connected equipment fastened in place, such as a permanently mounted projector.
Install flexible cords so tension, including that from the weight of the cord, won't be transmitted to the conductor terminals [400.10]. While the NEC allows you to accomplish this by knotting the cord or winding the cord with tape, it's best to use fittings designed for the purpose, such as strain-relief fittings (Fig. 4).
While a strain-relief fitting is optional, other types of fittings may not be. You must use bushings or fittings to protect cords where they pass through holes in covers, outlet boxes, or similar enclosures [400.14].
While you normally can't install flexible cords in raceways, there is a provision in the Code that allows cords inside raceways for a specific type of installation. In industrial establishments where the conditions of maintenance and supervision ensure that only qualified persons will service the installation, you can install flexible cords or flexible cables in aboveground raceways — but only if the cords/cables aren’t more than 50 ft long [400.14].
Flexible cords and cables must be protected against overcurrent in accordance with 240.5, which contains the following requirements:
How do you know which conductor in a cord is the neutral? It must be identified using one of the following methods [400.22]:
The equipment grounding conductor must be identified with one of the following methods [400.23]:
If a conductor in a cord or cable has insulation that's green or green with one or more yellow stripes, don't use it for an ungrounded or neutral conductor [250.119].
Fixture wires are flexible conductors used for wiring fixtures and control circuits, but not branch circuits. There are some special uses and requirements for fixture wires, and NEC Art. 402 deals especially with fixture wires. Some key points:
Raceways must be large enough to permit the installation and removal of conductors without damaging conductor insulation [402.7]. The number of fixture wires permitted in a single raceway must not exceed the percentage fill specified in Table 1 of Chapter 9.
When all conductors in a raceway are the same size and insulation, you can use Annex C to determine the number of conductors permitted for the raceway type.
When all conductors are not of the same insulation type or the same size, then Chapter 9, Table 5 is used to find the area of all conductors, and Chapter 9, Table 4 is then used to size the raceway that can accommodate that conductor fill.
The fixture wire used as a neutral conductor must have continuous white stripes or be identified by one of the means allowed for the neutral of flexible cords and cables as listed in 400.22(A) through (E) [402.8]. Always remember that in order to prevent electric shock, the screw shell of a luminaire or lampholder must be connected to the neutral conductor [200.10(C) and 410.50].
You can use fixture wires:
You can't use fixture wires, however, for branch-circuit wiring, except as permitted elsewhere in the Code [402.11].
Fixture wires are allowed to be tapped off of a branch-circuit conductor following the requirements of [240.5(B)(2)]:
Fixture wires used for motor control circuit taps must have overcurrent protection per 430.72(A), and Class 1 remote-control circuits must have overcurrent protection per 725.43.
The requirements for flexible cords, cables, and fixture wires aren't hard to meet; however, violating them can cause serious problems. Always remember that the rules for flexible cords and cables are in Art. 400 and fixture wires are covered by Art. 402, then start by using the tables in these articles for application and ampacity details.